ACS Publications. Most Trusted. Most Cited. Most Read
Quick View

OR SEARCH CITATIONS

My Activity
Publications
CONTENT TYPES

Figure 1Loading Img
Download Hi-Res ImageDownload to MS-PowerPointCite This:Nano Lett. 2022, 22, 13, 5252-5259
RETURN TO ISSUEPREVLetterNEXT

Moiré-Induced Transport in CVD-Based Small-Angle Twisted Bilayer Graphene

  • Giulia Piccinini
    Giulia Piccinini
    NEST, Scuola Normale Superiore, Piazza San Silvestro 12, 56127 Pisa, Italy
    Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, 56127 Pisa, Italy
    More by Giulia Piccinini
    Orcidhttps://orcid.org/0000-0002-2902-1361
  • , 
  • Vaidotas Mišeikis
    Vaidotas Mišeikis
    Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, 56127 Pisa, Italy
    Graphene Laboratories, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy
    More by Vaidotas Mišeikis
  • , 
  • Pietro Novelli
    Pietro Novelli
    Istituto Italiano di Tecnologia, Via Melen 83, 16152 Genova, Italy
    More by Pietro Novelli
    Orcidhttps://orcid.org/0000-0003-1623-5659
  • , 
  • Kenji Watanabe
    Kenji Watanabe
    Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
    More by Kenji Watanabe
    Orcidhttps://orcid.org/0000-0003-3701-8119
  • , 
  • Takashi Taniguchi
    Takashi Taniguchi
    International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
    More by Takashi Taniguchi
    Orcidhttps://orcid.org/0000-0002-1467-3105
  • , 
  • Marco Polini
    Marco Polini
    Dipartimento di Fisica, Università di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
    Graphene Laboratories, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy
    More by Marco Polini
  • , 
  • Camilla Coletti*
    Camilla Coletti
    Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, 56127 Pisa, Italy
    Graphene Laboratories, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy
    *Email: [email protected]
    More by Camilla Coletti
    Orcidhttps://orcid.org/0000-0002-8134-7633
  • , and 
  • Sergio Pezzini*
    Sergio Pezzini
    NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Piazza San Silvestro 12, 56127 Pisa, Italy
    *Email: [email protected]
    More by Sergio Pezzini
    Orcidhttps://orcid.org/0000-0003-4289-907X
Cite this: Nano Lett. 2022, 22, 13, 5252–5259
Publication Date (Web):July 1, 2022
https://doi.org/10.1021/acs.nanolett.2c01114

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY 4.0.
  • Open Access

Article Views

3797

Altmetric

-

Citations

1
LEARN ABOUT THESE METRICS
PDF (5 MB)
Get e-Alerts
Supporting Info (1)»
SUBJECTS:

Abstract

High Resolution Image
Download MS PowerPoint Slide

To realize the applicative potential of 2D twistronic devices, scalable synthesis and assembly techniques need to meet stringent requirements in terms of interface cleanness and twist-angle homogeneity. Here, we show that small-angle twisted bilayer graphene assembled from separated CVD-grown graphene single-crystals can ensure high-quality transport properties, determined by a device-scale-uniform moiré potential. Via low-temperature dual-gated magnetotransport, we demonstrate the hallmarks of a 2.4°-twisted superlattice, including tunable regimes of interlayer coupling, reduced Fermi velocity, large interlayer capacitance, and density-independent Brown-Zak oscillations. The observation of these moiré-induced electrical transport features establishes CVD-based twisted bilayer graphene as an alternative to “tear-and-stack” exfoliated flakes for fundamental studies, while serving as a proof-of-concept for future large-scale assembly.

This publication is licensed under

CC-BY 4.0.
  • cc licence
  • by licence
KEYWORDS:

Twisted 2D materials provide an extraordinarily rich platform for engineering emergent electronic, (1,2) magnetic, (3) and optical (4) properties. The van der Waals (vdW) stacking techniques, (5,6) which are not applicable to traditional low-dimensional condensed-matter systems, (7) are especially boosting this research field, allowing the realization of complex moiré structures involving multiple precisely aligned atomic layers. (8−10) As twistronics, that is, the understanding and control of the moiré-induced behaviors, rapidly advances, (11−13) novel perspectives of technological application arise. (14) For instance, the tantalizing superconducting phase of magic-angle (MA) twisted bilayer graphene (TBG) (1) has already been exploited for the fabrication of broadband photodetectors, (15) as well as gate-defined monolithic Josephson junctions (16−18) and quantum interference devices. (19) However, although technological integration of stand-alone 2D materials appears increasingly viable thanks to key advancements in the synthesis methods (20) (such as chemical vapor deposition (CVD) of high-mobility single-layer graphene (SLG) (21−24)), in the case of TBG, further challenges have to be addressed. Ideally, application-oriented TBG devices should simultaneously offer (i) a deterministically selectable small-angle (SA) twisting, (ii) a device-scale uniform twist angle, and (iii) an atomically clean interlayer enabling the formation of a moiré potential. In TBG, strong modifications in the electronic bands arise only for SA twisting (θ < 5°), (25,26) while the physics of two decoupled layers is reached asymptotically at larger twist angles. (27−29) SA twisting was observed in CVD-grown graphene films studied by scanning probe microscopy. (30) However, due to its polycrystalline nature and random grain orientations, this system is unsuitable for spatially averaging probes such as electrical transport. CVD-grown graphene single crystals, compatible with fabrication of high-quality devices, can incorporate TBG domains with uniform twisting. (31−34) Nonetheless, the twist angle preferentially locks to 0° (Bernal stacking) or 30° due to interactions with the growth substrate. (31)

Recent developments in the synthesis process (35) have allowed one to obtain a fraction of intermediate twist-angles (down to ∼3°), higher than in previous studies (36) but lacking however deterministic control, as well as moiré transport signatures. To overcome this issue, one can employ a hybrid approach by stacking two CVD-grown SLG to form TBG, obtaining either large or SA twisting, as demonstrated by photoemission (37−39) and scanning probe experiments, (40,41) respectively. Although permitting high rotational accuracy in analogy to exfoliated flakes, (6) sequentially stacked CVD-grown graphene layers tend to damage and accumulate contaminants at their interface. (42) As a consequence, transport experiments on CVD-based TBG with moiré effects are (to the best of our knowledge) unreported and, therefore a conclusive demonstration of TBG realizing the preliminary scalability conditions outlined above is lacking.

In this work, we fill this gap by introducing SA-TBG samples obtained by hBN-mediated stacking of isolated SLG crystals grown by CVD on a single Cu grain. The growth-determined crystallographic alignment of the SLG crystals (43) enables deterministic control on the twist angle at the vdW assembly stage. The interface cleanness and twist-angle uniformity are unambiguously supported by the observation of high-quality quantum transport features specific to TBG with a twist angle of ∼2.4°. By these means, we demonstrate the first moiré device based on CVD-grown crystals and set a cornerstone toward the application of 2D materials twistronics.

In Figure 1a, we present the vdW assembly sequence developed for CVD-based SA-TBG. As the pick-up medium, we employ a poly(bisphenol A carbonate) (PC) film deposited onto a few-mm thick polydimethylsiloxane (PDMS) block, supported by a glass slide, (44) that we control using a home-built transfer setup. (45) We start from an array of SLG crystals grown via CVD on Cu (see the SI file for details) and subsequently transferred to SiO2/Si using a polymer-assisted technique, as described in refs (43and45). We select two graphene crystals from the array, making sure that they were synthesized on the same Cu grain and, therefore, that they share the same crystallographic orientation, as demonstrated in ref (43) (Figure 1b). The use of two separated crystals extends the standard method for preparing SA-TBG samples for transport studies, which proceeds by stacking two portions of the same SLG flake. (6) Once the two crystals are selected, we adopt the procedure described in ref (44) to pick up the first graphene crystal from SiO2 using an hBN flake (10–50 nm thick). We then use the goniometer stage holding the sample with graphene on SiO2 (shown in SI Figure S1) to rotate the graphene array by an arbitrary angle θ, which is affected by an instrumental error of ∼0.01°. The twist angle θ determines the expected periodicity λ of the moiré pattern (Figure 1c), according to , where a ≃ 0.246 nm is the SLG lattice constant. Thereafter, we approach and pick up the second graphene crystal and a second hBN flake, completing the encapsulation. The temperature of the setup is kept at 40–60°C during all these steps, consistently ensuring the complete pick-up of the graphene regions approached by the hBN. Finally, the stack is released onto a SiO2/Si substrate by melting the PC film at 160–170°C, favoring cleaning of the vdW interfaces. (44) After the assembly, we nevertheless observe blisters where contaminants aggregate (Figure 1d, inset), which limit the lateral dimension of flat areas suitable for device processing (typically few micron-wide). Figure 1d shows the Raman spectrum of the assembled TBG, compared to that of an hBN-encapsulated SLG. The two spectra differ in several features. The large 2D/G intensity ratio characteristic of SLG (∼10) dramatically drops in TBG (∼1). In addition, the 2D peak width strongly increases, from ∼17 to ∼54 cm–1. At a closer inspection, the 2D peak of TBG reveals a multicomponent structure (46−48) with two broad subpeaks located at ∼2675 and ∼2700 cm–1. Overall, we observe striking similarities with the Raman spectrum at ∼2.6°-twisting reported in ref (46) in accordance with the angle θ = 2.5° set during the vdW assembly. The assembly of a second sample with the same target angle, showing analogous Raman response, is presented in SI. Raman data from a third sample with sub-MA twisting are shown in SI.

Figure 1

Figure 1. (a) Schematics of the dry pick-up process with stacking of separated CVD-grown graphene crystals. (b) Optical microscopy image of CVD SLG crystals on SiO2/Si. The dashed lines indicate their crystallographic alignment. (c) The θ-rotated graphene sheets form a moiré pattern with periodicity λ. (d) Representative Raman spectrum of TBG (dark red), compared to a SLG reference (gray). The light red and blue lines are the two Lorentzian components of the TBG 2D peak. Inset: optical microscopy image of hBN-encapsulated SA-TBG. The dark red spot indicates the point where the TBG spectrum in the main panel is acquired.

High Resolution Image
Download MS PowerPoint Slide

The target angle θ is chosen to fall in the intermediate twist-angle range where the Fermi velocity (vF) is reduced with respect to that of SLG, (25) while the interlayer coupling can be varied from weak to strong by experimentally available gate voltages. Such tunability was demonstrated by transport experiments on devices obtained by “tear-and-stack” exfoliated flakes in refs (47−51), which serve as a guideline for our investigation of CVD-based SA-TBG.

In Figure 2, we show low-temperature (magneto)transport data on a dual-gated device fabricated from the SA-TBG sample (see SI for details on the processing). The dual-gated configuration (Figure 2b) is essential in multilayer graphene devices, as it allows independent tuning of the total carrier density (ntot, determined by the sum of the gate potentials) and its distribution among the layers via the so-called displacement filed (D, determined by the difference of the gate potentials). However, this holds true as long as the interlayer coupling is small enough as to keep the layers’ Dirac cones independent, (27−29,34) while in the strong coupling regime D has no major effect. (52) By applying a perpendicular magnetic field (B = 3 T in Figure 2a) we observe a pattern of crossings in the derivative of the Hall conductivity σxy with respect to the voltage applied via the top gate (dσxy/dVtg), corresponding to alternating interlayer quantum Hall states (dσxy/dVtg = 0) and layer-resolved Landau levels (LLs, dσxy/dVtg ≠ 0). (34) This pattern can be modeled by considering the screening properties of two superimposed SLG subject to the top and bottom gate potentials─ Vtg and Vbg, respectively ─and coupled via an interlayer capacitance Cgg (27,29) (complete details on the electrostatic model employed can be found in ref (53)). Importantly, the exact gate dependence of the LLs is sensitive to both the carrier density n and Fermi energy EF in the individual layers, which for Dirac Fermions are related according to . Using Cgg and vF as free parameters, we simulate the LLs trajectories and make them converge to the experimental pattern of crossings. The results are shown as orange and red dotted lines in Figure 2a, for the upper and lower layers LLs, respectively. From this procedure, we can estimate a Fermi velocity vF = (0.47 ± 0.02) × 106 m/s and an interlayer capacitance Cgg = (17.5 ± 1.0) × 10–6 F/cm2.

Figure 2

Figure 2. (a) First derivative of the Hall conductivity as a function of top and back-gate voltages, measured for a fixed value of the applied perpendicular magnetic field (B = 3 T). The dotted orange (red) lines are the calculated positions of Landau levels from the upper (lower) graphene layers, employing vF = 0.47 × 106 m/s and Cgg = 17.5 × 10–6 F/cm2. (b) Schematics of the gating configuration. The optical microscopy image of the device is taken before the final etching step; the scale bar is 2.5 μm. (c) Fermi velocity of TBG as a function of the twist angle, calculated according to the theory described in refs (54and55) and references therein. Results in this figure have been obtained by setting u0 = 79.7 meV and u1 = 97.5 meV, where u0 and u1 are the intra- and intersublattice interlayer tunneling amplitudes, respectively. The blue circle corresponds to the vF value estimated for our device. (d) Hall conductivity as a function of the gate voltages (same gate ranges and magnetic field as in (a)). The sign changes in σxy correspond to the sample CNP and the two vHs. The black rectangle indicates the gate range considered in panel (e), the black and dark red dots are the gate values used for the measurements in Figure 4. (e) Zero-field longitudinal conductivity (lg scale) as a function of top-gate voltage relative to the sample CNP and back-gate voltage. The dotted orange (red) line is the calculated charge neutrality point for the upper (lower) layer. All the data in this figure have been acquired at T = 4.2 K.

High Resolution Image
Download MS PowerPoint Slide

The suppression of vF with respect to SLG is a well-known feature of SA-TBG. (25,30) Band structure calculations based on Bistritzer-MacDonald-type Hamiltonians (25,54,55) allow to estimate the corresponding twist angle to be ∼2.4° (Figure 2c).

Concerning the interlayer capacitance and in agreement with ref (48), our estimate is twice as large with respect to the accepted value of Cgg for large-angle TBG. (27,29) If one insists in using a classical-type formula for Cgg, that is, Cgg = ε0εr/deff, with deff a suitable effective interlayer distance, this finding could be interpreted in terms of a smaller effective interlayer spacing, signaling the increased coupling in this twist-angle range (eventually, toward MA such effective separation vanishes, leading to a complete suppression of the LLs crossings (52)). A less naïve approach should rely on analyzing microscopically all the nonclassical contributions to Cgg by using the profound relationship that exists between the ground-state energy of a double-layer system and linear response functions. (56) This has been recently done for example in ref (57), but no explicit calculations have been reported by the authors for TBG.

The crossing pattern in Figure 2a is abruptly interrupted in the vicinity of the upper right and lower left corners of the Vtg–Vbg map, that is, at high total carrier density (ntot > 5.88 × 1012 cm–2 with ntot being the sum of the carrier densities in the two layers obtained from the electrostatic modeling). The Hall conductivity σxy, plotted in Figure 2d, shows that the upper right (lower left) region corresponds to a transition from large electron (hole) density to large hole (electron) density. This change contrasts with the low-density switch at the charge-neutrality point (CNP, central diagonal), and it is characteristic of van Hove singularities (vHs) in the density of states, corresponding to the transition from layer-independent massless electrons (holes) to layer-coupled massive holes (electrons). (47−51) In Figure 2e, we show the zero-field longitudinal conductivity as a function of the gate potentials in the vicinity of the sample CNP (black-highlighted area in Figure 2d). In this zoomed plot, we can observe different regions of interlayer charge configuration, controlled by a splitting of the CNPs of the individual layers (similar data for large-angle TBG have been reported in refs (53and58)). The boundaries of these regions are perfectly reproduced by the neutrality conditions for the two layers (orange and red dotted lines), computed according to the extracted vF and Cgg, confirming the estimate obtained in the perpendicular magnetic field (in the SI file we show how the CNPs trajectories vary as functions of vF and Cgg). The observation of the CNPs splitting substantiates the reduction of the peak resistance at large displacement fields observed in previous experiments (47,50,51) which is due to coexisting charges of opposite sign in the two layers with relatively small concentration (<1011 cm–2).

In Figure 3a we present the longitudinal resistance of the device, measured as a function of Vtg and B, at two fixed values of Vbg (−60 V and +60 V, in the left and right panels, respectively, corresponding to the upper and lower limits of the gate map in Figure 2a). This configuration is chosen to span the largest possible density range, while keeping D finite. A fast Fourier transform (FFT) of these data, giving the frequency spectrum of the 1/B-periodic components of the resistance, (47) is shown in Figure 3b to ease the interpretation of the complicated pattern of experimental data reported in Figure 3a. The color map in Figure 3b represents the normalized amplitude of the FFT plotted as a function of the total carrier density, and the frequency BF, which is proportional to the extremal area of the Fermi surface perpendicular to the magnetic field. In the central part of the magneto-resistance data, close to the CNP, we observe two superimposed Landau fans, which can be attributed to the upper and lower layers’ Dirac cones centered at the Ks and Ks′ points in the superlattice Brillouin zone (see band structure calculations in Figure 3c, inset). Because of the finite displacement field, the two layers have different carrier concentrations and, therefore, the fans repeatedly cross each other. The corresponding frequencies in the FFT are well described by (orange and red dotted lines in Figure 3b, for the upper and lower layer, respectively), where the carrier concentration nlayer in each layer can be calculated by using the vF and Cgg values extracted previously, and the factor 4 accounts for the spin and valley degeneracies. The sum of the two components evolves as (dark red dotted line).

Figure 3

Figure 3. (a) Longitudinal resistance measured as a function of Vtg and B, at Vbg = −60 V (left panel, T = 2.5 K) and Vbg = +60 V (right panel, T = 4.2 K). (b) Normalized FFT amplitude of the data in panel (a), as a function of the total charge density and of the oscillation frequency BF. (c) Fan of quantized states originating from the Γs point. Inset: band structure calculations for TBG with θ = 2.4°, based on refs (25,54,and55). The same intra- and intersublattice interlayer tunneling amplitudes of Figure 2c are used. Hartree self-consistent corrections do not yield significant changes with respect to single-particle calculations because the twist angle considered in this work is sufficiently larger that the MA. (d) Hall conductivity in the vicinity of the hole-side vHs, as a function of Vtg and 1/B (left axis). The right axis scale shows the number of flux quanta per superlattice unit cell, that is, ϕ/ϕ0.

High Resolution Image
Download MS PowerPoint Slide

At the largest density reached in the experiment (left-most part in the left panel, right-most part in the right one), we observe LL fans with opposite dispersions with respect to the central ones, in accordance with the change in sign of the charge carriers detected in the Hall conductivity (Figure 2d). These features emerge due to progressive filling of the moiré band at Γs (Figure 3c, inset), which is completed at . In Figure 3b we show that the frequency components corresponding to the Γs fans evolve as (dark cyan dotted lines, where ns = ±13.3 × 1012 cm–2 for the right and left panel, respectively), indicating a single 4-fold degenerate Fermi surface, in agreement with ref (47). In Figure 3c, we show that the corresponding fan of quantized states, calculated using a zero Berry phase, (49) matches the resistance oscillations in panel a.

Close to the previously identified vHs (Vtg ∼ −4 V and +4 V, in the left and right panel, respectively) we observe two funnelling structures with large longitudinal resistance, associated with the coexistence of carries with opposite sign. Here, the oppositely dispersing fans of Landau levels coalesce, as expected from theoretical calculations in our twist-angle range. (26,59) Notably, we observe a series of horizontal strikes superimposed to the intersecting fans, which signal a density-independent oscillation of the resistance. The corresponding frequency is equal to 137 T (magenta dotted line in Figure 3b). Density-independent oscillations were discovered in graphene-hBN superlattices (60−62) and attributed to the periodic creation of so-called Brown-Zak (B-Z) particles moving along straight trajectories in finite magnetic field. (61) The characteristic frequency of this phenomenon allows a highly precise estimate of the moiré periodicity according to . Considering the average position of our FFT peak, we obtain a twist angle θ = (2.39 ± 0.01)°. Finally, in Figure 3d we show the Hall conductivity in the vicinity of the hole-side vHs, as a function of 1/B. In accordance to the B-Z periodicity, we observe sign changes at commensurate values of flux quanta per superlattice unit cell ϕ/ϕ0 = 1/q (where ϕ0 = h/e is the flux quantum, and q is an integer). In addition, toward the highest magnetic fields, we observe a nonmonotonic behavior as a function of both magnetic field and carrier density, a hallmark of the Hofstadter’s butterfly. (63−65) The appearance of these features coincides with the transition from the semiclassical regime (well-defined electron and hole-like oscillations) to the fractal regime, which is expected when the magnetic length () becomes comparable to the superlattice periodicity λ = 5.9 nm. (26)

A distinctive feature of the B-Z oscillations is their resilience to the thermal energy, which allows their observation up to boiling-water temperature. (60) In Figure 4, we present resistance data acquired at T = 35 K, where the standard Shubnikov–De Haas oscillations are strongly suppressed and the B-Z oscillations become more apparent. (50) We show two curves taken in the vicinity of the electron-side vHs, at D = 0 and D > 0 (dark red and black curves, respectively; the gate values are indicated by markers in Figure 2d). We observe a dominant fast oscillation corresponding to the B-Z frequency (BF = 137 T, see FFT spectra in the inset), whose amplitude and phase are unaffected by D (in addition to ntot, as already shown in Figure 3b). This contrasts with the slowly varying background (BF ∼ 30 T), attributed to the Ks–Ks′ Shubnikov–De Haas oscillations, whose phase reverts as a function of D as the charge distribution in the two layers is modified. Recently discovered D-dependent high-temperature oscillations from interminivalley scattering (66) are not observable in our current set of data.

Figure 4

Figure 4. Longitudinal resistance as a function of B, measured at T = 35 K in the vicinity of the electron-side vHs, at D = 0 (dark red curve) and D > 0 (black curve); the gate values are indicated by the dark red and black circles in Figure 2d. Inset: FFT spectra of the oscillatory resistance from the curves in the main panel.

High Resolution Image
Download MS PowerPoint Slide

The experimental observation of this collection of moiré-induced transport features necessarily implies the presence of a superlattice with uniform twist angle (within cent-of-degree accuracy) over the device area within the voltage probes (∼2 μm2). While local techniques have been successfully applied before to CVD-based SA-TBG, (40) transport studies are not available in the literature, to the best of our knowledge. The realization of device-scale moiré effects using CVD-grown crystals is of high relevance for different potential applications. In particular, TBG can be used for ultrafast, highly sensitive and selective photodetectors. (15,67) Moreover, moiré patterns in SA-TBG provide confined conducting channels that can be used for the directed propagation of surface plasmons (68) or for the study of moiré plasmons. (54,55,69)

In principle, our assembly approach could be up-scaled by employing multiple crystals from the same array simultaneously (that is, within the same pick-rotate-and-stack process). Nonetheless, two main limiting factors to scalability of CVD-based SA-TBG should be considered. First, the presence of blisters due to incomplete interface cleaning currently constrain the device dimensions: this could be mitigated by using dry polymer-free techniques for CVD graphene transfer, such as in ref (21). Second, the requirement of hBN flakes, acting both as pick-up carrier and high-quality electrostatic environment for TBG, which are limited to the lateral size currently yielded by micromechanical exfoliation (typically up to ∼100 μm).

In addition, while a path toward twisted N-layer graphene devices appears to be traced by recent results on flake-based quadrilayers and pentalayers, (70,71) a crucial experimental bottleneck arises. Exfoliated graphene flakes have limited lateral dimensions (up to ∼100 μm), which impede the realization of thick angle-controlled stacks with areas compatible with device fabrication. Since our CVD matrixes retain a single crystallographic orientation over millimeters-sized areas, stacking of graphene layers with N > 5 and device-compatible size could be pursued using the technique introduced here.

In conclusion, we demonstrated the first SA-TBG high-quality moiré device based on CVD-grown crystals. The use of aligned graphene crystals from CVD-grown arrays, together with the manual stacking approach, allows deterministically selectable twist angles. The existence of a moiré potential with uniform periodicity on a device-scale area is confirmed by the observation of density-independent Brown-Zak oscillations, which coexist with multiple Landau fans at low temperature, and survive up to tens of Kelvin. Overall, our results establish a novel tool for future developments of 2D materials twistronics and related technology.

Supporting Information

ARTICLE SECTIONS
Jump To

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.2c01114.

  • Details on the experimental methods for CVD growth, Raman spectroscopy, device fabrication, and low-temperature magnetotransport; assembly of a second SA-TBG sample; Raman and transport data on CVD-based bilayer graphene with sub-MA twisting; dependence of the CNPs splitting on Fermi velocity and interlayer capacitance; Figures S1–S4 (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

ARTICLE SECTIONS
Jump To
  • Corresponding Authors
  • Authors
    • Giulia Piccinini - NEST, Scuola Normale Superiore, Piazza San Silvestro 12, 56127 Pisa, Italy;  Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, 56127 Pisa, Italy;  Orcidhttps://orcid.org/0000-0002-2902-1361
    • Vaidotas Mišeikis - Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, 56127 Pisa, Italy;  Graphene Laboratories, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy
    • Pietro Novelli - Istituto Italiano di Tecnologia, Via Melen 83, 16152 Genova, Italy;  Orcidhttps://orcid.org/0000-0003-1623-5659
    • Kenji Watanabe - Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan;  Orcidhttps://orcid.org/0000-0003-3701-8119
    • Takashi Taniguchi - International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan;  Orcidhttps://orcid.org/0000-0002-1467-3105
    • Marco Polini - Dipartimento di Fisica, Università di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy;  Graphene Laboratories, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

ARTICLE SECTIONS
Jump To

We thank F. Rossella for technical support during the low-temperature experiments. Growth of hexagonal boron nitride crystals was supported by the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant JPMXP0112101001, JSPS KAKENHI Grant JP20H00354 and the CREST(JPMJCR15F3), JST. The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreements 785219-Graphene Core2 and 881603-Graphene Core3.

References

ARTICLE SECTIONS
Jump To

This article references 71 other publications.

  1. 1
    Cao, Y. Unconventional superconductivity in magic-angle graphene superlattices. Nature 2018, 556, 43– 50,  DOI: 10.1038/nature26160
    Google Scholar
    1
    Unconventional superconductivity in magic-angle graphene superlattices
    Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo
    Nature (London, United Kingdom) (2018), 556 (7699), 43-50CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    The behavior of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theor. understanding has motivated the development of exptl. techniques for studying such behavior, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional supercond.-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a crit. temp. of up to 1.7 K. The temp.-carrier-d. phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to supercond. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting crit. temp. of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier d. of about 1011 per square centimeter), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-crit.-temp. superconductors and quantum spin liqs.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXntVGjsbc%253D&md5=686b029f989784d0fa11ede1bfcecdbe
  2. 2
    Cao, Y. Correlated insulator behaviour at half-filling in magic-angle graphene superlattices. Nature 2018, 556, 80– 84,  DOI: 10.1038/nature26154
    Google Scholar
    2
    Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
    Cao, Yuan; Fatemi, Valla; Demir, Ahmet; Fang, Shiang; Tomarken, Spencer L.; Luo, Jason Y.; Sanchez-Yamagishi, Javier D.; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Ashoori, Ray C.; Jarillo-Herrero, Pablo
    Nature (London, United Kingdom) (2018), 556 (7699), 80-84CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials. One such property is the 'twist' angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analog in other types of heterostructure, such as semiconductors grown using mol. beam epitaxy. For small twist angles, the moire pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concd. mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride. Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show exptl. that, as predicted theor., when this angle is close to the 'magic' angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moire´ pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through elec. tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liqs.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXntVGjtr8%253D&md5=1697ebd09a673171514a527c17acf5b4
  3. 3
    Sharpe, A. L. Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene. Science 2019, 365, 605– 608,  DOI: 10.1126/science.aaw3780
    Google Scholar
    3
    Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene
    Sharpe, Aaron L.; Fox, Eli J.; Barnard, Arthur W.; Finney, Joe; Watanabe, Kenji; Taniguchi, Takashi; Kastner, M. A.; Goldhaber-Gordon, David
    Science (Washington, DC, United States) (2019), 365 (6453), 605-608CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    When two sheets of graphene are stacked at a small twist angle, the resulting flat superlattice minibands are expected to strongly enhance electron-electron interaction. Here, we present evidence that near three-quarters (3/4) filling of the conduction miniband, these enhanced interactions drive the twisted bilayer graphene into a ferromagnetic state. In a narrow d. range around an apparent insulating state at 3/4, we observe emergent ferromagnetic hysteresis, with a giant anomalous Hall (AH) effect as large as 10.4 kilohms and indications of chiral edge states. Notably, the magnetization of the sample can be reversed by applying a small d.c. Although the AH resistance is not quantized, and dissipation is present, our measurements suggest that the system may be an incipient Chern insulator.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsFCisLbF&md5=8b0eb08ac0a8e14c71cbbd8ff06e2168
  4. 4
    Tran, K. Evidence for moiré excitons in van der Waals heterostructures. Nature 2019, 567, 71– 75,  DOI: 10.1038/s41586-019-0975-z
    Google Scholar
    4
    Evidence for moire´ excitons in van der Waals heterostructures
    Tran, Kha; Moody, Galan; Wu, Fengcheng; Lu, Xiaobo; Choi, Junho; Kim, Kyounghwan; Rai, Amritesh; Sanchez, Daniel A.; Quan, Jiamin; Singh, Akshay; Embley, Jacob; Zepeda, Andre; Campbell, Marshall; Autry, Travis; Taniguchi, Takashi; Watanabe, Kenji; Lu, Nanshu; Banerjee, Sanjay K.; Silverman, Kevin L.; Kim, Suenne; Tutuc, Emanuel; Yang, Li; MacDonald, Allan H.; Li, Xiaoqin
    Nature (London, United Kingdom) (2019), 567 (7746), 71-75CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Recent advances in the isolation and stacking of monolayers of van der Waals materials have provided approaches for the prepn. of quantum materials in the ultimate two-dimensional limit. In van der Waals heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moire´ superlattice. It is widely recognized that the moire´ superlattice can modulate the electronic band structure of the material and lead to transport properties such as unconventional supercond. and insulating behavior driven by correlations; however, the influence of the moire´ superlattice on optical properties has not been investigated exptl. Here we report the observation of multiple interlayer exciton resonances with either pos. or neg. circularly polarized emission in a molybdenum diselenide/tungsten diselenide (MoSe2/WSe2) heterobilayer with a small twist angle. We attribute these resonances to excitonic ground and excited states confined within the moire´ potential. This interpretation is supported by recombination dynamics and by the dependence of these interlayer exciton resonances on twist angle and temp. These results suggest the feasibility of engineering artificial excitonic crystals using van der Waals heterostructures for nanophotonics and quantum information applications.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXmsVyrtLg%253D&md5=1d6baad81a874a558d08b1303d5970d8
  5. 5
    Wang, L. One-dimensional electrical contact to a two-dimensional material. Science 2013, 342, 614– 617,  DOI: 10.1126/science.1244358
    Google Scholar
    5
    One-Dimensional Electrical Contact to a Two-Dimensional Material
    Wang, L.; Meric, I.; Huang, P. Y.; Gao, Q.; Gao, Y.; Tran, H.; Taniguchi, T.; Watanabe, K.; Campos, L. M.; Muller, D. A.; Guo, J.; Kim, P.; Hone, J.; Shepard, K. L.; Dean, C. R.
    Science (Washington, DC, United States) (2013), 342 (6158), 614-617CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal B nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality elec. contact. Here, the authors report a contact geometry in which the authors metalize only the 1-dimensional edge of a 2-dimensional graphene layer. In addn. to outperforming conventional surface contacts, the edge-contact geometry allows a complete sepn. of the layer assembly and contact metalization processes. In graphene heterostructures, this enables high electronic performance, including low-temp. ballistic transport over distances longer than 15 μm, and room-temp. mobility comparable to the theor. phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2-dimensional materials.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhs1yrs7fJ&md5=de354f7f8a0425d61230545f9afd5e80
  6. 6
    Kim, K. van der Waals heterostructures with high accuracy rotational alignment. Nano Lett. 2016, 16, 1989– 1995,  DOI: 10.1021/acs.nanolett.5b05263
    Google Scholar
    6
    van der Waals Heterostructures with High Accuracy Rotational Alignment
    Kim, Kyounghwan; Yankovitz, Matthew; Fallahazad, Babak; Kang, Sangwoo; Movva, Hema C. P.; Huang, Shengqiang; Larentis, Stefano; Corbet, Chris M.; Taniguchi, Takashi; Watanabe, Kenji; Banerjee, Sanjay K.; LeRoy, Brian J.; Tutuc, Emanuel
    Nano Letters (2016), 16 (3), 1989-1995CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We describe the realization of van der Waals (vdW) heterostructures with accurate rotational alignment of individual layer crystal axes. We illustrate the approach by demonstrating a Bernal-stacked bilayer graphene formed using successive transfers of monolayer graphene flakes. The Raman spectra of this artificial bilayer graphene possess a wide 2D band, which is best fit by four Lorentzians, consistent with Bernal stacking. Scanning tunneling microscopy reveals no moir´e pattern on the artificial bilayer graphene, and tunneling spectroscopy as a function of gate voltage reveals a const. d. of states, also in agreement with Bernal stacking. In addn., electron transport probed in dual-gated samples reveals a band gap opening as a function of transverse elec. field. To illustrate the applicability of this technique to realize vdW heterostructures in which the functionality is critically dependent on rotational alignment, we demonstrate resonant tunneling double bilayer graphene heterostructures sepd. by hexagonal boron-nitride dielec.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XitF2ns7w%253D&md5=28d1713bd561cf80dea282840f96369d
  7. 7
    Geim, A. K.; Grigorieva, I. V. Van der Waals heterostructures. Nature 2013, 499, 419– 425,  DOI: 10.1038/nature12385
    Google Scholar
    7
    Van der Waals heterostructures
    Geim, A. K.; Grigorieva, I. V.
    Nature (London, United Kingdom) (2013), 499 (7459), 419-425CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    A review. Research on graphene and other two-dimensional at. crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated at. planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as van der Waals') have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene's springboard, van der Waals heterostructures should develop into a large field of their own.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtFKnu7rN&md5=58b3fc8bf8d8e656719bfaa23ab0e99b
  8. 8
    Wang, Z.; Wang, Y. B.; Yin, J.; Tovari, E.; Yang, Y.; Lin, L.; Holwill, M.; Birkbeck, J.; Perello, D. J.; Xu, S.; Zultak, J.; Gorbachev, R. V.; Kretinin, A. V.; Taniguchi, T.; Watanabe, K.; Morozov, S. V.; Anđelkovic, M.; Milovanovic, S. P.; Covaci, L.; Peeters, F. M.; Mishchenko, A.; Geim, A. K.; Novoselov, K. S.; Fal’ko, V. I.; Knothe, A.; Woods, C. R. Composite super-moiré lattices in double-aligned graphene heterostructures. Sci. Adv. 2019, 5, eaay8897  DOI: 10.1126/sciadv.aay8897
    Google Scholar
    There is no corresponding record for this reference.
  9. 9
    Park, J. M.; Cao, Y.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P. Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene. Nature 2021, 590, 249– 255,  DOI: 10.1038/s41586-021-03192-0
    Google Scholar
    9
    Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene
    Park, Jeong Min; Cao, Yuan; Watanabe, Kenji; Taniguchi, Takashi; Jarillo-Herrero, Pablo
    Nature (London, United Kingdom) (2021), 590 (7845), 249-255CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Abstr.: Moire´ superlattices1,2 have recently emerged as a platform upon which correlated physics and supercond. can be studied with unprecedented tunability3-6. Although correlated effects have been obsd. in several other moire´ systems7-17, magic-angle twisted bilayer graphene remains the only one in which robust supercond. has been reproducibly measured4-6. Here we realize a moire´ superconductor in magic-angle twisted trilayer graphene (MATTG)18, which has better tunability of its electronic structure and superconducting properties than magic-angle twisted bilayer graphene. Measurements of the Hall effect and quantum oscillations as a function of d. and elec. field enable us to det. the tunable phase boundaries of the system in the normal metallic state. Zero-magnetic-field resistivity measurements reveal that the existence of supercond. is intimately connected to the broken-symmetry phase that emerges from two carriers per moire´ unit cell. We find that the superconducting phase is suppressed and bounded at the Van Hove singularities that partially surround the broken-symmetry phase, which is difficult to reconcile with weak-coupling BCS theory. Moreover, the extensive in situ tunability of our system allows us to reach the ultrastrong-coupling regime, characterized by a Ginzburg-Landau coherence length that reaches the av. inter-particle distance, and very large TBKT/TF values, in excess of 0.1 (where TBKT and TF are the Berezinskii-Kosterlitz-Thouless transition and Fermi temps., resp.). These observations suggest that MATTG can be elec. tuned close to the crossover to a two-dimensional Bose-Einstein condensate. Our results establish a family of tunable moire´ superconductors that have the potential to revolutionize our fundamental understanding of and the applications for strongly coupled supercond.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXis1Ont7w%253D&md5=89912784d6b6cdd3421cffc5251bc484
  10. 10
    Hao, Z. Electric field–tunable superconductivity in alternating-twist magic-angle trilayer graphene. Science 2021, 371, 1133– 1138,  DOI: 10.1126/science.abg0399
    Google Scholar
    10
    Electric field-tunable superconductivity in alternating-twist magic-angle trilayer graphene
    Hao, Zeyu; Zimmerman, A. M.; Ledwith, Patrick; Khalaf, Eslam; Najafabadi, Danial Haie; Watanabe, Kenji; Taniguchi, Takashi; Vishwanath, Ashvin; Kim, Philip
    Science (Washington, DC, United States) (2021), 371 (6534), 1133-1138CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    Engineering moire superlattices by twisting layers in van der Waals (vdW) heterostructures has uncovered a wide array of quantum phenomena. We constructed a vdW heterostructure that consists of three graphene layers stacked with alternating twist angles ±θ. At the av. twist angle θ ∼ 1.56°, a theor. predicted "magic angle" for the formation of flat electron bands, we obsd. displacement field-tunable supercond. with a max. crit. temp. of 2.1 K. By tuning the doping level and displacement field, we found that superconducting regimes occur in conjunction with flavor polarization of moire bands and are bounded by a van Hove singularity (vHS) at high displacement fields. Our findings display inconsistencies with a weak coupling description, suggesting that the obsd. moire supercond. has an unconventional nature.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXmsVOlu78%253D&md5=877d0a2958ec738d6024179ea7b90400
  11. 11
    Carr, S.; Massatt, D.; Fang, S.; Cazeaux, P.; Luskin, M.; Kaxiras, E. Twistronics: Manipulating the electronic properties of two-dimensional layered structures through their twist angle. Phys. Rev. B 2017, 95, 075420  DOI: 10.1103/PhysRevB.95.075420
    Google Scholar
    11
    Twistronics: manipulating the electronic properties of two-dimensional layered structures through their twist angle
    Carr, Stephen; Massatt, Daniel; Fang, Shiang; Cazeaux, Paul; Luskin, Mitchell; Kaxiras, Efthimios
    Physical Review B (2017), 95 (7), 075420/1-075420/6CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    The ability in expts. to control the relative twist angle between successive layers in two-dimensional (2D) materials offers an approach to manipulating their electronic properties; we refer to this approach as "twistronics." A major challenge to theory is that, for arbitrary twist angles, the resulting structure involves incommensurate (aperiodic) 2D lattices. Here, we present a general method for the calcn. of the electronic d. of states of aperiodic 2D layered materials, using parameter-free Hamiltonians derived from ab initio d.-functional theory. We use graphene, a semimetal, and MoS2, a representative of the transition-metal dichalcogenide family of 2D semiconductors, to illustrate the application of our method, which enables fast and efficient simulation of multilayered stacks in the presence of local disorder and external fields. We comment on the interesting features of their d. of states as a function of twist angle and local configuration and on how these features can be exptl. obsd.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsVeqt7vN&md5=41b65e533c5ac799d2eeb29de1ada5d5
  12. 12
    Ribeiro-Palau, R.; Zhang, C.; Watanabe, K.; Taniguchi, T.; Hone, J.; Dean, C. R. Twistable electronics with dynamically rotatable heterostructures. Science 2018, 361, 690– 693,  DOI: 10.1126/science.aat6981
    Google Scholar
    12
    Twistable electronics with dynamically rotatable heterostructures
    Ribeiro-Palau, Rebeca; Zhang, Changjian; Watanabe, Kenji; Taniguchi, Takashi; Hone, James; Dean, Cory R.
    Science (Washington, DC, United States) (2018), 361 (6403), 690-693CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    In heterostructures of two-dimensional materials, electronic properties can vary dramatically with relative interlayer angle. This effect makes it theor. possible to realize a new class of twistable electronics in which properties can be manipulated on demand by means of rotation. We demonstrate a device architecture in which a layered heterostructure can be dynamically twisted in situ. We study graphene encapsulated by boron nitride, where, at small rotation angles, the device characteristics are dominated by coupling to a long-wavelength moire superlattice. The ability to investigate arbitrary rotation angle in a single device reveals features of the optical, mech., and electronic response in this system not captured in static rotation studies. Our results establish the capability to fabricate twistable electronic devices with dynamically tunable properties.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsFagt7zK&md5=e65e7ec302bc04f0691f5b04c7b484c4
  13. 13
    Yang, Y.; Li, J.; Yin, J.; Xu, S.; Mullan, C.; Taniguchi, T.; Watanabe, K.; Geim, A. K.; Novoselov, K. S.; Mishchenko, A. In situ manipulation of van der Waals heterostructures for twistronics. Sci. Adv. 2020, 6, eabd3655  DOI: 10.1126/sciadv.abd3655
    Google Scholar
    There is no corresponding record for this reference.
  14. 14
    Polini, M.; et al. Materials and devices for fundamental quantum science and quantum technologies. arXiv 2201.09260, 2022, accessed 2022-06-27.
    Google Scholar
    There is no corresponding record for this reference.
  15. 15
    Seifert, P. Magic-Angle Bilayer Graphene Nanocalorimeters: Toward Broadband, Energy-Resolving Single Photon Detection. Nano Lett. 2020, 20, 3459– 3464,  DOI: 10.1021/acs.nanolett.0c00373
    Google Scholar
    15
    Magic-Angle Bilayer Graphene Nanocalorimeters: Toward Broadband, Energy-Resolving Single Photon Detection
    Seifert, Paul; Lu, Xiaobo; Stepanov, Petr; Duran Retamal, Jose Ramon; Moore, John N.; Fong, Kin-Chung; Principi, Alessandro; Efetov, Dmitri K.
    Nano Letters (2020), 20 (5), 3459-3464CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Because of the ultralow photon energies at mid-IR and terahertz frequencies, in these bands photodetectors are notoriously underdeveloped, and broadband single photon detectors (SPDs) are nonexistent. Advanced SPDs exploit thermal effects in nanostructured superconductors, and their performance is currently limited to the more energetic near-IR photons due to their high electronic heat capacity. Here, we demonstrate a superconducting magic-angle bilayer graphene (MAG) device that is theor. capable of detecting single photons of ultralow energies by utilizing its record-low heat capacity and sharp superconducting transition. We theor. quantify its calorimetric photoresponse and est. its detection limits. This device allows the detection of ultrabroad range single photons from the visible to sub-terahertz with a response time around 4 ns and energy resoln. better than 1 THz. These attributes position MAG as an exceptional material for long-wavelength single photon sensing, which could revolutionize such disparate fields as quantum information processing and radio astronomy.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXns1eqsrk%253D&md5=fa69f8004e08829303a769e097f832b9
  16. 16
    Rodan-Legrain, D. Highly tunable junctions and non-local Josephson effect in magic-angle graphene tunnelling devices. Nat. Nanotechnol. 2021, 16, 769– 775,  DOI: 10.1038/s41565-021-00894-4
    Google Scholar
    16
    Highly tunable junctions and non-local Josephson effect in magic-angle graphene tunnelling devices
    Rodan-Legrain, Daniel; Cao, Yuan; Park, Jeong Min; de la Barrera, Sergio C.; Randeria, Mallika T.; Watanabe, Kenji; Taniguchi, Takashi; Jarillo-Herrero, Pablo
    Nature Nanotechnology (2021), 16 (7), 769-775CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
    Magic-angle twisted bilayer graphene (MATBG) has recently emerged as a highly tunable two-dimensional material platform exhibiting a wide range of phases, such as metal, insulator and superconductor states. Local electrostatic control over these phases may enable the creation of versatile quantum devices that were previously not achievable in other single-material platforms. Here we engineer Josephson junctions and tunnelling transistors in MATBG, solely defined by electrostatic gates. Our multi-gated device geometry offers independent control of the weak link, barriers and tunnelling electrodes. These purely two-dimensional MATBG Josephson junctions exhibit non-local electrodynamics in a magnetic field, in agreement with the Pearl theory for ultrathin superconductors. Utilizing the intrinsic bandgaps of MATBG, we also demonstrate monolithic edge tunnelling spectroscopy within the same MATBG devices and measure the energy spectrum of MATBG in the superconducting phase. Furthermore, by inducing a double-barrier geometry, the devices can be operated as a single-electron transistor, exhibiting Coulomb blockade. With versatile functionality encompassed within a single material, these MATBG tunnelling devices may find applications in graphene-based tunable superconducting qubits, on-chip superconducting circuits and electromagnetic sensing.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhtVensbnL&md5=7d44a3a8935df7e2d66f59e28477079c
  17. 17
    de Vries, F. K. Gate-defined Josephson junctions in magic-angle twisted bilayer graphene. Nat. Nanotechnol. 2021, 16, 760– 763,  DOI: 10.1038/s41565-021-00896-2
    Google Scholar
    17
    Gate-defined Josephson junctions in magic-angle twisted bilayer graphene
    de Vries, Folkert K.; Portoles, Elias; Zheng, Giulia; Taniguchi, Takashi; Watanabe, Kenji; Ihn, Thomas; Ensslin, Klaus; Rickhaus, Peter
    Nature Nanotechnology (2021), 16 (7), 760-763CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
    In situ electrostatic control of two-dimensional supercond.1 is commonly limited due to large charge carrier densities, and gate-defined Josephson junctions are therefore rare 2,3. Magic-angle twisted bilayer graphene (MATBG)4-8 has recently emerged as a versatile platform that combines metallic, superconducting, magnetic and insulating phases in a single crystal9-14. Although MATBG appears to be an ideal two-dimensional platform for gate-tunable supercond.9,11,13, progress towards practical implementations has been hindered by the need for well-defined gated regions. Here we use multilayer gate technol. to create a device based on two distinct phases in adjustable regions of MATBG. We electrostatically define the superconducting and insulating regions of a Josephson junction and observe tunable d.c. and a.c. Josephson effects15,16. The ability to tune the superconducting state within a single material circumvents interface and fabrication challenges, which are common in multimaterial nanostructures. This work is an initial step towards devices where gate-defined correlated states are connected in single-crystal nanostructures. We envision applications in superconducting electronics17,18 and quantum information technol.19,20.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhtVensbjE&md5=a00230b4d00caff23ba0c88083d08ee7
  18. 18
    Diez-Merida, J.; et al. Magnetic Josephson Junctions and Superconducting Diodes in Magic Angle Twisted Bilayer Graphene. arXiv 2110.01067, 2021, accessed 2022-06-27.
    Google Scholar
    There is no corresponding record for this reference.
  19. 19
    Portolés, E.; et al. A Tunable Monolithic SQUID in Twisted Bilayer Graphene. arXiv 2201.13276, 2022, accessed 2022-06-27.
    Google Scholar
    There is no corresponding record for this reference.
  20. 20
    Backes, C. Production and processing of graphene and related materials. 2D Mater. 2020, 7, 022001  DOI: 10.1088/2053-1583/ab1e0a
    Google Scholar
    20
    Production and processing of graphene and related materials
    Backes, Claudia; Abdelkader, Amr M.; Alonso, Concepcion; Andrieux-Ledier, Amandine; Arenal, Raul; Azpeitia, Jon; Balakrishnan, Nilanthy; Banszerus, Luca; Barjon, Julien; Bartali, Ruben; Bellani, Sebastiano; Berger, Claire; Berger, Reinhard; Ortega, M. M. Bernal; Bernard, Carlo; Beton, Peter H.; Beyer, Andre; Bianco, Alberto; Boeggild, Peter B.; Bonaccorso, Francesco; Barin, Gabriela Borin; Botas, Cristina; Bueno, Rebeca A.; Carriazo, Daniel; Gomez, Andres Castellanos; Christian, Meganne; Ciesielski, Artur; Ciuk, Tymoteusz; Cole, Matthew T.; Coleman, Jonathan; Coletti, Camilla; Crema, Luigi; Cun, Huanyao; Dasler, Daniela; De Fazio, Domenico; Diez, Noel; Drieschner, Simon; Duesberg, Georg S.; Fasel, Roman; Feng, Xinliang; Fina, Alberto; Forti, Stiven; Galiotis, Costas; Garberoglio, Giovanni; Garcia, Jorge M.; Garrido, Jose Antonio; Gibertini, Marco; Goelzhaeuser, Armin; Gomez, Julio; Greber, Thomas; Hauke, Frank; Hemmi, Adrian; Hernandez-Rodriguez, Irene; Hirsch, Andreas; Hodge, Stephen A.; Huttel, Yves; Jepsen, Peter U.; Jimenez, Ignacio; Kaiser, Ute; Kaplas, Tommi; Kim, Hokwon; Kis, Andras; Papagelis, Konstantinos; Kostarelos, Kostas; Krajewska, Aleksandra; Lee, Kangho; Li, Changfeng; Lipsanen, Harri; Liscio, Andrea; Lohe, Martin R.; Loiseau, Annick; Lombardi, Lucia; Lopez, Maria Francisca; Martin, Oliver; Martin, Cristina; Martinez, Lidia; Martin-Gago, Joseangel; Martinez, Jose Ignacio; Marzari, Nicola; Mayoral, Alvaro; Mcmanus, John; Melucci, Manuela; Mendez, Javier; Merino, Cesar; Merino, Pablo; Meyer, Andreas P.; Miniussi, Elisa; Miseikis, Vaidotas; Mishra, Neeraj; Morandi, Vittorio; Munuera, Carmen; Munoz, Roberto; Nolan, Hugo; Ortolani, Luca; Ott, Annak; Palacio, Irene; Palermo, Vincenzo; Parthenios, John; Pasternak, Iwona; Patane, Amalia; Prato, Maurizio; Prevost, Henri; Prudkovskiy, Vladimir; Pugno, Nicola; Rojo, Teofilo; Rossi, Antonio; Ruffieux, Pascal; Samori, Paolo; Schue, Leonard; Setijadi, Eki; Seyller, Thomas; Speranza, Giorgio; Stampfer, Christoph; Stenger, Ingrid; Strupinski, Wlodek; Svirko, Yuri; Taioli, Simone; Bkteo, Kenneth; Testi, Matteo; Tomarchio, Flavia; Tortello, Mauro; Treossi, Emanuele; Turchanin, Andrey; Vazquez, Ester; Villaro, Elvira; Whelan, Patrick R.; Xia, Zhenyuan; Yakimova, Rositza; Yang, Sheng; Yazdi, G. Reza; Yim, Chanyoung; Yoon, Duhee; Zhang, Xianghui; Zhuang, Xiaodong; Colombo, Luigi; Ferrari, Andrea C.; Garcia-Hernandez, Mar
    2D Materials (2020), 7 (2), 022001CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    We present an overview of the main techniques for prodn. and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitFCiu77E&md5=d1231e8040840587aab88ec261c48a19
  21. 21
    Banszerus, L.; Schmitz, M.; Engels, S.; Dauber, J.; Oellers, M.; Haupt, F.; Watanabe, K.; Taniguchi, T.; Beschoten, B.; Stampfer, C. Ultrahigh-mobility graphene devices from chemical vapor deposition on reusable copper. Sci. Adv. 2015, 1, e1500222  DOI: 10.1126/sciadv.1500222
    Google Scholar
    21
    Ultrahigh-mobility graphene devices from chemical vapor deposition on reusable copper
    Banszerus, Luca; Schmitz, Michael; Engels, Stephan; Dauber, Jan; Oellers, Martin; Haupt, Federica; Watanabe, Kenji; Taniguchi, Takashi; Beschoten, Bernd; Stampfer, Christoph
    Science Advances (2015), 1 (6), e1500222/1-e1500222/6CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
    Graphene research has prospered impressively in the past few years, and promising applications such as high frequency transistors, magnetic field sensors, and flexible optoelectronics are just waiting for a scalable and cost efficient fabrication technol. to produce high-mobility graphene. Although significant progress has been made in chem. vapor deposition (CVD) and epitaxial growth of graphene, the carrier mobility obtained with these techniques is still significantly lower than what is achieved using exfoliated graphene. We show that the quality of CVD-grown graphene depends critically on the used transfer process, and we report on an advanced transfer technique that allows both reusing the copper substrate of the CVD growth and making devices with mobilities as high as 350,000 cm2 V-1 s-1, thus rivaling exfoliated graphene.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XnsFGhsb4%253D&md5=afccf1cc5730fcf64f1861aeaf40bac9
  22. 22
    De Fazio, D. High-mobility, wet-transferred graphene grown by chemical vapour deposition. ACS Nano 2019, 13, 8926– 8935,  DOI: 10.1021/acsnano.9b02621
    Google Scholar
    22
    High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition
    De Fazio, Domenico; Purdie, David G.; Ott, Anna K.; Braeuninger-Weimer, Philipp; Khodkov, Timofiy; Goossens, Stijn; Taniguchi, Takashi; Watanabe, Kenji; Livreri, Patrizia; Koppens, Frank H. L.; Hofmann, Stephan; Goykhman, Ilya; Ferrari, Andrea C.; Lombardo, Antonio
    ACS Nano (2019), 13 (8), 8926-8935CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    We report high room-temp. mobility in single-layer graphene grown by chem. vapor deposition (CVD) after wet transfer on SiO2 and hexagonal boron nitride (hBN) encapsulation. By removing contaminations, trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to ∼70000 cm2 V-1 s-1 at room temp. and ∼120 000 cm2 V-1 s-1 at 9K. These are more than twice those of previous wet-transferred graphene and comparable to samples prepd. by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycryst. graphene, achieving room-temp. mobilities of ∼30 000 cm2 V-1 s-1. These results show that, with appropriate encapsulation and cleaning, room-temp. mobilities well above 10 000 cm2 V-1 s-1 can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsVSqsbnM&md5=9c06d8950fc9402af9d63dfc1a520686
  23. 23
    Pezzini, S. High-quality electrical transport using scalable CVD graphene. 2D Mater. 2020, 7, 041003  DOI: 10.1088/2053-1583/aba645
    Google Scholar
    23
    High-quality electrical transport using scalable CVD graphene
    Pezzini, Sergio; Miseikis, Vaidotas; Pace, Simona; Rossella, Francesco; Watanabe, Kenji; Taniguchi, Takashi; Coletti, Camilla
    2D Materials (2020), 7 (4), 041003CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    Producing and manipulating graphene on fab-compatible scale, while maintaining its remarkable carrier mobility, is key to finalize its technol. application. We show that a large-scale approach (chem. vapor deposition on Cu followed by polymer-mediated semi-dry transfer) yields single-layer graphene crystals fully comparable, in terms of electronic transport, to micro-mech. exfoliated flakes. Hexagonal boron nitride is used to encapsulate the graphene crystals-without taking part to their detachment from the growth catalyst-and study their intrinsic properties in field-effect devices. At room temp., the electron-phonon coupling sets the mobility to ∼ 1.3 x 105 cm2 V-1 s-1 at ∼ 1011 cm-2 concn. At T = 4.2 K, the mobility (>6 x 105 cm2 V-1 s-1 at ∼ 1011 cm-2) is limited by the devices' phys. edges, and charge fluctuations < 7 x 109 cm-2 are detected. Under perpendicular magnetic fields, we observe early onset of Landau quantization (B ∼ 50 mT) and signatures of electronic correlation, including the fractional quantum Hall effect.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXit1Sqs7rJ&md5=a9690c10926630e0f589491316bf3035
  24. 24
    Schmitz, M. Fractional quantum Hall effect in CVD-grown graphene. 2D Mater. 2020, 7, 041007  DOI: 10.1088/2053-1583/abae7b
    Google Scholar
    24
    Fractional quantum Hall effect in CVD-grown graphene
    Schmitz, M.; Ouaj, T.; Winter, Z.; Rubi, K.; Watanabe, K.; Taniguchi, T.; Zeitler, U.; Beschoten, B.; Stampfer, C.
    2D Materials (2020), 7 (4), 041007CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    A review. We show the emergence of fractional quantum Hall states in graphene grown by chem. vapor deposition (CVD) for magnetic fields from below 3 T to 35 T where the CVD-graphene was dry-transferred. Effective composite-fermion filling factors up to ν* = 4 are visible and higher order composite-fermion states (with four flux quanta attached) start to emerge at the highest fields. Our results show that the quantum mobility of CVD-grown graphene is comparable to that of exfoliated graphene and, more specifically, that the p/3 fractional quantum Hall states have energy gaps of up to 30 K, well comparable to those obsd. in other silicon-gated devices based on exfoliated graphene.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXit1Sqs73N&md5=a873725d115bc9f1fc490a278ea3f841
  25. 25
    Bistritzer, R.; MacDonald, A. H. Moiré bands in twisted double-layer graphene. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 12233– 12237,  DOI: 10.1073/pnas.1108174108
    Google Scholar
    25
    Moire bands in twisted double-layer graphene
    Bistritzer, Rafi; MacDonald, Allan H.
    Proceedings of the National Academy of Sciences of the United States of America (2011), 108 (30), 12233-12237CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    A moire pattern is formed when two copies of a periodic pattern are overlaid with a relative twist. We address the electronic structure of a twisted two-layer graphene system, showing that in its continuum Dirac model the moire pattern periodicity leads to moire Bloch bands. The two layers become more strongly coupled and the Dirac velocity crosses zero several times as the twist angle is reduced. For a discrete set of magic angles the velocity vanishes, the lowest moire band flattens, and the Dirac-point d.-of-states and the counterflow cond. are strongly enhanced.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXpvFagtLs%253D&md5=11c357756459c64b402f461c4c40c9d4
  26. 26
    Moon, P.; Koshino, M. Energy spectrum and quantum Hall effect in twisted bilayer graphene. Phys. Rev. B 2012, 85, 195458,  DOI: 10.1103/PhysRevB.85.195458
    Google Scholar
    26
    Energy spectrum and quantum Hall effect in twisted bilayer graphene
    Moon, Pilkyung; Koshino, Mikito
    Physical Review B: Condensed Matter and Materials Physics (2012), 85 (19), 195458/1-195458/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We investigate the electronic structure and the quantum Hall effect in twisted bilayer graphenes with various rotation angles in the presence of magnetic field. Using a low-energy approxn., which incorporates the rigorous interlayer interaction, we computed the energy spectrum and the quantized Hall cond. in a wide range of magnetic field from the semiclassical regime to the fractal spectrum regime. In weak magnetic fields, the low-energy conduction band is quantized into electronlike and holelike Landau levels at energies below and above the van Hove singularity, resp., and the Hall cond. sharply drops from pos. to neg. when the Fermi energy goes through the transition point. In increasing magnetic field, the spectrum gradually evolves into a fractal band structure called Hofstadter's butterfly, where the Hall cond. exhibits a nonmonotonic behavior as a function of Fermi energy. The typical electron d. and magnetic field amplitude characterizing the spectrum monotonically decrease as the rotation angle is reduced, indicating that the rich electronic structure may be obsd. in a moderate condition.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVWjurfF&md5=ff97101c64be6ff120a173d42308953c
  27. 27
    Sanchez-Yamagishi, J. D.; Taychatanapat, T.; Watanabe, K.; Taniguchi, T.; Yacoby, A.; Jarillo-Herrero, P. Quantum Hall Effect, Screening, and Layer-Polarized Insulating States in Twisted Bilayer Graphene. Phys. Rev. Lett. 2012, 108, 076601  DOI: 10.1103/PhysRevLett.108.076601
    Google Scholar
    27
    Quantum Hall effect, screening, and layer-polarized insulating states in twisted bilayer graphene
    Sanchez-Yamagishi, Javier D.; Taychatanapat, Thiti; Watanabe, Kenji; Taniguchi, Takashi; Yacoby, Amir; Jarillo-Herrero, Pablo
    Physical Review Letters (2012), 108 (7), 076601/1-076601/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We investigate electronic transport in dual-gated twisted-bilayer graphene. Despite the subnanometer proximity between the layers, we identify independent contributions to the magnetoresistance from the graphene Landau level spectrum of each layer. We demonstrate that the filling factor of each layer can be independently controlled via the dual gates, which we use to induce Landau level crossings between the layers. By analyzing the gate dependence of the Landau level crossings, we characterize the finite interlayer screening and ext. the capacitance between the atomically spaced layers. At zero filling factor, we observe an insulating state at large displacement fields, which can be explained by the presence of counterpropagating edge states with interlayer coupling.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XjvVejsLY%253D&md5=7ac0deb65221b851a9b771f008127b17
  28. 28
    Sanchez-Yamagishi, J. D. Helical edge states and fractional quantum Hall effect in a graphene electron–hole bilayer. Nat. Nanotechnol. 2017, 12, 118– 122,  DOI: 10.1038/nnano.2016.214
    Google Scholar
    28
    Helical edge states and fractional quantum Hall effect in a graphene electron-hole bilayer
    Sanchez-Yamagishi, Javier D.; Luo, Jason Y.; Young, Andrea F.; Hunt, Benjamin M.; Watanabe, Kenji; Taniguchi, Takashi; Ashoori, Raymond C.; Jarillo-Herrero, Pablo
    Nature Nanotechnology (2017), 12 (2), 118-122CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Helical 1D electronic systems are a promising route towards realizing circuits of topol. quantum states that exhibit non-Abelian statistics. Here, we demonstrate a versatile platform to realize 1D systems made by combining quantum Hall (QH) edge states of opposite chiralities in a graphene electron-hole bilayer at moderate magnetic fields. Using this approach, we engineer helical 1D edge conductors where the counter-propagating modes are localized in sep. electron and hole layers by a tunable elec. field. These helical conductors exhibit strong non-local transport signals and suppressed backscattering due to the opposite spin polarizations of the counter-propagating modes. Unlike other approaches used for realizing helical states, the graphene electron-hole bilayer can be used to build new 1D systems incorporating fractional edge states. Indeed, we are able to tune the bilayer devices into a regime hosting fractional and integer edge states of opposite chiralities, paving the way towards 1D helical conductors with fractional quantum statistics.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhsl2hsb7O&md5=7d6a6d3cf5ff5338b01550333e5dd39d
  29. 29
    Rickhaus, P.; Liu, M.-H.; Kurpas, M.; Kurzmann, A.; Lee, Y.; Overweg, H.; Eich, M.; Pisoni, R.; Taniguchi, T.; Watanabe, K.; Richter, K.; Ensslin, K.; Ihn, T. The electronic thickness of graphene. Sci. Adv. 2020, 6, eaay8409  DOI: 10.1126/sciadv.aay8409
    Google Scholar
    There is no corresponding record for this reference.
  30. 30
    Luican, A. Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers. Phys. Rev. Lett. 2011, 106, 126802,  DOI: 10.1103/PhysRevLett.106.126802
    Google Scholar
    30
    Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers
    Luican, A.; Li, Guohong; Reina, A.; Kong, J.; Nair, R. R.; Novoselov, K. S.; Geim, A. K.; Andrei, E. Y.
    Physical Review Letters (2011), 106 (12), 126802/1-126802/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chem. vapor deposition. For twist angles exceeding ∼3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is obsd. which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXkt1artLc%253D&md5=c452f1a55946efd7ab283c4cf3642124
  31. 31
    Yan, Z. Large Hexagonal Bi-and Trilayer Graphene Single Crystals with Varied Interlayer Rotations. Angew. Chemie Int. Ed. 2014, 53, 1565– 1569,  DOI: 10.1002/anie.201306317
    Google Scholar
    31
    Large hexagonal bi- and trilayer graphene single crystals with varied interlayer rotations
    Yan, Zheng; Liu, Yuanyue; Ju, Long; Peng, Zhiwei; Lin, Jian; Wang, Gunuk; Zhou, Haiqing; Xiang, Changsheng; Samuel, E. L. G.; Kittrell, Carter; Artyukhov, Vasilii I.; Wang, Feng; Yakobson, Boris I.; Tour, James M.
    Angewandte Chemie, International Edition (2014), 53 (6), 1565-1569CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Bi- and trilayer graphene have attracted intensive interest due to their rich electronic and optical properties, which are dependent on interlayer rotations. However, the prepn. of high-quality large-size bi- and trilayer graphene single crystals still remains a challenge. Here, the prepn. of 100 μm pyramid-like hexagonal bi- and trilayer graphene single-crystal domains on Cu foils using chem. vapor deposition is reported. The as-produced graphene domains show almost exclusively either 0° or 30° interlayer rotations. Raman spectroscopy, transmission electron microscopy, and Fourier-transformed IR spectroscopy were used to demonstrate that bilayer graphene domains with 0° interlayer stacking angles were Bernal stacked. Based on first-principle calcns., it is proposed that rotations originate from the graphene nucleation at the Cu step, which explains the origin of the interlayer rotations and agrees well with the exptl. observations.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVygurs%253D&md5=4dfcaae423d30e1d08976d2f187b0617
  32. 32
    Gao, Z. Crystalline bilayer graphene with preferential stacking from Ni–Cu gradient alloy. ACS Nano 2018, 12, 2275– 2282,  DOI: 10.1021/acsnano.7b06992
    Google Scholar
    32
    Crystalline bilayer graphene with preferential stacking from Ni-Cu gradient alloy
    Gao, Zhaoli; Zhang, Qicheng; Naylor, Carl H.; Kim, Youngkuk; Abidi, Irfan Haider; Ping, Jinglei; Ducos, Pedro; Zauberman, Jonathan; Zhao, Meng-Qiang; Rappe, Andrew M.; Luo, Zhengtang; Ren, Li; Johnson, Alan T. Charlie
    ACS Nano (2018), 12 (3), 2275-2282CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The authors developed a high-yield synthesis of highly cryst. bilayer graphene (BLG) with two preferential stacking modes using a Ni-Cu gradient alloy growth substrate. Previously reported approaches for BLG growth include flat growth substrates of Cu or Ni-Cu uniform alloys and "copper pocket" structures. Use of flat substrates has the advantage of being scalable, but the growth mechanism is either "surface limited" (for Cu) or carbon pptn. (for uniform Ni-Cu), which results in multicryst. BLG grains. For copper pockets, growth proceeds through a carbon back-diffusion mechanism, which leads to the formation of highly cryst. BLG, but scaling of the copper pocket structure is expected to be difficult. Here, the authors demonstrate a Ni-Cu gradient alloy that combines the advantages of these earlier methods: the substrate is flat, so easy to scale, while growth proceeds by a carbon back-diffusion mechanism leading to high-yield growth of BLG with high crystallinity. The BLG layer stacking was almost exclusively Bernal or twisted with an angle of 30°, consistent with first-principles calcns. the authors conducted. Furthermore, we demonstrated scalable prodn. of transistor arrays based cryst. Bernal-stacked BLG with a band gap that was tunable at room temp.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXjvFGmsLc%253D&md5=138f16fadb7cae462a47dae52259b827
  33. 33
    Deng, B. Interlayer Decoupling in 30° Twisted Bilayer Graphene Quasicrystal. ACS Nano 2020, 14, 1656– 1664,  DOI: 10.1021/acsnano.9b07091
    Google Scholar
    33
    Interlayer decoupling in 30° twisted bilayer graphene quasicrystal
    Deng, Bing; Wang, Binbin; Li, Ning; Li, Rongtan; Wang, Yani; Tang, Jilin; Fu, Qiang; Tian, Zhen; Gao, Peng; Xue, Jiamin; Peng, Hailin
    ACS Nano (2020), 14 (2), 1656-1664CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Stacking order has a strong influence on the coupling between the two layers of twisted bilayer graphene (BLG), which in turn dets. its phys. properties. Here, we report the investigation of the interlayer coupling of the epitaxially grown single-crystal 30°-twisted BLG on Cu(111) at the at. scale. The stacking order and morphol. of BLG is controlled by a rationally designed two-step growth process, i.e., the thermodynamically controlled nucleation and kinetically controlled growth. The crystal structure of the 30°-twisted bilayer graphene (30°-tBLG) is detd. to have quasicrystal-like symmetry. The electronic properties and interlayer coupling of the 30°-tBLG are investigated using scanning tunneling microscopy and spectroscopy. The energy-dependent local d. of states with in situ electrostatic doping shows that the electronic states in two graphene layers are decoupled near the Dirac point. A linear dispersion originated from the constituent graphene monolayers is discovered with doubled degeneracy. This study contributes to controlled growth of twist-angle-defined BLG and provides insights on the electronic properties and interlayer coupling in this intriguing system.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhtlWhtb0%253D&md5=b9c97ccc9d9faf254183acfbae5ed32f
  34. 34
    Pezzini, S. 30°-Twisted Bilayer Graphene Quasicrystals From Chemical Vapor Deposition. Nano Lett. 2020, 20, 3313– 3319,  DOI: 10.1021/acs.nanolett.0c00172
    Google Scholar
    34
    30°-Twisted bilayer graphene quasicrystals from chemical vapor deposition
    Pezzini, Sergio; Miseikis, Vaidotas; Piccinini, Giulia; Forti, Stiven; Pace, Simona; Engelke, Rebecca; Rossella, Francesco; Watanabe, Kenji; Taniguchi, Takashi; Kim, Philip; Coletti, Camilla
    Nano Letters (2020), 20 (5), 3313-3319CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The artificial stacking of atomically thin crystals suffers from intrinsic limitations in terms of control and reproducibility of the relative orientation of exfoliated flakes. This drawback is particularly severe when the properties of the system critically depends on the twist angle, as in the case of the dodecagonal quasicrystal formed by two graphene layers rotated by 30°. Here we show that large-area 30°-rotated bilayer graphene can be grown deterministically by chem. vapor deposition on Cu, eliminating the need of artificial assembly. The quasicrystals are easily transferred to arbitrary substrates and integrated in high-quality hexagonal boron nitride-encapsulated heterostructures, which we process into dual-gated devices exhibiting carrier mobility up to 105 cm2/(V s). From low-temp. magnetotransport, we find that the graphene quasicrystals effectively behave as uncoupled graphene layers, showing 8-fold degenerate quantum Hall states. This result indicates that the Dirac cones replica detected by previous photoemission expts. do not contribute to the elec. transport.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXnt1Sqs7o%253D&md5=75952a93c94935d7385e199d2ba6f313
  35. 35
    Sun, Y.; Polani, S.; Luo, F.; Ott, S.; Strasser, P.; Dionigi, F. Hetero-site nucleation for growing twisted bilayer graphene with a wide range of twist angles. Nat. Commun. 2021, 12, 2391,  DOI: 10.1038/s41467-021-25911-x
    Google Scholar
    35
    Hetero-site nucleation for growing twisted bilayer graphene with a wide range of twist angles
    Sun, Luzhao; Wang, Zihao; Wang, Yuechen; Zhao, Liang; Li, Yanglizhi; Chen, Buhang; Huang, Shenghong; Zhang, Shishu; Wang, Wendong; Pei, Ding; Fang, Hongwei; Zhong, Shan; Liu, Haiyang; Zhang, Jincan; Tong, Lianming; Chen, Yulin; Li, Zhenyu; Rummeli, Mark H.; Novoselov, Kostya S.; Peng, Hailin; Lin, Li; Liu, Zhongfan
    Nature Communications (2021), 12 (1), 2391CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Twisted bilayer graphene (tBLG) has recently attracted growing interest due to its unique twist-angle-dependent electronic properties. The prepn. of high-quality large-area bilayer graphene with rich rotation angles would be important for the investigation of angle-dependent physics and applications, which, however, is still challenging. Here, we demonstrate a chem. vapor deposition (CVD) approach for growing high-quality tBLG using a hetero-site nucleation strategy, which enables the nucleation of the second layer at a different site from that of the first layer. The fraction of tBLGs in bilayer graphene domains with twist angles ranging from 0° to 30° was found to be improved to 88%, which is significantly higher than those reported previously. The hetero-site nucleation behavior was carefully investigated using an isotope-labeling technique. Furthermore, the clear Moire patterns and ultrahigh room-temp. carrier mobility of 68,000 cm2 V-1 s-1 confirmed the high cryst. quality of our tBLG. Our study opens an avenue for the controllable growth of tBLGs for both fundamental research and practical applications.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXpsVOgur0%253D&md5=a79ccfb5ea76a338671b92dceb0c3da5
  36. 36
    Lu, C.-C.; Lin, Y.-C.; Liu, Z.; Yeh, C.-H.; Suenaga, K.; Chiu, P.-W. Twisting Bilayer Graphene Superlattices. ACS Nano 2013, 7, 2587– 2594,  DOI: 10.1021/nn3059828
    Google Scholar
    36
    Twisting Bilayer Graphene Superlattices
    Lu, Chun-Chieh; Lin, Yung-Chang; Liu, Zheng; Yeh, Chao-Hui; Suenaga, Kazu; Chiu, Po-Wen
    ACS Nano (2013), 7 (3), 2587-2594CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Bilayer graphene is an intriguing material in that its electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of low-dimensional carbon system. Twisted bilayer graphene can be obtained by (i) thermal decompn. of SiC; (ii) CVD on metal catalysts; (iii) folding graphene; or (iv) stacking graphene layers one atop the other, the latter of which suffers from interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycryst. structures. The present study studies bilayer graphene grown by ambient pressure CVD on polycryst. Cu. Controlling the nucleation in early stage growth allows the constituent layers to form single hexagonal crystals. New Raman active modes result from the twist, with the angle detd. by TEM. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXjt1Wlsb8%253D&md5=fc7e5afd6cb530217d653070c5325582
  37. 37
    Ohta, T.; Robinson, J. T.; Feibelman, P. J.; Bostwick, A.; Rotenberg, E.; Beechem, T. E. Evidence for Interlayer Coupling and Moiré Periodic Potentials in Twisted Bilayer Graphene. Phys. Rev. Lett. 2012, 109, 186807,  DOI: 10.1103/PhysRevLett.109.186807
    Google Scholar
    37
    Evidence for interlayer coupling and Moire periodic potentials in twisted bilayer graphene
    Ohta, Taisuke; Robinson, Jeremy T.; Feibelman, Peter J.; Bostwick, Aaron; Rotenberg, Eli; Beechem, Thomas E.
    Physical Review Letters (2012), 109 (18), 186807/1-186807/6CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We report a study of the valence band dispersion of twisted bilayer graphene using angle-resolved photoemission spectroscopy and ab initio calcns. We observe two noninteracting cones near the Dirac crossing energy and the emergence of van Hove singularities where the cones overlap for large twist angles (>5°). Besides the expected interaction between the Dirac cones, minigaps appeared at the Brillouin zone boundaries of the moire superlattice formed by the misorientation of the two graphene layers. We attribute the emergence of these minigaps to a periodic potential induced by the moire. These anticrossing features point to coupling between the two graphene sheets, mediated by moire periodic potentials.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslClsL%252FK&md5=8f47fe5f9da9daefcd72261fa016161e
  38. 38
    Tan, Z. Building Large-Domain Twisted Bilayer Graphene with van Hove Singularity. ACS Nano 2016, 10, 6725– 6730,  DOI: 10.1021/acsnano.6b02046
    Google Scholar
    38
    Building large-domain twisted bilayer graphene with van Hove singularity
    Tan, Zhenjun; Yin, Jianbo; Chen, Cheng; Wang, Huan; Lin, Li; Sun, Luzhao; Wu, Jinxiong; Sun, Xiao; Yang, Haifeng; Chen, Yulin; Peng, Hailin; Liu, Zhongfan
    ACS Nano (2016), 10 (7), 6725-6730CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Twisted bilayer graphene (tBLG) with van Hove Singularity (VHS) has exhibited novel twist-angle-dependent chem. and phys. phenomena. However, scalable prodn. of high-quality tBLG is still in its infancy, esp. lacking the angle controlled prepn. methods. Here, we report a facile approach to prep. tBLG with large domain sizes (>100 μm) and controlled twist angles by a clean layer-by-layer transfer of two constituent graphene monolayers. The whole process without interfacial polymer contamination in two monolayers guarantees the interlayer interaction of the π-bond electrons, which gives rise to the existence of minigaps in electronic structures and the consequent formation of VHSs in d. of state. Such perturbation on band structure was directly obsd. by angle-resolved photoemission spectroscopy with submicrometer spatial resoln. (micro-ARPES). The VHSs lead to a strong light-matter interaction and thus introduce ∼20-fold enhanced intensity of Raman G-band, which is a characteristic of high-quality tBLG. The as-prepd. tBLG with strong light-matter interaction was further fabricated into high-performance photodetectors with selectively enhanced photocurrent generation (up to ∼6 times compared with monolayer in our device).
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XnslCrtbg%253D&md5=3841b709e2fb1b87e5f71aa1dc0045fd
  39. 39
    Yang, S.-J. Wafer-Scale Programmed Assembly of One-Atom-Thick Crystals. Nano Lett. 2022, 22, 1518– 1524,  DOI: 10.1021/acs.nanolett.1c04139
    Google Scholar
    39
    Wafer-Scale Programmed Assembly of One-Atom-Thick Crystals
    Yang, Seong-Jun; Jung, Ju-Hyun; Lee, Eunsook; Han, Edmund; Choi, Min-Yeong; Jung, Daesung; Choi, Shinyoung; Park, Jun-Ho; Oh, Dongseok; Noh, Siwoo; Kim, Ki-Jeong; Huang, Pinshane Y.; Hwang, Chan-Cuk; Kim, Cheol-Joo
    Nano Letters (2022), 22 (4), 1518-1524CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Cryst. films offer various phys. properties based on the modulation of their thicknesses and at. structures. The layer-by-layer assembly of atomically thin crystals provides powerful means to arbitrarily design films at the at.-level, which are unattainable with existing growth technols. Atomically clean assembly of the materials with high scalability and reproducibility remains challenging. Programmed crystal assembly (PCA) of graphene and monolayer hexagonal BN (ML hBN), assisted by van der Waals interactions, to form wafer-scale films of pristine interfaces with near-unity yield are reported. The at. configurations of the films are tailored with layer-resolved compns. and in-plane cryst. orientations. Batch-fabricated tunnel device arrays are demonstrated with modulation of the resistance over orders of magnitude by thickness-control of the hBN barrier with single-atom precision, and large-scale, twisted multilayer graphene with programmable electronic band structures and crystal symmetries. The results constitute an important development in the artificial design of large-scale films.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XislertL8%253D&md5=3b9133ef25e943608637d185e0ee8ca3
  40. 40
    Wong, D. Local spectroscopy of moiré-induced electronic structure in gate-tunable twisted bilayer graphene. Phys. Rev. B 2015, 92, 155409,  DOI: 10.1103/PhysRevB.92.155409
    Google Scholar
    40
    Local spectroscopy of moir´e-induced electronic structure in gate-tunable twisted bilayer graphene
    Wong, Dillon; Wang, Yang; Jung, Jeil; Pezzini, Sergio; Da Silva, Ashley M.; Tsai, Hsin-Zon; Jung, Han Sae; Khajeh, Ramin; Kim, Youngkyou; Lee, Juwon; Kahn, Salman; Tollabimazraehno, Sajjad; Rasool, Haider; Watanabe, Kenji; Taniguchi, Takashi; Zettl, Alex; Adam, Shaffique; MacDonald, Allan H.; Crommie, Michael F.
    Physical Review B: Condensed Matter and Materials Physics (2015), 92 (15), 155409/1-155409/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Twisted bilayer graphene (tBLG) forms a quasicrystal whose structural and electronic properties depend on the angle of rotation between its layers. Here, we present a scanning tunneling microscopy study of gate-tunable tBLG devices supported by atomically smooth and chem. inert hexagonal boron nitride (BN). The high quality of these tBLG devices allows identification of coexisting moir´e patterns and moir´e super-superlattices produced by graphene-graphene and graphene-BN interlayer interactions. Furthermore, we examine addnl. tBLG spectroscopic features in the local d. of states beyond the first van Hove singularity. Our exptl. data are explained by a theory of moir´e bands that incorporates ab initio calcns. and confirms the strongly nonperturbative character of tBLG interlayer coupling in the small twist-angle regime.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xos1entLw%253D&md5=8f85f2ebde7e3a0f45a12a3aa8713284
  41. 41
    Ren, Y.-N. Spectroscopic Evidence for a Spin- and Valley-Polarized Metallic State in a Nonmagic-Angle Twisted Bilayer Graphene. ACS Nano 2020, 14, 13081– 13090,  DOI: 10.1021/acsnano.0c04631
    Google Scholar
    41
    Spectroscopic Evidence for a Spin- and Valley-Polarized Metallic State in a Nonmagic-Angle Twisted Bilayer Graphene
    Ren, Ya-Ning; Lu, Chen; Zhang, Yu; Li, Si-Yu; Liu, Yi-Wen; Yan, Chao; Guo, Zi-Han; Liu, Cheng-Cheng; Yang, Fan; He, Lin
    ACS Nano (2020), 14 (10), 13081-13090CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    In the magic-angle twisted bilayer graphene (MA-TBG), strong electron-electron (e-e) correlations caused by the band-flattening lead to many exotic quantum phases such as supercond., correlated insulator, ferromagnetism, and quantum anomalous Hall effects, when its low-energy van Hove singularities (VHSs) are partially filled. Here our high-resoln. scanning tunneling microscope and spectroscopy measurements demonstrate that the e-e correlation in a nonmagic-angle TBG with a twist angle θ = 1.49° still plays an important role in detg. its electronic properties. Our most interesting observation on that sample is when one of its VHSs is partially filled, the one assocd. peak in the spectrum splits into four peaks. Simultaneously, the spatial symmetry of electronic states around the split VHSs is broken by the e-e correlation. Our anal. based on the continuum model suggests that such a one-to-four split of the VHS originates from the formation of an interaction-driven spin-valley-polarized metallic state near the VHS, which is a symmetry-breaking phase that has not been identified in the MA-TBG or in other systems.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitVGlu7bK&md5=c8a6671b10b45130d2b99473048caf9e
  42. 42
    Shivayogimath, A. Do-It-Yourself Transfer of Large-Area Graphene Using an Office Laminator and Water. Chem. Mater. 2019, 31, 2328– 2336,  DOI: 10.1021/acs.chemmater.8b04196
    Google Scholar
    42
    Do-It-Yourself Transfer of Large-Area Graphene Using an Office Laminator and Water
    Shivayogimath, Abhay; Whelan, Patrick Rebsdorf; MacKenzie, David M. A.; Luo, Birong; Huang, Deping; Luo, Da; Wang, Meihui; Gammelgaard, Lene; Shi, Haofei; Ruoff, Rodney S.; Boeggild, Peter; Booth, Timothy J.
    Chemistry of Materials (2019), 31 (7), 2328-2336CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The authors demonstrate a simple method for transferring large areas (up to A4-size sheets) of CVD graphene from Cu foils onto a target substrate using a com. available polyvinyl alc. polymer foil as a carrier substrate and com. hot-roll office laminator. Through the use of terahertz time-domain spectroscopy and Raman spectroscopy, large-area quant. optical contrast mapping, and the fabrication and elec. characterization of ∼50 individual centimeter-scale van der Pauw field effect devices, the authors show a nondestructive technique to transfer large-area graphene with low residual doping that is scalable, economical, reproducible, and easy to use and that results in less doping and transfer-induced damage than etching or electrochem. delamination transfers. The Cu substrate can be used multiple times with minimal loss of material and no observable redn. in graphene quality. The authors have addnl. demonstrated the transfer of multilayer hexagonal B nitride from Cu and Fe foils. Finally, this approach allows graphene to be supplied on stand-alone polymer supports by CVD graphene manufacturers to end users, with the only equipment and consumables required to transfer graphene onto target substrates being a com. office laminator and H2O.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXktlOjtr4%253D&md5=e23f9158b101c9f2b99444c6e21bb67e
  43. 43
    Miseikis, V. Deterministic patterned growth of high-mobility large-crystal graphene: A path towards wafer scale integration. 2D Mater. 2017, 4, 021004  DOI: 10.1088/2053-1583/aa5481
    Google Scholar
    43
    Deterministic patterned growth of high-mobility large-crystal graphene: a path towards wafer scale integration
    Miseikis, Vaidotas; Bianco, Federica; David, Jeremy; Gemmi, Mauro; Pellegrini, Vittorio; Romagnoli, Marco; Coletti, Camilla
    2D Materials (2017), 4 (2), 021004/1-021004/8CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    We demonstrate rapid deterministic (seeded) growth of large single-crystals of graphene by chem. vapor deposition (CVD) utilizing pre-patterned copper substrates with chromium nucleation sites. Arrays of graphene single-crystals as large as several hundred microns are grown with a periodicity of up to 1 mm. The graphene is transferred to target substrates using aligned and contaminationfree semi-dry transfer. The high quality of the synthesized graphene is confirmed by Raman spectroscopy and transport measurements, demonstrating room-temp. carrier mobility of 21 000 cm2 V-1 s-1 when transferred on top of hexagonal boron nitride. By tailoring the nucleation of large single-crystals according to the desired device geometry, it will be possible to produce complex device architectures based on single-crystal graphene, thus paving the way to the adoption of CVD graphene in wafer-scale fabrication.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXnsVyisrs%253D&md5=98a0b57b5639191f352dc016d6983dbb
  44. 44
    Purdie, D. G.; Pugno, N. M.; Taniguchi, T.; Watanabe, K.; Ferrari, A. C.; Lombardo, A. Cleaning interfaces in layered materials heterostructures. Nat. Commun. 2018, 9, 5387,  DOI: 10.1038/s41467-018-07558-3
    Google Scholar
    44
    Cleaning interfaces in layered materials heterostructures
    Purdie, D. G.; Pugno, N. M.; Taniguchi, T.; Watanabe, K.; Ferrari, A. C.; Lombardo, A.
    Nature Communications (2018), 9 (1), 5387CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Heterostructures formed by stacking layered materials require atomically clean interfaces. However, contaminants are usually trapped between the layers, aggregating into randomly located blisters, incompatible with scalable fabrication processes. Here we report a process to remove blisters from fully formed heterostructures. Our method is over an order of magnitude faster than those previously reported and allows multiple interfaces to be cleaned simultaneously. We fabricate blister-free regions of graphene encapsulated in hexagonal boron nitride with an area ∼ 5000μm2, achieving mobilities up to 180,000 cm2 V-1 s-1 at room temp., and 1.8 × 106 cm2 V-1 s-1 at 9 K. We also assemble heterostructures using graphene intentionally exposed to polymers and solvents. After cleaning, these samples reach similar mobilities. This demonstrates that exposure of graphene to process-related contaminants is compatible with the realization of high mobility samples, paving the way to the development of wafer-scale processes for the integration of layered materials in (opto)electronic devices.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXisFKht7fI&md5=9218b0901bfcaa32530c6827d335fb83
  45. 45
    Giambra, M. A. Wafer-Scale Integration of Graphene-Based Photonic Devices. ACS Nano 2021, 15, 3171– 3187,  DOI: 10.1021/acsnano.0c09758
    Google Scholar
    45
    Wafer-Scale Integration of Graphene-Based Photonic Devices
    Giambra, Marco A.; Miseikis, Vaidotas; Pezzini, Sergio; Marconi, Simone; Montanaro, Alberto; Fabbri, Filippo; Sorianello, Vito; Ferrari, Andrea C.; Coletti, Camilla; Romagnoli, Marco
    ACS Nano (2021), 15 (2), 3171-3187CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Graphene and related materials can lead to disruptive advances in next-generation photonics and optoelectronics. The challenge is to devise growth, transfer and fabrication protocols providing high (≥5000 cm2 V-1 s-1) mobility devices with reliable performance at the wafer scale. Here, we present a flow for the integration of graphene in photonics circuits. This relies on chem. vapor deposition (CVD) of single layer graphene (SLG) matrixes comprising up to ~ 12000 individual single crystals, grown to match the geometrical configuration of the devices in the photonic circuit. This is followed by a transfer approach which guarantees coverage over ~ 80% of the device area, and integrity for up to 150 mm wafers, with room temp. mobility ~ 5000 cm2 V-1 s-1. We use this process flow to demonstrate double SLG electro-absorption modulators with modulation efficiency ~ 0.25, 0.45, 0.75, 1 dB V-1 for device lengths ~ 30, 60, 90, 120μm. The data rate is up to 20 Gbps. Encapsulation with single-layer hexagonal boron nitride (hBN) is used to protect SLG during plasma-enhanced CVD of Si3N4, ensuring reproducible device performance. The processes are compatible with full automation. This paves the way for large scale prodn. of graphene-based photonic devices.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXisVWgsLs%253D&md5=daefda6abc1764a849dd867736cc0836
  46. 46
    Gadelha, A. C.; Ohlberg, D. A. A.; Rabelo, C.; Neto, E. G. S.; Vasconcelos, T. L.; Campos, J. L.; Lemos, J. S.; Ornelas, V.; Miranda, D.; Nadas, R.; Santana, F. C.; Watanabe, K.; Taniguchi, T.; van Troeye, B.; Lamparski, M.; Meunier, V.; Nguyen, V.-H.; Paszko, D.; Charlier, J.-C.; Campos, L. C.; Cancado, L. G.; Medeiros-Ribeiro, G.; Jorio, A. Localization of lattice dynamics in low-angle twisted bilayer graphene. Nature 2021, 590, 405– 409,  DOI: 10.1038/s41586-021-03252-5
    Google Scholar
    46
    Localization of lattice dynamics in low-angle twisted bilayer graphene
    Gadelha, Andreij C.; Ohlberg, Douglas A. A.; Rabelo, Cassiano; Neto, Eliel G. S.; Vasconcelos, Thiago L.; Campos, Joao L.; Lemos, Jessica S.; Ornelas, Vinicius; Miranda, Daniel; Nadas, Rafael; Santana, Fabiano C.; Watanabe, Kenji; Taniguchi, Takashi; van Troeye, Benoit; Lamparski, Michael; Meunier, Vincent; Nguyen, Viet-Hung; Paszko, Dawid; Charlier, Jean-Christophe; Campos, Leonardo C.; Cancado, Luiz G.; Medeiros-Ribeiro, Gilberto; Jorio, Ado
    Nature (London, United Kingdom) (2021), 590 (7846), 405-409CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Abstr.: Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain1-3. The resulting superlattice modulates the vibrational3,4 and electronic5,6 structures within the material, leading to changes in the behavior of electron-phonon coupling7,8 and to the observation of strong correlations and supercond.9. However, accessing these modulations and understanding the related effects are challenging, because the modulations are too small for exptl. techniques to accurately resolve the relevant energy levels and too large for theor. models to properly describe the localized effects. Here we report hyperspectral optical images, generated by a nano-Raman spectroscope10, of the crystal superlattice in reconstructed (low-angle) twisted bilayer graphene. Observations of the crystallog. structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topol. points1 causing detectable spectral variations. The results are rationalized by an atomistic model that enables evaluation of the local d. of the electronic and vibrational states of the superlattice. This evaluation highlights the relevance of solitons and topol. points for the vibrational and electronic properties of the structures, particularly for small twist angles. Our results are an important step towards understanding phonon-related effects at at. and nanometric scales, such as Jahn-Teller effects11 and electronic Cooper pairing12-14, and may help to improve device characterization15 in the context of the rapidly developing field of twistronics16.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXktlGls7g%253D&md5=7440b55893e62ff8e9458bb37ffb0206
  47. 47
    Cao, Y. Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene. Phys. Rev. Lett. 2016, 117, 116804,  DOI: 10.1103/PhysRevLett.117.116804
    Google Scholar
    47
    Superlattice-Induced insulating states and valley-protected orbits in twisted bilayer Graphene
    Cao, Y.; Luo, J. Y.; Fatemi, V.; Fang, S.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P.
    Physical Review Letters (2016), 117 (11), 116804/1-116804/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moir´e physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theor. predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calcns. reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moir´e wave vector.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVWku7nE&md5=000d518b94f35535bbddfb29ca40c5b8
  48. 48
    Chung, T.-F.; Xu, Y.; Chen, Y. P. Transport measurements in twisted bilayer graphene: Electron-phonon coupling and Landau level crossing. Phys. Rev. B 2018, 98, 035425  DOI: 10.1103/PhysRevB.98.035425
    Google Scholar
    48
    Transport measurements in twisted bilayer graphene: Electron-phonon coupling and Landau level crossing
    Chung, Ting-Fung; Xu, Yang; Chen, Yong P.
    Physical Review B (2018), 98 (3), 035425CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    We investigate electronic transport in twisted bilayer graphene (tBLG) under variable temps. (T), carrier densities (n), and transverse magnetic fields, focusing on samples with small twist angles (θ). These samples show prominent signatures assocd. with the van Hove singularities (VHSs) and superlattice-induced minigaps (SMGs). Temp.-dependent field-effect measurement shows that the difference between temp.-dependent resistivity and residual resistivity, ρxx(T,n)-ρ0(n), follows ∼Tβ for n between the main Dirac point (DP) and SMG. The evolution of the temp. exponent β with n exhibits a W-shaped dependence, with min. of β∼0.9 near the VHSs and maxima of β∼1.7 toward the SMGs. This W-shaped behavior can be qual. understood with a theor. picture that considers both the Fermi surface smearing near the VHSs and flexural-acoustic phonon scattering. In the quantum Hall regime, we observe only Landau level crossings in the massless Dirac spectrum originating from the main DP but not in the parabolic band near the SMG. Such crossings enable the measurement of an enhanced interlayer dielec. const., attributed to a reduced Fermi velocity. Moreover, we measure the Fermi velocity, interlayer coupling strength, VHS energy relative to the DP, and gap size of SMG, four important parameters used to describe the peculiar band structure of the small-θ tBLG.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXltVGmsrs%253D&md5=2257f760d6fb5746b1fd991397426a8a
  49. 49
    Kim, Y. Charge Inversion and Topological Phase Transition at a Twist Angle Induced van Hove Singularity of Bilayer Graphene. Nano Lett. 2016, 16, 5053– 5059,  DOI: 10.1021/acs.nanolett.6b01906
    Google Scholar
    49
    Charge Inversion and Topological Phase Transition at a Twist Angle Induced van Hove Singularity of Bilayer Graphene
    Kim, Youngwook; Herlinger, Patrick; Moon, Pilkyung; Koshino, Mikito; Taniguchi, Takashi; Watanabe, Kenji; Smet, Jurgen H.
    Nano Letters (2016), 16 (8), 5053-5059CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Van Hove singularities (VHS's) in the d. of states play an outstanding and diverse role for the electronic and thermodn. properties of cryst. solids. At the crit. point the Fermi surface connectivity changes, and topol. properties undergo a transition. Opportunities to systematically pass a VHS at the turn of a voltage knob and study its diverse impact are however rare. With the advent of van der Waals heterostructures, control over the at. registry of neighboring graphene layers offers an unprecedented tool to generate a low energy VHS easily accessible with conventional gating. Here the authors have addressed magnetotransport when the chem. potential crosses the twist angle induced VHS in twisted bilayer graphene. A topol. phase transition is exptl. disclosed in the abrupt conversion of electrons to holes or vice versa, a loss of a nonzero Berry phase and distinct sequences of integer quantum Hall states above and below the singularity.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtFeku7bK&md5=dfd392a11beddb12980900909133f71a
  50. 50
    Berdyugin, A. I.; Tsim, B.; Kumaravadivel, P.; Xu, S. G.; Ceferino, A.; Knothe, A.; Kumar, R. K.; Taniguchi, T.; Watanabe, K.; Geim, A. K.; Grigorieva, I. V.; Fal’ko, V. I. Minibands in twisted bilayer graphene probed by magnetic focusing. Sci. Adv. 2020, 6, eaay7838  DOI: 10.1126/sciadv.aay7838
    Google Scholar
    There is no corresponding record for this reference.
  51. 51
    Kim, Y.; Moon, P.; Watanabe, K.; Taniguchi, T.; Smet, J. H. Odd Integer Quantum Hall States with Interlayer Coherence in Twisted Bilayer Graphene. Nano Lett. 2021, 21, 4249– 4254,  DOI: 10.1021/acs.nanolett.1c00360
    Google Scholar
    51
    Odd integer quantum hall states with interlayer coherence in twisted bilayer graphene
    Kim, Youngwook; Moon, Pilkyung; Watanabe, Kenji; Taniguchi, Takashi; Smet, Jurgen H.
    Nano Letters (2021), 21 (10), 4249-4254CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We report on the quantum Hall effect in two stacked graphene layers rotated by 2°. The tunneling strength among the layers can be varied from very weak to strong via the mechanism of magnetic breakdown when tuning the d. Odd-integer quantum Hall physics is not anticipated in the regime of suppressed tunneling for balanced layer densities, yet it is obsd. We interpret this as a signature of Coulomb interaction induced interlayer coherence and Bose-Einstein condensation of excitons that form at half filling of each layer. A d. imbalance gives rise to reentrant behavior due to a phase transition from the interlayer coherent state to incompressible behavior caused by simultaneous condensation of both layers in different quantum Hall states. With increasing overall d., magnetic breakdown gains the upper hand. As a consequence of the enhanced interlayer tunneling, the interlayer coherent state and the phase transition vanish.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhtVWrt7jL&md5=17b4780057a4b93db1b5fc17541d9085
  52. 52
    Yankowitz, M. Tuning superconductivity in twisted bilayer graphene. Science 2019, 363, 1059– 1064,  DOI: 10.1126/science.aav1910
    Google Scholar
    52
    Tuning superconductivity in twisted bilayer graphene
    Yankowitz, Matthew; Chen, Shaowen; Polshyn, Hryhoriy; Zhang, Yuxuan; Watanabe, K.; Taniguchi, T.; Graf, David; Young, Andrea F.; Dean, Cory R.
    Science (Washington, DC, United States) (2019), 363 (6431), 1059-1064CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Materials with flat electronic bands often exhibit exotic quantum phenomena owing to strong correlations. An isolated low-energy flat band can be induced in bilayer graphene by simply rotating the layers by 1.1°, resulting in the appearance of gate-tunable superconducting and correlated insulating phases. In this study, the authors demonstrate that in addn. to the twist angle, the interlayer coupling can be varied to precisely tune these phases. The authors induce supercond. at a twist angle larger than 1.1°-in which correlated phases are otherwise absent-by varying the interlayer spacing with hydrostatic pressure. Their low-disorder devices reveal details about the superconducting phase diagram and its relation to the nearby insulator. The results demonstrate twisted bilayer graphene to be a distinctively tunable platform for exploring correlated states.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXktFagt7c%253D&md5=c98ef0256319b9a3b8a1d2617c35ae87
  53. 53
    Piccinini, G.; Mišeikis, V.; Watanabe, K.; Taniguchi, T.; Coletti, C.; Pezzini, S. Parallel transport and layer-resolved thermodynamic measurements in twisted bilayer graphene. Phys. Rev. B 2021, 104, L241410,  DOI: 10.1103/PhysRevB.104.L241410
    Google Scholar
    53
    Parallel transport and layer-resolved thermodynamic measurements in twisted bilayer graphene
    Piccinini, G.; Miseikis, V.; Watanabe, K.; Taniguchi, T.; Coletti, C.; Pezzini, S.
    Physical Review B (2021), 104 (24), L241410CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    We employ dual-gated 30°-twisted bilayer graphene to demonstrate simultaneous ultrahigh mobility and cond. (up to 40 mS at room temp.), unattainable in a single layer of graphene. We find quant. agreement with a simple phenomenol. of parallel conduction between two pristine graphene sheets, with a gate-controlled carrier distribution. Based on the parallel transport mechanism, we then introduce a method for in situ measurements of the chem. potential of the two layers. This twist-enabled approach, neither requiring a dielec. spacer, nor sep. contacting, has the potential to greatly simplify the measurement of thermodn. quantities in graphene-based systems of high current interest.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhtV2jtr8%253D&md5=52373f01c13ff999c4427bc36f1e3e3d
  54. 54
    Novelli, P.; Torre, I.; Koppens, F. H. L.; Taddei, F.; Polini, M. Optical and plasmonic properties of twisted bilayer graphene: Impact of interlayer tunneling asymmetry and ground-state charge inhomogeneity. Phys. Rev. B 2020, 102, 125403,  DOI: 10.1103/PhysRevB.102.125403
    Google Scholar
    54
    Optical and plasmonic properties of twisted bilayer graphene: Impact of interlayer tunneling asymmetry and ground-state charge inhomogeneity
    Novelli, Pietro; Torre, Iacopo; Koppens, Frank H. L.; Taddei, Fabio; Polini, Marco
    Physical Review B (2020), 102 (12), 125403CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Twisted bilayer graphene (TBG) at twist angles θ≈1o has recently attracted a great deal of interest for its rich transport phenomenol. We present a theor. study of the local optical cond., plasmon spectra, and thermoelec. properties of TBG at different filling factors and twist angles θ. Our calcns. are based on the electronic band structures obtained from a continuum model that has two tunable parameters u0 and u1 which parametrize the intrasublattice interlayer and intersublattice interlayer tunneling rate, resp. In this article we focus on two key aspects: (i) we study the dependence of our results on the value of u0, exploring the whole range 0≤u0≤u1; and (ii) we take into account effects arising from the intrinsic charge d. inhomogeneity present in TBG, by calcg. the band structures within the self-consistent Hartree approxn. At zero filling factor, i.e., at the charge neutrality point, the optical cond. is quite sensitive to the value of u0 and twist angle, whereas the charge inhomogeneity brings about only modest corrections. On the other hand, away from zero filling, static screening dominates and the optical cond. is appreciably affected by the charge inhomogeneity, the largest effects being seen on the intraband contribution to it. These findings are also reflected by the plasmonic spectra. We compare our results with existing ones in the literature, where effects (i) and (ii) above have not been studied systematically. As natural byproducts of our calcns., we obtain the Drude wt. and Seebeck coeff. The former displays an enhanced particle-hole asymmetry stemming from the inhomogeneous ground-state charge distribution. The latter is shown to display a broad sign-changing feature even at low temps. (≈5K) due to the reduced slope of the bands, as compared to those of single-layer graphene.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitVOiu7rE&md5=301ca4d6806eb0952ca6771cb1415c05
  55. 55
    Hesp, N. C. H. Observation of interband collective excitations in twisted bilayer graphene. Nat. Phys. 2021, 17, 1162– 1168,  DOI: 10.1038/s41567-021-01327-8
    Google Scholar
    55
    Observation of interband collective excitations in twisted bilayer graphene
    Hesp, Niels C. H.; Torre, Iacopo; Rodan-Legrain, Daniel; Novelli, Pietro; Cao, Yuan; Carr, Stephen; Fang, Shiang; Stepanov, Petr; Barcons-Ruiz, David; Herzig Sheinfux, Hanan; Watanabe, Kenji; Taniguchi, Takashi; Efetov, Dmitri K.; Kaxiras, Efthimios; Jarillo-Herrero, Pablo; Polini, Marco; Koppens, Frank H. L.
    Nature Physics (2021), 17 (10), 1162-1168CODEN: NPAHAX; ISSN:1745-2473. (Nature Portfolio)
    The single-particle and many-body properties of twisted bilayer graphene (TBG) can be dramatically different from those of a single graphene layer, particularly when the two layers are rotated relative to each other by a small angle (θ ≈ 1°), owing to the moire potential induced by the twist. Here we probe the collective excitations of TBG with a spatial resoln. of 20 nm, by applying mid-IR near-field optical microscopy. We find a propagating plasmon mode in charge-neutral TBG for θ = 1.1-1.7°, which is different from the intraband plasmon in single-layer graphene. We interpret it as an interband plasmon assocd. with the optical transitions between minibands originating from the moire superlattice. The details of the plasmon dispersion are directly related to the motion of electrons in the moire superlattice and offer an insight into the phys. properties of TBG, such as band nesting between the flat band and remote band, local interlayer coupling, and losses. We find a strongly reduced interlayer coupling in the regions with AA stacking, pointing at screening due to electron-electron interactions. Optical nano-imaging of TBG allows the spatial probing of interaction effects at the nanoscale and potentially elucidates the contribution of collective excitations to many-body ground states.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXitFamtrvJ&md5=c30b2b8cf46afb06c451b8e1f04ca10e
  56. 56
    Giuliani, G. F.; Vignale, G. Quantum Theory of the Electron Liquid; Cambridge Univ. Press: Cambridge, England, 2005.
    Google Scholar
    There is no corresponding record for this reference.
  57. 57
    Berthod, C.; Zhang, H.; Morpurgo, A. F.; Giamarchi, T. Theory of cross quantum capacitance. Phys. Rev. Res. 2021, 3, 043036  DOI: 10.1103/PhysRevResearch.3.043036
    Google Scholar
    57
    Theory of cross quantum capacitance
    Berthod, Christophe; Zhang, Haijing; Morpurgo, Alberto F.; Giamarchi, Thierry
    Physical Review Research (2021), 3 (4), 043036CODEN: PRRHAI; ISSN:2643-1564. (American Physical Society)
    Impressive progress in the control of atomically thin crystals is now enabling the realization of gated structures in which two electrodes are sepd. by at. scale distances. The elec. capacitance of these structures is detd. by phenomena that are not relevant in capacitors with larger electrode sepn. With the aim to analyze these phenomena, we use linear-response theory to develop a systematic description of capacitance for two coupled electron liqs., accounting for the wave nature of electrons, as well as for the effect of both intra- and interlayer Coulomb interactions. Our theory leads to a general expression for the elec. capacitance in terms of both intra- and interlayer electronic polarizabilities. The intralayer polarizability is directly related to the conventional expression for the quantum capacitance, whereas the interlayer polarizability term accounts for interaction-induced correlations between charges hosted by opposite capacitor plates. We refer to this latter term-which has not been considered earlier-as to the cross quantum capacitance. We discuss the implications of the general expression for the capacitance, show that it leads to established results when the effect of interlayer correlations is negligible, and that the intra- and interlayer polarizabilities play a comparable role for capacitors with very small electrode sepn. (i.e., cross quantum capacitance effects can be large and cannot be neglected). Using two different approaches, we calc. the capacitance in specific cases, and find that the interlayer polarizability can be either pos. or neg., so that-depending on the regime considered-the cross quantum capacitance can either increase or decrease the total capacitance. We conclude by showing that the cross quantum capacitance term can lead to a nonmonotonic evolution of the total capacitance with increasing sepn. between the capacitor plates, which would represent an unambiguous manifestation of the cross quantum capacitance if obsd. exptl.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhtFShs7w%253D&md5=fe17d6c36655fd19a871cfd23126b8fc
  58. 58
    Slizovskiy, S. Out-of-Plane Dielectric Susceptibility of Graphene in Twistronic and Bernal Bilayers. Nano Lett. 2021, 21, 6678– 6683,  DOI: 10.1021/acs.nanolett.1c02211
    Google Scholar
    58
    Out-of-plane dielectric susceptibility of graphene in twistronic and bernal bilayers
    Slizovskiy, Sergey; Garcia-Ruiz, Aitor; Berdyugin, Alexey I.; Xin, Na; Taniguchi, Takashi; Watanabe, Kenji; Geim, Andre K.; Drummond, Neil D.; Fal'ko, Vladimir I.
    Nano Letters (2021), 21 (15), 6678-6683CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We describe how the out-of-plane dielec. polarizability of monolayer graphene influences the electrostatics of bilayer graphene-both Bernal (BLG) and twisted (tBLG). We compare the polarizability value computed using d. functional theory with the output from previously published exptl. data on the electrostatically controlled interlayer asymmetry potential in BLG and data on the on-layer d. distribution in tBLG. We show that monolayers in tBLG are described well by polarizability αexp = 10.8 Å3 and effective out-of-plane dielec. susceptibility εz = 2.5, including their on-layer electron d. distribution at zero magnetic field and the interlayer Landau level pinning at quantizing magnetic fields.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhs1SgsbvP&md5=f60d06525a8d1c542c0f25afb6ba1c0e
  59. 59
    Hejazi, K.; Liu, C.; Balents, L. Landau levels in twisted bilayer graphene and semiclassical orbits. Phys. Rev. B 2019, 100, 035115  DOI: 10.1103/PhysRevB.100.035115
    Google Scholar
    59
    Landau levels in twisted bilayer graphene and semiclassical orbits
    Hejazi, Kasra; Liu, Chunxiao; Balents, Leon
    Physical Review B (2019), 100 (3), 035115CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Twisted bilayer graphene has been argued theor. to host exceptionally flat bands when the angle between the two layers falls within a magic range near 1.1o. This is now strongly supported by expt., which furthermore reveals dramatic correlation effects in the magic range due to the relative dominance of interactions when the bandwidth is suppressed. Exptl., quantum oscillations exhibit different Landau level degeneracies when the angles fall in or outside the magic range; these observations can contain crucial information about the low-energy physics. In this paper, we report a thorough theor. study of the Landau level structure of the noninteracting continuum model for twisted bilayer graphene as the magnetic field and the twist angle are tuned. We first show that a discernible difference exists in the butterfly spectra when twist angle falls in and outside the magic range. Next, we carry out semiclassical anal. in detail, which quant. dets. the origin of the low-energy Landau levels from the zero field band structure. We find that the Landau level degeneracy predicted in the above analyses is capable of partially explaining features of the quantum oscillation expts. in a natural way. Finally, topol. aspects, validity, and other subtle points of the model are discussed.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhvFelsbnO&md5=38de8892eb1e86934688aabceafb5dac
  60. 60
    Krishna Kumar, R.; Chen, X.; Auton, G. H.; Mishchenko, A.; Bandurin, D. A.; Morozov, S. V.; Cao, Y.; Khestanova, E.; Ben Shalom, M.; Kretinin, A. V.; Novoselov, K. S.; Eaves, L.; Grigorieva, I. V.; Ponomarenko, L. A.; Fal’ko, V. I.; Geim, A. K. High-temperature quantum oscillations caused by recurring Bloch states in graphene superlattices. Science 2017, 357, 181– 184,  DOI: 10.1126/science.aal3357
    Google Scholar
    60
    High-temperature quantum oscillations caused by recurring Bloch states in graphene superlattices
    Krishna Kumar, R.; Chen, X.; Auton, G. H.; Mishchenko, A.; Bandurin, D. A.; Morozov, S. V.; Cao, Y.; Khestanova, E.; Ben Shalom, M.; Kretinin, A. V.; Novoselov, K. S.; Eaves, L.; Grigorieva, I. V.; Ponomarenko, L. A.; Fal'ko, V. I.; Geim, A. K.
    Science (Washington, DC, United States) (2017), 357 (6347), 181-184CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Cyclotron motion of charge carriers in metals and semiconductors leads to Landau quantization and magneto-oscillatory behavior in their properties. Cryogenic temps. are usually required to observe these oscillations. We show that graphene superlattices support a different type of quantum oscillation that does not rely on Landau quantization. The oscillations are extremely robust and persist well above room temp. in magnetic fields of only a few tesla. We attribute this phenomenon to repetitive changes in the electronic structure of superlattices such that charge carriers experience effectively no magnetic field at simple fractions of the flux quantum per superlattice unit cell. Our work hints at unexplored physics in Hofstadter butterfly systems at high temps.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtFOjs73J&md5=5a7b5d8db342f35e55d35295c8bfd289
  61. 61
    Kumar, R. K.; Mishchenko, A.; Chen, X.; Pezzini, S.; Auton, G. H.; Ponomarenko, L. A.; Zeitler, U.; Eaves, L.; Fal’ko, V. I.; Geim, A. K. High-order fractal states in graphene superlattices. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 5135– 5139,  DOI: 10.1073/pnas.1804572115
    Google Scholar
    61
    High-order fractal states in graphene superlattices
    Kumar, R. Krishna; Mishchenko, A.; Chen, X.; Pezzini, S.; Auton, G. H.; Ponomarenko, L. A.; Zeitler, U.; Eaves, L.; Fal'ko, V. I.; Geim, A. K.
    Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (20), 5135-5139CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Graphene superlattices were shown to exhibit high-temp. quantum oscillations due to periodic emergence of delocalized Bloch states in high magnetic fields such that unit fractions of the flux quantum pierce a superlattice unit cell. Under these conditions, semiclassical electron trajectories become straight again, similar to the case of zero magnetic field. Here, we report magnetotransport measurements that reveal second-, third-, and fourth-order magnetic Bloch states at high electron densities and temps. above 100 K. The recurrence of these states creates a fractal pattern intimately related to the origin of Hofstadter butterflies. The hierarchy of the fractal states is detd. by the width of magnetic minibands, in qual. agreement with our band-structure calcns.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvVGht7fF&md5=16e9fb066d124af6693837bc5c8d2297
  62. 62
    Barrier, J.; Kumaravadivel, P.; Krishna Kumar, R.; Ponomarenko, L. A.; Xin, N.; Holwill, M.; Mullan, C.; Kim, M.; Gorbachev, R. V.; Thompson, M. D.; Prance, J. R.; Taniguchi, T.; Watanabe, K.; Grigorieva, I. V.; Novoselov, K. S.; Mishchenko, A.; Fal’ko, V. I.; Geim, A. K.; Berdyugin, A. I. Long-range ballistic transport of Brown-Zak fermions in graphene superlattices. Nat. Commun. 2020, 11, 5756,  DOI: 10.1038/s41467-020-19604-0
    Google Scholar
    62
    Long-range ballistic transport of Brown-Zak fermions in graphene superlattices
    Barrier, Julien; Kumaravadivel, Piranavan; Krishna Kumar, Roshan; Ponomarenko, L. A.; Xin, Na; Holwill, Matthew; Mullan, Ciaran; Kim, Minsoo; Gorbachev, R. V.; Thompson, M. D.; Prance, J. R.; Taniguchi, T.; Watanabe, K.; Grigorieva, I. V.; Novoselov, K. S.; Mishchenko, A.; Fal'ko, V. I.; Geim, A. K.; Berdyugin, A. I.
    Nature Communications (2020), 11 (1), 5756CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    In quantizing magnetic fields, graphene superlattices exhibit a complex fractal spectrum often referred to as the Hofstadter butterfly. It can be viewed as a collection of Landau levels that arise from quantization of Brown-Zak minibands recurring at rational (p/q) fractions of the magnetic flux quantum per superlattice unit cell. Here we show that, in graphene-on-boron-nitride superlattices, Brown-Zak fermions can exhibit mobilities above 106 cm2 V-1 s-1 and the mean free path exceeding several micrometers. The exceptional quality of our devices allows us to show that Brown-Zak minibands are 4q times degenerate and all the degeneracies (spin, valley and mini-valley) can be lifted by exchange interactions below 1 K. We also found neg. bend resistance at 1/q fractions for elec. probes placed as far as several micrometers apart. The latter observation highlights the fact that Brown-Zak fermions are Bloch quasiparticles propagating in high fields along straight trajectories, just like electrons in zero field.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitlKltrfO&md5=882fe77d97079ff01a99d9c2be0b77c3
  63. 63
    Ponomarenko, L. A. Cloning of Dirac fermions in graphene superlattices. Nature 2013, 497, 594– 597,  DOI: 10.1038/nature12187
    Google Scholar
    63
    Cloning of Dirac fermions in graphene superlattices
    Ponomarenko, L. A.; Gorbachev, R. V.; Yu, G. L.; Elias, D. C.; Jalil, R.; Patel, A. A.; Mishchenko, A.; Mayorov, A. S.; Woods, C. R.; Wallbank, J. R.; Mucha-Kruczynski, M.; Piot, B. A.; Potemski, M.; Grigorieva, I. V.; Novoselov, K. S.; Guinea, F.; Fal'ko, V. I.; Geim, A. K.
    Nature (London, United Kingdom) (2013), 497 (7451), 594-597CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Superlattices have attracted great interest because their use may make it possible to modify the spectra of two-dimensional electron systems and, ultimately, create materials with tailored electronic properties. In previous studies (see, for example, refs 1, 2, 3, 4, 5, 6, 7, 8), it proved difficult to realize superlattices with short periodicities and weak disorder, and most of their obsd. features could be explained in terms of cyclotron orbits commensurate with the superlattice. Evidence for the formation of superlattice minibands (forming a fractal spectrum known as Hofstadter's butterfly) has been limited to the observation of new low-field oscillations and an internal structure within Landau levels. Here we report transport properties of graphene placed on a boron nitride substrate and accurately aligned along its crystallog. directions. The substrate's moire potential acts as a superlattice and leads to profound changes in the graphene's electronic spectrum. Second-generation Dirac points appear as pronounced peaks in resistivity, accompanied by reversal of the Hall effect. The latter indicates that the effective sign of the charge carriers changes within graphene's conduction and valence bands. Strong magnetic fields lead to Zak-type cloning of the third generation of Dirac points, which are obsd. as numerous neutrality points in fields where a unit fraction of the flux quantum pierces the superlattice unit cell. Graphene superlattices such as this one provide a way of studying the rich physics expected in incommensurable quantum systems and illustrate the possibility of controllably modifying the electronic spectra of two-dimensional at. crystals by varying their crystallog. alignment within van der Waals heterostuctures.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXotFSgtbo%253D&md5=3a155ffd5b544c2b4f870a6931e23072
  64. 64
    Dean, C. R. Hofstadter’s butterfly and the fractal quantum Hall effect in moiré superlattices. Nature 2013, 497, 598– 602,  DOI: 10.1038/nature12186
    Google Scholar
    64
    Hofstadter's butterfly and the fractal quantum Hall effect in moire superlattices
    Dean, C. R.; Wang, L.; Maher, P.; Forsythe, C.; Ghahari, F.; Gao, Y.; Katoch, J.; Ishigami, M.; Moon, P.; Koshino, M.; Taniguchi, T.; Watanabe, K.; Shepard, K. L.; Hone, J.; Kim, P.
    Nature (London, United Kingdom) (2013), 497 (7451), 598-602CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Electrons moving through a spatially periodic lattice potential develop a quantized energy spectrum consisting of discrete Bloch bands. In 2 dimensions, electrons moving through a magnetic field also develop a quantized energy spectrum, consisting of highly degenerate Landau energy levels. When subject to both a magnetic field and a periodic electrostatic potential, 2D systems of electrons exhibit a self-similar recursive energy spectrum. Known as Hofstadter's butterfly, this complex spectrum results from an interplay between the characteristic lengths assocd. with the 2 quantizing fields, and is one of the first quantum fractals discovered in physics. In the decades since its prediction, exptl. attempts to study this effect were limited by difficulties in reconciling the 2 length scales. Typical at. lattices (with periodicities of less than one nanometer) require unfeasibly large magnetic fields to reach the commensurability condition, and in artificially engineered structures (with periodicities greater than about 100 nm) the corresponding fields are too small to overcome disorder completely. Moire superlattices arising in bilayer graphene coupled to hexagonal BN provide a periodic modulation with ideal length scales of the order of ten nanometers, enabling unprecedented exptl. access to the fractal spectrum. Quantum Hall features assocd. with the fractal gaps are described by 2 integer topol. quantum nos., and report evidence of their recursive structure. Observation of a Hofstadter spectrum in bilayer graphene means that it is possible to investigate emergent behavior within a fractal energy landscape in a system with tunable internal degrees of freedom.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXotFSntb8%253D&md5=507cf9b9e6769653489870f59f6c1a17
  65. 65
    Hunt, B. Massive Dirac Fermions and Hofstadter Butterfly in a van der Waals Heterostructure. Science 2013, 340, 1427– 1430,  DOI: 10.1126/science.1237240
    Google Scholar
    65
    Massive Dirac Fermions and Hofstadter Butterfly in a van der Waals Heterostructure
    Hunt, B.; Sanchez-Yamagishi, J. D.; Young, A. F.; Yankowitz, M.; Le Roy, B. J.; Watanabe, K.; Taniguchi, T.; Moon, P.; Koshino, M.; Jarillo-Herrero, P.; Ashoori, R. C.
    Science (Washington, DC, United States) (2013), 340 (6139), 1427-1430CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Van der Waals heterostructures constitute a new class of artificial materials formed by stacking atomically thin planar crystals. We demonstrated band structure engineering in a van der Waals heterostructure composed of a monolayer graphene flake coupled to a rotationally aligned hexagonal boron nitride substrate. The spatially varying interlayer at. registry results in both a local breaking of the carbon sublattice symmetry and a long-range moire superlattice potential in the graphene. In our samples, this interplay between short- and long-wavelength effects resulted in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality. This picture is confirmed by our observation of fractional quantum Hall states at ±53 filling and features assocd. with the Hofstadter butterfly at ultrahigh magnetic fields.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXpsFKlt70%253D&md5=96c441306af443349b95d6f4cb8df576
  66. 66
    Phinney, I. Y. Strong interminivalley scattering in twisted bilayer graphene revealed by high-temperature magneto-oscillations. Phys. Rev. Lett. 2021, 127, 056802  DOI: 10.1103/PhysRevLett.127.056802
    Google Scholar
    66
    Strong Interminivalley Scattering in Twisted Bilayer Graphene Revealed by High-Temperature Magneto-Oscillations
    Phinney, I. Y.; Bandurin, D. A.; Collignon, C.; Dmitriev, I. A.; Taniguchi, T.; Watanabe, K.; Jarillo-Herrero, P.
    Physical Review Letters (2021), 127 (5), 056802CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
    Twisted bilayer graphene (TBG) provides an example of a system in which the interplay of interlayer interactions and superlattice structure impacts electron transport in a variety of nontrivial ways and gives rise to a plethora of interesting effects. Understanding the mechanisms of electron scattering in TBG has, however, proven challenging, raising many questions about the origins of resistivity in this system. Here we show that TBG exhibits high-temp. magneto-oscillations originating from the scattering of charge carriers between TBG minivalleys. The amplitude of these oscillations reveals that interminivalley scattering is strong, and its characteristic timescale is comparable to that of its intraminivalley counterpart. Furthermore, by exploring the temp. dependence of these oscillations, we est. the electron-electron collision rate in TBG and find that it exceeds that of monolayer graphene. Our study demonstrates the consequences of the relatively small size of the superlattice Brillouin zone and Fermi velocity redn. on lateral transport in TBG.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhvVamtbrJ&md5=c83f2230f9ca4787dc51a8b6aabbded2
  67. 67
    Yin, J.; Wang, H.; Peng, H.; Tan, Z.; Liao, L.; Lin, L.; Sun, X.; Koh, A. L.; Chen, Y.; Peng, H.; Liu, Z. Selectively enhanced photocurrent generation in twisted bilayer graphene with van Hove singularity. Nat. Commun. 2016, 7, 10699,  DOI: 10.1038/ncomms10699
    Google Scholar
    67
    Selectively enhanced photocurrent generation in twisted bilayer graphene with van Hove singularity
    Yin, Jianbo; Wang, Huan; Peng, Han; Tan, Zhenjun; Liao, Lei; Lin, Li; Sun, Xiao; Koh, Ai Leen; Chen, Yulin; Peng, Hailin; Liu, Zhongfan
    Nature Communications (2016), 7 (), 10699CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Graphene with ultra-high carrier mobility and ultra-short photoresponse time has shown remarkable potential in ultrafast photodetection. However, the broad and weak optical absorption (∼2.3%) of monolayer graphene hinders its practical application in photodetectors with high responsivity and selectivity. Here we demonstrate that twisted bilayer graphene, a stack of two graphene monolayers with an interlayer twist angle, exhibits a strong light-matter interaction and selectively enhanced photocurrent generation. Such enhancement is attributed to the emergence of unique twist-angle-dependent van Hove singularities, which are directly revealed by spatially resolved angle-resolved photoemission spectroscopy. When the energy interval between the van Hove singularities of the conduction and valance bands matches the energy of incident photons, the photocurrent generated can be significantly enhanced (up to ∼80 times with the integration of plasmonic structures in our devices). These results provide valuable insight for designing graphene photodetectors with enhanced sensitivity for variable wavelength.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XjslGntr4%253D&md5=e04571669b7fcd0ca9aa62ae2c739ed7
  68. 68
    Sunku, S. S. Photonic crystals for nano-light in moiré graphene superlattices. Science 2018, 362, 1153– 1156,  DOI: 10.1126/science.aau5144
    Google Scholar
    68
    Photonic crystals for nano-light in Moire graphene superlattices
    Sunku, S. S.; Ni, G. X.; Jiang, B. Y.; Yoo, H.; Sternbach, A.; McLeod, A. S.; Stauber, T.; Xiong, L.; Taniguchi, T.; Watanabe, K.; Kim, P.; Fogler, M. M.; Basov, D. N.
    Science (Washington, DC, United States) (2018), 362 (6419), 1153-1156CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Graphene is an atomically thin plasmonic medium that supports highly confined plasmon polaritons, or nano-light, with very low loss. Electronic properties of graphene can be drastically altered when it is laid upon another graphene layer, resulting in a moire superlattice. The relative twist angle between the 2 layers is a key tuning parameter of the interlayer coupling in thus-obtained twisted bilayer graphene (TBG). The authors studied the propagation of plasmon polaritons in TBG by IR nano-imaging. The at. reconstruction occurring at small twist angles transforms the TBG into a natural plasmon photonic crystal for propagating nano-light. This discovery points to a pathway for controlling nano-light by exploiting quantum properties of graphene and other atomically layered van der Waals materials, eliminating the need for arduous top-down nanofabrication.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXisVegu7zP&md5=52e76b884a18585e17b5c2727e3f3c54
  69. 69
    Tomadin, A.; Guinea, F.; Polini, M. Generation and morphing of plasmons in graphene superlattices. Phys. Rev. B 2014, 90, 161406,  DOI: 10.1103/PhysRevB.90.161406
    Google Scholar
    69
    Generation and morphing of plasmons in graphene superlattices
    Tomadin, Andrea; Guinea, Francisco; Polini, Marco
    Physical Review B: Condensed Matter and Materials Physics (2014), 90 (16), 161406/1-161406/5, 5 pp.CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Recent exptl. studies on graphene on hexagonal boron nitride (hBN) have demonstrated that hBN is not only a passive substrate that ensures superb electronic properties of graphene's carriers, but that it actively modifies their massless Dirac fermion character through a periodic moir´e potential. Here we present a theory of the plasmon excitation spectrum of massless Dirac fermions in a moir´e superlattice. We demonstrate that graphene-hBN stacks offer a rich platform for plasmonics in which control of plasmon modes can occur not only via electrostatic gating but also by adjusting, e.g., the relative crystallog. alignment.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhvVKjt7Y%253D&md5=0e9d2f9b9de300c9b955919dd012846f
  70. 70
    Zhang, Y.; et al. Ascendance of Superconductivity in Magic-Angle Graphene Multilayers. arXiv 2112.09270, 2021, accessed 2022-06-27.
    Google Scholar
    There is no corresponding record for this reference.
  71. 71
    Park, J. M.; et al. Magic-Angle Multilayer Graphene: A Robust Family of Moiré Superconductors. arXiv 2112.10760, 2021, accessed 2022-06-27.
    Google Scholar
    There is no corresponding record for this reference.

Cited By

ARTICLE SECTIONS
Jump To

This article is cited by 1 publications.

  1. Xiangbin Cai, Weibo Gao. Moiré Synergy: An Emerging Playground by Coupled Moirés. ACS Nano 2023, 17 (11) , 9673-9680. https://doi.org/10.1021/acsnano.3c01740
Download PDF

  • Abstract

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 1

    Figure 1. (a) Schematics of the dry pick-up process with stacking of separated CVD-grown graphene crystals. (b) Optical microscopy image of CVD SLG crystals on SiO2/Si. The dashed lines indicate their crystallographic alignment. (c) The θ-rotated graphene sheets form a moiré pattern with periodicity λ. (d) Representative Raman spectrum of TBG (dark red), compared to a SLG reference (gray). The light red and blue lines are the two Lorentzian components of the TBG 2D peak. Inset: optical microscopy image of hBN-encapsulated SA-TBG. The dark red spot indicates the point where the TBG spectrum in the main panel is acquired.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 2

    Figure 2. (a) First derivative of the Hall conductivity as a function of top and back-gate voltages, measured for a fixed value of the applied perpendicular magnetic field (B = 3 T). The dotted orange (red) lines are the calculated positions of Landau levels from the upper (lower) graphene layers, employing vF = 0.47 × 106 m/s and Cgg = 17.5 × 10–6 F/cm2. (b) Schematics of the gating configuration. The optical microscopy image of the device is taken before the final etching step; the scale bar is 2.5 μm. (c) Fermi velocity of TBG as a function of the twist angle, calculated according to the theory described in refs (54and55) and references therein. Results in this figure have been obtained by setting u0 = 79.7 meV and u1 = 97.5 meV, where u0 and u1 are the intra- and intersublattice interlayer tunneling amplitudes, respectively. The blue circle corresponds to the vF value estimated for our device. (d) Hall conductivity as a function of the gate voltages (same gate ranges and magnetic field as in (a)). The sign changes in σxy correspond to the sample CNP and the two vHs. The black rectangle indicates the gate range considered in panel (e), the black and dark red dots are the gate values used for the measurements in Figure 4. (e) Zero-field longitudinal conductivity (lg scale) as a function of top-gate voltage relative to the sample CNP and back-gate voltage. The dotted orange (red) line is the calculated charge neutrality point for the upper (lower) layer. All the data in this figure have been acquired at T = 4.2 K.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 3

    Figure 3. (a) Longitudinal resistance measured as a function of Vtg and B, at Vbg = −60 V (left panel, T = 2.5 K) and Vbg = +60 V (right panel, T = 4.2 K). (b) Normalized FFT amplitude of the data in panel (a), as a function of the total charge density and of the oscillation frequency BF. (c) Fan of quantized states originating from the Γs point. Inset: band structure calculations for TBG with θ = 2.4°, based on refs (25,54,and55). The same intra- and intersublattice interlayer tunneling amplitudes of Figure 2c are used. Hartree self-consistent corrections do not yield significant changes with respect to single-particle calculations because the twist angle considered in this work is sufficiently larger that the MA. (d) Hall conductivity in the vicinity of the hole-side vHs, as a function of Vtg and 1/B (left axis). The right axis scale shows the number of flux quanta per superlattice unit cell, that is, ϕ/ϕ0.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 4

    Figure 4. Longitudinal resistance as a function of B, measured at T = 35 K in the vicinity of the electron-side vHs, at D = 0 (dark red curve) and D > 0 (black curve); the gate values are indicated by the dark red and black circles in Figure 2d. Inset: FFT spectra of the oscillatory resistance from the curves in the main panel.

    High Resolution Image
    Download MS PowerPoint Slide

  • References

    ARTICLE SECTIONS
    Jump To

    This article references 71 other publications.

    1. 1
      Cao, Y. Unconventional superconductivity in magic-angle graphene superlattices. Nature 2018, 556, 43– 50,  DOI: 10.1038/nature26160
      Google Scholar
      1
      Unconventional superconductivity in magic-angle graphene superlattices
      Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo
      Nature (London, United Kingdom) (2018), 556 (7699), 43-50CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      The behavior of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theor. understanding has motivated the development of exptl. techniques for studying such behavior, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional supercond.-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a crit. temp. of up to 1.7 K. The temp.-carrier-d. phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to supercond. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting crit. temp. of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier d. of about 1011 per square centimeter), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-crit.-temp. superconductors and quantum spin liqs.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXntVGjsbc%253D&md5=686b029f989784d0fa11ede1bfcecdbe
    2. 2
      Cao, Y. Correlated insulator behaviour at half-filling in magic-angle graphene superlattices. Nature 2018, 556, 80– 84,  DOI: 10.1038/nature26154
      Google Scholar
      2
      Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
      Cao, Yuan; Fatemi, Valla; Demir, Ahmet; Fang, Shiang; Tomarken, Spencer L.; Luo, Jason Y.; Sanchez-Yamagishi, Javier D.; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Ashoori, Ray C.; Jarillo-Herrero, Pablo
      Nature (London, United Kingdom) (2018), 556 (7699), 80-84CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials. One such property is the 'twist' angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analog in other types of heterostructure, such as semiconductors grown using mol. beam epitaxy. For small twist angles, the moire pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concd. mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride. Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show exptl. that, as predicted theor., when this angle is close to the 'magic' angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moire´ pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through elec. tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liqs.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXntVGjtr8%253D&md5=1697ebd09a673171514a527c17acf5b4
    3. 3
      Sharpe, A. L. Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene. Science 2019, 365, 605– 608,  DOI: 10.1126/science.aaw3780
      Google Scholar
      3
      Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene
      Sharpe, Aaron L.; Fox, Eli J.; Barnard, Arthur W.; Finney, Joe; Watanabe, Kenji; Taniguchi, Takashi; Kastner, M. A.; Goldhaber-Gordon, David
      Science (Washington, DC, United States) (2019), 365 (6453), 605-608CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      When two sheets of graphene are stacked at a small twist angle, the resulting flat superlattice minibands are expected to strongly enhance electron-electron interaction. Here, we present evidence that near three-quarters (3/4) filling of the conduction miniband, these enhanced interactions drive the twisted bilayer graphene into a ferromagnetic state. In a narrow d. range around an apparent insulating state at 3/4, we observe emergent ferromagnetic hysteresis, with a giant anomalous Hall (AH) effect as large as 10.4 kilohms and indications of chiral edge states. Notably, the magnetization of the sample can be reversed by applying a small d.c. Although the AH resistance is not quantized, and dissipation is present, our measurements suggest that the system may be an incipient Chern insulator.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsFCisLbF&md5=8b0eb08ac0a8e14c71cbbd8ff06e2168
    4. 4
      Tran, K. Evidence for moiré excitons in van der Waals heterostructures. Nature 2019, 567, 71– 75,  DOI: 10.1038/s41586-019-0975-z
      Google Scholar
      4
      Evidence for moire´ excitons in van der Waals heterostructures
      Tran, Kha; Moody, Galan; Wu, Fengcheng; Lu, Xiaobo; Choi, Junho; Kim, Kyounghwan; Rai, Amritesh; Sanchez, Daniel A.; Quan, Jiamin; Singh, Akshay; Embley, Jacob; Zepeda, Andre; Campbell, Marshall; Autry, Travis; Taniguchi, Takashi; Watanabe, Kenji; Lu, Nanshu; Banerjee, Sanjay K.; Silverman, Kevin L.; Kim, Suenne; Tutuc, Emanuel; Yang, Li; MacDonald, Allan H.; Li, Xiaoqin
      Nature (London, United Kingdom) (2019), 567 (7746), 71-75CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Recent advances in the isolation and stacking of monolayers of van der Waals materials have provided approaches for the prepn. of quantum materials in the ultimate two-dimensional limit. In van der Waals heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moire´ superlattice. It is widely recognized that the moire´ superlattice can modulate the electronic band structure of the material and lead to transport properties such as unconventional supercond. and insulating behavior driven by correlations; however, the influence of the moire´ superlattice on optical properties has not been investigated exptl. Here we report the observation of multiple interlayer exciton resonances with either pos. or neg. circularly polarized emission in a molybdenum diselenide/tungsten diselenide (MoSe2/WSe2) heterobilayer with a small twist angle. We attribute these resonances to excitonic ground and excited states confined within the moire´ potential. This interpretation is supported by recombination dynamics and by the dependence of these interlayer exciton resonances on twist angle and temp. These results suggest the feasibility of engineering artificial excitonic crystals using van der Waals heterostructures for nanophotonics and quantum information applications.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXmsVyrtLg%253D&md5=1d6baad81a874a558d08b1303d5970d8
    5. 5
      Wang, L. One-dimensional electrical contact to a two-dimensional material. Science 2013, 342, 614– 617,  DOI: 10.1126/science.1244358
      Google Scholar
      5
      One-Dimensional Electrical Contact to a Two-Dimensional Material
      Wang, L.; Meric, I.; Huang, P. Y.; Gao, Q.; Gao, Y.; Tran, H.; Taniguchi, T.; Watanabe, K.; Campos, L. M.; Muller, D. A.; Guo, J.; Kim, P.; Hone, J.; Shepard, K. L.; Dean, C. R.
      Science (Washington, DC, United States) (2013), 342 (6158), 614-617CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal B nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality elec. contact. Here, the authors report a contact geometry in which the authors metalize only the 1-dimensional edge of a 2-dimensional graphene layer. In addn. to outperforming conventional surface contacts, the edge-contact geometry allows a complete sepn. of the layer assembly and contact metalization processes. In graphene heterostructures, this enables high electronic performance, including low-temp. ballistic transport over distances longer than 15 μm, and room-temp. mobility comparable to the theor. phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2-dimensional materials.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhs1yrs7fJ&md5=de354f7f8a0425d61230545f9afd5e80
    6. 6
      Kim, K. van der Waals heterostructures with high accuracy rotational alignment. Nano Lett. 2016, 16, 1989– 1995,  DOI: 10.1021/acs.nanolett.5b05263
      Google Scholar
      6
      van der Waals Heterostructures with High Accuracy Rotational Alignment
      Kim, Kyounghwan; Yankovitz, Matthew; Fallahazad, Babak; Kang, Sangwoo; Movva, Hema C. P.; Huang, Shengqiang; Larentis, Stefano; Corbet, Chris M.; Taniguchi, Takashi; Watanabe, Kenji; Banerjee, Sanjay K.; LeRoy, Brian J.; Tutuc, Emanuel
      Nano Letters (2016), 16 (3), 1989-1995CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We describe the realization of van der Waals (vdW) heterostructures with accurate rotational alignment of individual layer crystal axes. We illustrate the approach by demonstrating a Bernal-stacked bilayer graphene formed using successive transfers of monolayer graphene flakes. The Raman spectra of this artificial bilayer graphene possess a wide 2D band, which is best fit by four Lorentzians, consistent with Bernal stacking. Scanning tunneling microscopy reveals no moir´e pattern on the artificial bilayer graphene, and tunneling spectroscopy as a function of gate voltage reveals a const. d. of states, also in agreement with Bernal stacking. In addn., electron transport probed in dual-gated samples reveals a band gap opening as a function of transverse elec. field. To illustrate the applicability of this technique to realize vdW heterostructures in which the functionality is critically dependent on rotational alignment, we demonstrate resonant tunneling double bilayer graphene heterostructures sepd. by hexagonal boron-nitride dielec.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XitF2ns7w%253D&md5=28d1713bd561cf80dea282840f96369d
    7. 7
      Geim, A. K.; Grigorieva, I. V. Van der Waals heterostructures. Nature 2013, 499, 419– 425,  DOI: 10.1038/nature12385
      Google Scholar
      7
      Van der Waals heterostructures
      Geim, A. K.; Grigorieva, I. V.
      Nature (London, United Kingdom) (2013), 499 (7459), 419-425CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      A review. Research on graphene and other two-dimensional at. crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated at. planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as van der Waals') have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene's springboard, van der Waals heterostructures should develop into a large field of their own.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtFKnu7rN&md5=58b3fc8bf8d8e656719bfaa23ab0e99b
    8. 8
      Wang, Z.; Wang, Y. B.; Yin, J.; Tovari, E.; Yang, Y.; Lin, L.; Holwill, M.; Birkbeck, J.; Perello, D. J.; Xu, S.; Zultak, J.; Gorbachev, R. V.; Kretinin, A. V.; Taniguchi, T.; Watanabe, K.; Morozov, S. V.; Anđelkovic, M.; Milovanovic, S. P.; Covaci, L.; Peeters, F. M.; Mishchenko, A.; Geim, A. K.; Novoselov, K. S.; Fal’ko, V. I.; Knothe, A.; Woods, C. R. Composite super-moiré lattices in double-aligned graphene heterostructures. Sci. Adv. 2019, 5, eaay8897  DOI: 10.1126/sciadv.aay8897
      Google Scholar
      There is no corresponding record for this reference.
    9. 9
      Park, J. M.; Cao, Y.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P. Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene. Nature 2021, 590, 249– 255,  DOI: 10.1038/s41586-021-03192-0
      Google Scholar
      9
      Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene
      Park, Jeong Min; Cao, Yuan; Watanabe, Kenji; Taniguchi, Takashi; Jarillo-Herrero, Pablo
      Nature (London, United Kingdom) (2021), 590 (7845), 249-255CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Abstr.: Moire´ superlattices1,2 have recently emerged as a platform upon which correlated physics and supercond. can be studied with unprecedented tunability3-6. Although correlated effects have been obsd. in several other moire´ systems7-17, magic-angle twisted bilayer graphene remains the only one in which robust supercond. has been reproducibly measured4-6. Here we realize a moire´ superconductor in magic-angle twisted trilayer graphene (MATTG)18, which has better tunability of its electronic structure and superconducting properties than magic-angle twisted bilayer graphene. Measurements of the Hall effect and quantum oscillations as a function of d. and elec. field enable us to det. the tunable phase boundaries of the system in the normal metallic state. Zero-magnetic-field resistivity measurements reveal that the existence of supercond. is intimately connected to the broken-symmetry phase that emerges from two carriers per moire´ unit cell. We find that the superconducting phase is suppressed and bounded at the Van Hove singularities that partially surround the broken-symmetry phase, which is difficult to reconcile with weak-coupling BCS theory. Moreover, the extensive in situ tunability of our system allows us to reach the ultrastrong-coupling regime, characterized by a Ginzburg-Landau coherence length that reaches the av. inter-particle distance, and very large TBKT/TF values, in excess of 0.1 (where TBKT and TF are the Berezinskii-Kosterlitz-Thouless transition and Fermi temps., resp.). These observations suggest that MATTG can be elec. tuned close to the crossover to a two-dimensional Bose-Einstein condensate. Our results establish a family of tunable moire´ superconductors that have the potential to revolutionize our fundamental understanding of and the applications for strongly coupled supercond.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXis1Ont7w%253D&md5=89912784d6b6cdd3421cffc5251bc484
    10. 10
      Hao, Z. Electric field–tunable superconductivity in alternating-twist magic-angle trilayer graphene. Science 2021, 371, 1133– 1138,  DOI: 10.1126/science.abg0399
      Google Scholar
      10
      Electric field-tunable superconductivity in alternating-twist magic-angle trilayer graphene
      Hao, Zeyu; Zimmerman, A. M.; Ledwith, Patrick; Khalaf, Eslam; Najafabadi, Danial Haie; Watanabe, Kenji; Taniguchi, Takashi; Vishwanath, Ashvin; Kim, Philip
      Science (Washington, DC, United States) (2021), 371 (6534), 1133-1138CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      Engineering moire superlattices by twisting layers in van der Waals (vdW) heterostructures has uncovered a wide array of quantum phenomena. We constructed a vdW heterostructure that consists of three graphene layers stacked with alternating twist angles ±θ. At the av. twist angle θ ∼ 1.56°, a theor. predicted "magic angle" for the formation of flat electron bands, we obsd. displacement field-tunable supercond. with a max. crit. temp. of 2.1 K. By tuning the doping level and displacement field, we found that superconducting regimes occur in conjunction with flavor polarization of moire bands and are bounded by a van Hove singularity (vHS) at high displacement fields. Our findings display inconsistencies with a weak coupling description, suggesting that the obsd. moire supercond. has an unconventional nature.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXmsVOlu78%253D&md5=877d0a2958ec738d6024179ea7b90400
    11. 11
      Carr, S.; Massatt, D.; Fang, S.; Cazeaux, P.; Luskin, M.; Kaxiras, E. Twistronics: Manipulating the electronic properties of two-dimensional layered structures through their twist angle. Phys. Rev. B 2017, 95, 075420  DOI: 10.1103/PhysRevB.95.075420
      Google Scholar
      11
      Twistronics: manipulating the electronic properties of two-dimensional layered structures through their twist angle
      Carr, Stephen; Massatt, Daniel; Fang, Shiang; Cazeaux, Paul; Luskin, Mitchell; Kaxiras, Efthimios
      Physical Review B (2017), 95 (7), 075420/1-075420/6CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      The ability in expts. to control the relative twist angle between successive layers in two-dimensional (2D) materials offers an approach to manipulating their electronic properties; we refer to this approach as "twistronics." A major challenge to theory is that, for arbitrary twist angles, the resulting structure involves incommensurate (aperiodic) 2D lattices. Here, we present a general method for the calcn. of the electronic d. of states of aperiodic 2D layered materials, using parameter-free Hamiltonians derived from ab initio d.-functional theory. We use graphene, a semimetal, and MoS2, a representative of the transition-metal dichalcogenide family of 2D semiconductors, to illustrate the application of our method, which enables fast and efficient simulation of multilayered stacks in the presence of local disorder and external fields. We comment on the interesting features of their d. of states as a function of twist angle and local configuration and on how these features can be exptl. obsd.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsVeqt7vN&md5=41b65e533c5ac799d2eeb29de1ada5d5
    12. 12
      Ribeiro-Palau, R.; Zhang, C.; Watanabe, K.; Taniguchi, T.; Hone, J.; Dean, C. R. Twistable electronics with dynamically rotatable heterostructures. Science 2018, 361, 690– 693,  DOI: 10.1126/science.aat6981
      Google Scholar
      12
      Twistable electronics with dynamically rotatable heterostructures
      Ribeiro-Palau, Rebeca; Zhang, Changjian; Watanabe, Kenji; Taniguchi, Takashi; Hone, James; Dean, Cory R.
      Science (Washington, DC, United States) (2018), 361 (6403), 690-693CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      In heterostructures of two-dimensional materials, electronic properties can vary dramatically with relative interlayer angle. This effect makes it theor. possible to realize a new class of twistable electronics in which properties can be manipulated on demand by means of rotation. We demonstrate a device architecture in which a layered heterostructure can be dynamically twisted in situ. We study graphene encapsulated by boron nitride, where, at small rotation angles, the device characteristics are dominated by coupling to a long-wavelength moire superlattice. The ability to investigate arbitrary rotation angle in a single device reveals features of the optical, mech., and electronic response in this system not captured in static rotation studies. Our results establish the capability to fabricate twistable electronic devices with dynamically tunable properties.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsFagt7zK&md5=e65e7ec302bc04f0691f5b04c7b484c4
    13. 13
      Yang, Y.; Li, J.; Yin, J.; Xu, S.; Mullan, C.; Taniguchi, T.; Watanabe, K.; Geim, A. K.; Novoselov, K. S.; Mishchenko, A. In situ manipulation of van der Waals heterostructures for twistronics. Sci. Adv. 2020, 6, eabd3655  DOI: 10.1126/sciadv.abd3655
      Google Scholar
      There is no corresponding record for this reference.
    14. 14
      Polini, M.; et al. Materials and devices for fundamental quantum science and quantum technologies. arXiv 2201.09260, 2022, accessed 2022-06-27.
      Google Scholar
      There is no corresponding record for this reference.
    15. 15
      Seifert, P. Magic-Angle Bilayer Graphene Nanocalorimeters: Toward Broadband, Energy-Resolving Single Photon Detection. Nano Lett. 2020, 20, 3459– 3464,  DOI: 10.1021/acs.nanolett.0c00373
      Google Scholar
      15
      Magic-Angle Bilayer Graphene Nanocalorimeters: Toward Broadband, Energy-Resolving Single Photon Detection
      Seifert, Paul; Lu, Xiaobo; Stepanov, Petr; Duran Retamal, Jose Ramon; Moore, John N.; Fong, Kin-Chung; Principi, Alessandro; Efetov, Dmitri K.
      Nano Letters (2020), 20 (5), 3459-3464CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Because of the ultralow photon energies at mid-IR and terahertz frequencies, in these bands photodetectors are notoriously underdeveloped, and broadband single photon detectors (SPDs) are nonexistent. Advanced SPDs exploit thermal effects in nanostructured superconductors, and their performance is currently limited to the more energetic near-IR photons due to their high electronic heat capacity. Here, we demonstrate a superconducting magic-angle bilayer graphene (MAG) device that is theor. capable of detecting single photons of ultralow energies by utilizing its record-low heat capacity and sharp superconducting transition. We theor. quantify its calorimetric photoresponse and est. its detection limits. This device allows the detection of ultrabroad range single photons from the visible to sub-terahertz with a response time around 4 ns and energy resoln. better than 1 THz. These attributes position MAG as an exceptional material for long-wavelength single photon sensing, which could revolutionize such disparate fields as quantum information processing and radio astronomy.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXns1eqsrk%253D&md5=fa69f8004e08829303a769e097f832b9
    16. 16
      Rodan-Legrain, D. Highly tunable junctions and non-local Josephson effect in magic-angle graphene tunnelling devices. Nat. Nanotechnol. 2021, 16, 769– 775,  DOI: 10.1038/s41565-021-00894-4
      Google Scholar
      16
      Highly tunable junctions and non-local Josephson effect in magic-angle graphene tunnelling devices
      Rodan-Legrain, Daniel; Cao, Yuan; Park, Jeong Min; de la Barrera, Sergio C.; Randeria, Mallika T.; Watanabe, Kenji; Taniguchi, Takashi; Jarillo-Herrero, Pablo
      Nature Nanotechnology (2021), 16 (7), 769-775CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
      Magic-angle twisted bilayer graphene (MATBG) has recently emerged as a highly tunable two-dimensional material platform exhibiting a wide range of phases, such as metal, insulator and superconductor states. Local electrostatic control over these phases may enable the creation of versatile quantum devices that were previously not achievable in other single-material platforms. Here we engineer Josephson junctions and tunnelling transistors in MATBG, solely defined by electrostatic gates. Our multi-gated device geometry offers independent control of the weak link, barriers and tunnelling electrodes. These purely two-dimensional MATBG Josephson junctions exhibit non-local electrodynamics in a magnetic field, in agreement with the Pearl theory for ultrathin superconductors. Utilizing the intrinsic bandgaps of MATBG, we also demonstrate monolithic edge tunnelling spectroscopy within the same MATBG devices and measure the energy spectrum of MATBG in the superconducting phase. Furthermore, by inducing a double-barrier geometry, the devices can be operated as a single-electron transistor, exhibiting Coulomb blockade. With versatile functionality encompassed within a single material, these MATBG tunnelling devices may find applications in graphene-based tunable superconducting qubits, on-chip superconducting circuits and electromagnetic sensing.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhtVensbnL&md5=7d44a3a8935df7e2d66f59e28477079c
    17. 17
      de Vries, F. K. Gate-defined Josephson junctions in magic-angle twisted bilayer graphene. Nat. Nanotechnol. 2021, 16, 760– 763,  DOI: 10.1038/s41565-021-00896-2
      Google Scholar
      17
      Gate-defined Josephson junctions in magic-angle twisted bilayer graphene
      de Vries, Folkert K.; Portoles, Elias; Zheng, Giulia; Taniguchi, Takashi; Watanabe, Kenji; Ihn, Thomas; Ensslin, Klaus; Rickhaus, Peter
      Nature Nanotechnology (2021), 16 (7), 760-763CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
      In situ electrostatic control of two-dimensional supercond.1 is commonly limited due to large charge carrier densities, and gate-defined Josephson junctions are therefore rare 2,3. Magic-angle twisted bilayer graphene (MATBG)4-8 has recently emerged as a versatile platform that combines metallic, superconducting, magnetic and insulating phases in a single crystal9-14. Although MATBG appears to be an ideal two-dimensional platform for gate-tunable supercond.9,11,13, progress towards practical implementations has been hindered by the need for well-defined gated regions. Here we use multilayer gate technol. to create a device based on two distinct phases in adjustable regions of MATBG. We electrostatically define the superconducting and insulating regions of a Josephson junction and observe tunable d.c. and a.c. Josephson effects15,16. The ability to tune the superconducting state within a single material circumvents interface and fabrication challenges, which are common in multimaterial nanostructures. This work is an initial step towards devices where gate-defined correlated states are connected in single-crystal nanostructures. We envision applications in superconducting electronics17,18 and quantum information technol.19,20.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhtVensbjE&md5=a00230b4d00caff23ba0c88083d08ee7
    18. 18
      Diez-Merida, J.; et al. Magnetic Josephson Junctions and Superconducting Diodes in Magic Angle Twisted Bilayer Graphene. arXiv 2110.01067, 2021, accessed 2022-06-27.
      Google Scholar
      There is no corresponding record for this reference.
    19. 19
      Portolés, E.; et al. A Tunable Monolithic SQUID in Twisted Bilayer Graphene. arXiv 2201.13276, 2022, accessed 2022-06-27.
      Google Scholar
      There is no corresponding record for this reference.
    20. 20
      Backes, C. Production and processing of graphene and related materials. 2D Mater. 2020, 7, 022001  DOI: 10.1088/2053-1583/ab1e0a
      Google Scholar
      20
      Production and processing of graphene and related materials
      Backes, Claudia; Abdelkader, Amr M.; Alonso, Concepcion; Andrieux-Ledier, Amandine; Arenal, Raul; Azpeitia, Jon; Balakrishnan, Nilanthy; Banszerus, Luca; Barjon, Julien; Bartali, Ruben; Bellani, Sebastiano; Berger, Claire; Berger, Reinhard; Ortega, M. M. Bernal; Bernard, Carlo; Beton, Peter H.; Beyer, Andre; Bianco, Alberto; Boeggild, Peter B.; Bonaccorso, Francesco; Barin, Gabriela Borin; Botas, Cristina; Bueno, Rebeca A.; Carriazo, Daniel; Gomez, Andres Castellanos; Christian, Meganne; Ciesielski, Artur; Ciuk, Tymoteusz; Cole, Matthew T.; Coleman, Jonathan; Coletti, Camilla; Crema, Luigi; Cun, Huanyao; Dasler, Daniela; De Fazio, Domenico; Diez, Noel; Drieschner, Simon; Duesberg, Georg S.; Fasel, Roman; Feng, Xinliang; Fina, Alberto; Forti, Stiven; Galiotis, Costas; Garberoglio, Giovanni; Garcia, Jorge M.; Garrido, Jose Antonio; Gibertini, Marco; Goelzhaeuser, Armin; Gomez, Julio; Greber, Thomas; Hauke, Frank; Hemmi, Adrian; Hernandez-Rodriguez, Irene; Hirsch, Andreas; Hodge, Stephen A.; Huttel, Yves; Jepsen, Peter U.; Jimenez, Ignacio; Kaiser, Ute; Kaplas, Tommi; Kim, Hokwon; Kis, Andras; Papagelis, Konstantinos; Kostarelos, Kostas; Krajewska, Aleksandra; Lee, Kangho; Li, Changfeng; Lipsanen, Harri; Liscio, Andrea; Lohe, Martin R.; Loiseau, Annick; Lombardi, Lucia; Lopez, Maria Francisca; Martin, Oliver; Martin, Cristina; Martinez, Lidia; Martin-Gago, Joseangel; Martinez, Jose Ignacio; Marzari, Nicola; Mayoral, Alvaro; Mcmanus, John; Melucci, Manuela; Mendez, Javier; Merino, Cesar; Merino, Pablo; Meyer, Andreas P.; Miniussi, Elisa; Miseikis, Vaidotas; Mishra, Neeraj; Morandi, Vittorio; Munuera, Carmen; Munoz, Roberto; Nolan, Hugo; Ortolani, Luca; Ott, Annak; Palacio, Irene; Palermo, Vincenzo; Parthenios, John; Pasternak, Iwona; Patane, Amalia; Prato, Maurizio; Prevost, Henri; Prudkovskiy, Vladimir; Pugno, Nicola; Rojo, Teofilo; Rossi, Antonio; Ruffieux, Pascal; Samori, Paolo; Schue, Leonard; Setijadi, Eki; Seyller, Thomas; Speranza, Giorgio; Stampfer, Christoph; Stenger, Ingrid; Strupinski, Wlodek; Svirko, Yuri; Taioli, Simone; Bkteo, Kenneth; Testi, Matteo; Tomarchio, Flavia; Tortello, Mauro; Treossi, Emanuele; Turchanin, Andrey; Vazquez, Ester; Villaro, Elvira; Whelan, Patrick R.; Xia, Zhenyuan; Yakimova, Rositza; Yang, Sheng; Yazdi, G. Reza; Yim, Chanyoung; Yoon, Duhee; Zhang, Xianghui; Zhuang, Xiaodong; Colombo, Luigi; Ferrari, Andrea C.; Garcia-Hernandez, Mar
      2D Materials (2020), 7 (2), 022001CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      We present an overview of the main techniques for prodn. and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitFCiu77E&md5=d1231e8040840587aab88ec261c48a19
    21. 21
      Banszerus, L.; Schmitz, M.; Engels, S.; Dauber, J.; Oellers, M.; Haupt, F.; Watanabe, K.; Taniguchi, T.; Beschoten, B.; Stampfer, C. Ultrahigh-mobility graphene devices from chemical vapor deposition on reusable copper. Sci. Adv. 2015, 1, e1500222  DOI: 10.1126/sciadv.1500222
      Google Scholar
      21
      Ultrahigh-mobility graphene devices from chemical vapor deposition on reusable copper
      Banszerus, Luca; Schmitz, Michael; Engels, Stephan; Dauber, Jan; Oellers, Martin; Haupt, Federica; Watanabe, Kenji; Taniguchi, Takashi; Beschoten, Bernd; Stampfer, Christoph
      Science Advances (2015), 1 (6), e1500222/1-e1500222/6CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
      Graphene research has prospered impressively in the past few years, and promising applications such as high frequency transistors, magnetic field sensors, and flexible optoelectronics are just waiting for a scalable and cost efficient fabrication technol. to produce high-mobility graphene. Although significant progress has been made in chem. vapor deposition (CVD) and epitaxial growth of graphene, the carrier mobility obtained with these techniques is still significantly lower than what is achieved using exfoliated graphene. We show that the quality of CVD-grown graphene depends critically on the used transfer process, and we report on an advanced transfer technique that allows both reusing the copper substrate of the CVD growth and making devices with mobilities as high as 350,000 cm2 V-1 s-1, thus rivaling exfoliated graphene.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XnsFGhsb4%253D&md5=afccf1cc5730fcf64f1861aeaf40bac9
    22. 22
      De Fazio, D. High-mobility, wet-transferred graphene grown by chemical vapour deposition. ACS Nano 2019, 13, 8926– 8935,  DOI: 10.1021/acsnano.9b02621
      Google Scholar
      22
      High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition
      De Fazio, Domenico; Purdie, David G.; Ott, Anna K.; Braeuninger-Weimer, Philipp; Khodkov, Timofiy; Goossens, Stijn; Taniguchi, Takashi; Watanabe, Kenji; Livreri, Patrizia; Koppens, Frank H. L.; Hofmann, Stephan; Goykhman, Ilya; Ferrari, Andrea C.; Lombardo, Antonio
      ACS Nano (2019), 13 (8), 8926-8935CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      We report high room-temp. mobility in single-layer graphene grown by chem. vapor deposition (CVD) after wet transfer on SiO2 and hexagonal boron nitride (hBN) encapsulation. By removing contaminations, trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to ∼70000 cm2 V-1 s-1 at room temp. and ∼120 000 cm2 V-1 s-1 at 9K. These are more than twice those of previous wet-transferred graphene and comparable to samples prepd. by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycryst. graphene, achieving room-temp. mobilities of ∼30 000 cm2 V-1 s-1. These results show that, with appropriate encapsulation and cleaning, room-temp. mobilities well above 10 000 cm2 V-1 s-1 can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsVSqsbnM&md5=9c06d8950fc9402af9d63dfc1a520686
    23. 23
      Pezzini, S. High-quality electrical transport using scalable CVD graphene. 2D Mater. 2020, 7, 041003  DOI: 10.1088/2053-1583/aba645
      Google Scholar
      23
      High-quality electrical transport using scalable CVD graphene
      Pezzini, Sergio; Miseikis, Vaidotas; Pace, Simona; Rossella, Francesco; Watanabe, Kenji; Taniguchi, Takashi; Coletti, Camilla
      2D Materials (2020), 7 (4), 041003CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      Producing and manipulating graphene on fab-compatible scale, while maintaining its remarkable carrier mobility, is key to finalize its technol. application. We show that a large-scale approach (chem. vapor deposition on Cu followed by polymer-mediated semi-dry transfer) yields single-layer graphene crystals fully comparable, in terms of electronic transport, to micro-mech. exfoliated flakes. Hexagonal boron nitride is used to encapsulate the graphene crystals-without taking part to their detachment from the growth catalyst-and study their intrinsic properties in field-effect devices. At room temp., the electron-phonon coupling sets the mobility to ∼ 1.3 x 105 cm2 V-1 s-1 at ∼ 1011 cm-2 concn. At T = 4.2 K, the mobility (>6 x 105 cm2 V-1 s-1 at ∼ 1011 cm-2) is limited by the devices' phys. edges, and charge fluctuations < 7 x 109 cm-2 are detected. Under perpendicular magnetic fields, we observe early onset of Landau quantization (B ∼ 50 mT) and signatures of electronic correlation, including the fractional quantum Hall effect.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXit1Sqs7rJ&md5=a9690c10926630e0f589491316bf3035
    24. 24
      Schmitz, M. Fractional quantum Hall effect in CVD-grown graphene. 2D Mater. 2020, 7, 041007  DOI: 10.1088/2053-1583/abae7b
      Google Scholar
      24
      Fractional quantum Hall effect in CVD-grown graphene
      Schmitz, M.; Ouaj, T.; Winter, Z.; Rubi, K.; Watanabe, K.; Taniguchi, T.; Zeitler, U.; Beschoten, B.; Stampfer, C.
      2D Materials (2020), 7 (4), 041007CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      A review. We show the emergence of fractional quantum Hall states in graphene grown by chem. vapor deposition (CVD) for magnetic fields from below 3 T to 35 T where the CVD-graphene was dry-transferred. Effective composite-fermion filling factors up to ν* = 4 are visible and higher order composite-fermion states (with four flux quanta attached) start to emerge at the highest fields. Our results show that the quantum mobility of CVD-grown graphene is comparable to that of exfoliated graphene and, more specifically, that the p/3 fractional quantum Hall states have energy gaps of up to 30 K, well comparable to those obsd. in other silicon-gated devices based on exfoliated graphene.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXit1Sqs73N&md5=a873725d115bc9f1fc490a278ea3f841
    25. 25
      Bistritzer, R.; MacDonald, A. H. Moiré bands in twisted double-layer graphene. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 12233– 12237,  DOI: 10.1073/pnas.1108174108
      Google Scholar
      25
      Moire bands in twisted double-layer graphene
      Bistritzer, Rafi; MacDonald, Allan H.
      Proceedings of the National Academy of Sciences of the United States of America (2011), 108 (30), 12233-12237CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      A moire pattern is formed when two copies of a periodic pattern are overlaid with a relative twist. We address the electronic structure of a twisted two-layer graphene system, showing that in its continuum Dirac model the moire pattern periodicity leads to moire Bloch bands. The two layers become more strongly coupled and the Dirac velocity crosses zero several times as the twist angle is reduced. For a discrete set of magic angles the velocity vanishes, the lowest moire band flattens, and the Dirac-point d.-of-states and the counterflow cond. are strongly enhanced.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXpvFagtLs%253D&md5=11c357756459c64b402f461c4c40c9d4
    26. 26
      Moon, P.; Koshino, M. Energy spectrum and quantum Hall effect in twisted bilayer graphene. Phys. Rev. B 2012, 85, 195458,  DOI: 10.1103/PhysRevB.85.195458
      Google Scholar
      26
      Energy spectrum and quantum Hall effect in twisted bilayer graphene
      Moon, Pilkyung; Koshino, Mikito
      Physical Review B: Condensed Matter and Materials Physics (2012), 85 (19), 195458/1-195458/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We investigate the electronic structure and the quantum Hall effect in twisted bilayer graphenes with various rotation angles in the presence of magnetic field. Using a low-energy approxn., which incorporates the rigorous interlayer interaction, we computed the energy spectrum and the quantized Hall cond. in a wide range of magnetic field from the semiclassical regime to the fractal spectrum regime. In weak magnetic fields, the low-energy conduction band is quantized into electronlike and holelike Landau levels at energies below and above the van Hove singularity, resp., and the Hall cond. sharply drops from pos. to neg. when the Fermi energy goes through the transition point. In increasing magnetic field, the spectrum gradually evolves into a fractal band structure called Hofstadter's butterfly, where the Hall cond. exhibits a nonmonotonic behavior as a function of Fermi energy. The typical electron d. and magnetic field amplitude characterizing the spectrum monotonically decrease as the rotation angle is reduced, indicating that the rich electronic structure may be obsd. in a moderate condition.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVWjurfF&md5=ff97101c64be6ff120a173d42308953c
    27. 27
      Sanchez-Yamagishi, J. D.; Taychatanapat, T.; Watanabe, K.; Taniguchi, T.; Yacoby, A.; Jarillo-Herrero, P. Quantum Hall Effect, Screening, and Layer-Polarized Insulating States in Twisted Bilayer Graphene. Phys. Rev. Lett. 2012, 108, 076601  DOI: 10.1103/PhysRevLett.108.076601
      Google Scholar
      27
      Quantum Hall effect, screening, and layer-polarized insulating states in twisted bilayer graphene
      Sanchez-Yamagishi, Javier D.; Taychatanapat, Thiti; Watanabe, Kenji; Taniguchi, Takashi; Yacoby, Amir; Jarillo-Herrero, Pablo
      Physical Review Letters (2012), 108 (7), 076601/1-076601/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      We investigate electronic transport in dual-gated twisted-bilayer graphene. Despite the subnanometer proximity between the layers, we identify independent contributions to the magnetoresistance from the graphene Landau level spectrum of each layer. We demonstrate that the filling factor of each layer can be independently controlled via the dual gates, which we use to induce Landau level crossings between the layers. By analyzing the gate dependence of the Landau level crossings, we characterize the finite interlayer screening and ext. the capacitance between the atomically spaced layers. At zero filling factor, we observe an insulating state at large displacement fields, which can be explained by the presence of counterpropagating edge states with interlayer coupling.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XjvVejsLY%253D&md5=7ac0deb65221b851a9b771f008127b17
    28. 28
      Sanchez-Yamagishi, J. D. Helical edge states and fractional quantum Hall effect in a graphene electron–hole bilayer. Nat. Nanotechnol. 2017, 12, 118– 122,  DOI: 10.1038/nnano.2016.214
      Google Scholar
      28
      Helical edge states and fractional quantum Hall effect in a graphene electron-hole bilayer
      Sanchez-Yamagishi, Javier D.; Luo, Jason Y.; Young, Andrea F.; Hunt, Benjamin M.; Watanabe, Kenji; Taniguchi, Takashi; Ashoori, Raymond C.; Jarillo-Herrero, Pablo
      Nature Nanotechnology (2017), 12 (2), 118-122CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Helical 1D electronic systems are a promising route towards realizing circuits of topol. quantum states that exhibit non-Abelian statistics. Here, we demonstrate a versatile platform to realize 1D systems made by combining quantum Hall (QH) edge states of opposite chiralities in a graphene electron-hole bilayer at moderate magnetic fields. Using this approach, we engineer helical 1D edge conductors where the counter-propagating modes are localized in sep. electron and hole layers by a tunable elec. field. These helical conductors exhibit strong non-local transport signals and suppressed backscattering due to the opposite spin polarizations of the counter-propagating modes. Unlike other approaches used for realizing helical states, the graphene electron-hole bilayer can be used to build new 1D systems incorporating fractional edge states. Indeed, we are able to tune the bilayer devices into a regime hosting fractional and integer edge states of opposite chiralities, paving the way towards 1D helical conductors with fractional quantum statistics.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhsl2hsb7O&md5=7d6a6d3cf5ff5338b01550333e5dd39d
    29. 29
      Rickhaus, P.; Liu, M.-H.; Kurpas, M.; Kurzmann, A.; Lee, Y.; Overweg, H.; Eich, M.; Pisoni, R.; Taniguchi, T.; Watanabe, K.; Richter, K.; Ensslin, K.; Ihn, T. The electronic thickness of graphene. Sci. Adv. 2020, 6, eaay8409  DOI: 10.1126/sciadv.aay8409
      Google Scholar
      There is no corresponding record for this reference.
    30. 30
      Luican, A. Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers. Phys. Rev. Lett. 2011, 106, 126802,  DOI: 10.1103/PhysRevLett.106.126802
      Google Scholar
      30
      Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers
      Luican, A.; Li, Guohong; Reina, A.; Kong, J.; Nair, R. R.; Novoselov, K. S.; Geim, A. K.; Andrei, E. Y.
      Physical Review Letters (2011), 106 (12), 126802/1-126802/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chem. vapor deposition. For twist angles exceeding ∼3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is obsd. which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXkt1artLc%253D&md5=c452f1a55946efd7ab283c4cf3642124
    31. 31
      Yan, Z. Large Hexagonal Bi-and Trilayer Graphene Single Crystals with Varied Interlayer Rotations. Angew. Chemie Int. Ed. 2014, 53, 1565– 1569,  DOI: 10.1002/anie.201306317
      Google Scholar
      31
      Large hexagonal bi- and trilayer graphene single crystals with varied interlayer rotations
      Yan, Zheng; Liu, Yuanyue; Ju, Long; Peng, Zhiwei; Lin, Jian; Wang, Gunuk; Zhou, Haiqing; Xiang, Changsheng; Samuel, E. L. G.; Kittrell, Carter; Artyukhov, Vasilii I.; Wang, Feng; Yakobson, Boris I.; Tour, James M.
      Angewandte Chemie, International Edition (2014), 53 (6), 1565-1569CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Bi- and trilayer graphene have attracted intensive interest due to their rich electronic and optical properties, which are dependent on interlayer rotations. However, the prepn. of high-quality large-size bi- and trilayer graphene single crystals still remains a challenge. Here, the prepn. of 100 μm pyramid-like hexagonal bi- and trilayer graphene single-crystal domains on Cu foils using chem. vapor deposition is reported. The as-produced graphene domains show almost exclusively either 0° or 30° interlayer rotations. Raman spectroscopy, transmission electron microscopy, and Fourier-transformed IR spectroscopy were used to demonstrate that bilayer graphene domains with 0° interlayer stacking angles were Bernal stacked. Based on first-principle calcns., it is proposed that rotations originate from the graphene nucleation at the Cu step, which explains the origin of the interlayer rotations and agrees well with the exptl. observations.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVygurs%253D&md5=4dfcaae423d30e1d08976d2f187b0617
    32. 32
      Gao, Z. Crystalline bilayer graphene with preferential stacking from Ni–Cu gradient alloy. ACS Nano 2018, 12, 2275– 2282,  DOI: 10.1021/acsnano.7b06992
      Google Scholar
      32
      Crystalline bilayer graphene with preferential stacking from Ni-Cu gradient alloy
      Gao, Zhaoli; Zhang, Qicheng; Naylor, Carl H.; Kim, Youngkuk; Abidi, Irfan Haider; Ping, Jinglei; Ducos, Pedro; Zauberman, Jonathan; Zhao, Meng-Qiang; Rappe, Andrew M.; Luo, Zhengtang; Ren, Li; Johnson, Alan T. Charlie
      ACS Nano (2018), 12 (3), 2275-2282CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The authors developed a high-yield synthesis of highly cryst. bilayer graphene (BLG) with two preferential stacking modes using a Ni-Cu gradient alloy growth substrate. Previously reported approaches for BLG growth include flat growth substrates of Cu or Ni-Cu uniform alloys and "copper pocket" structures. Use of flat substrates has the advantage of being scalable, but the growth mechanism is either "surface limited" (for Cu) or carbon pptn. (for uniform Ni-Cu), which results in multicryst. BLG grains. For copper pockets, growth proceeds through a carbon back-diffusion mechanism, which leads to the formation of highly cryst. BLG, but scaling of the copper pocket structure is expected to be difficult. Here, the authors demonstrate a Ni-Cu gradient alloy that combines the advantages of these earlier methods: the substrate is flat, so easy to scale, while growth proceeds by a carbon back-diffusion mechanism leading to high-yield growth of BLG with high crystallinity. The BLG layer stacking was almost exclusively Bernal or twisted with an angle of 30°, consistent with first-principles calcns. the authors conducted. Furthermore, we demonstrated scalable prodn. of transistor arrays based cryst. Bernal-stacked BLG with a band gap that was tunable at room temp.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXjvFGmsLc%253D&md5=138f16fadb7cae462a47dae52259b827
    33. 33
      Deng, B. Interlayer Decoupling in 30° Twisted Bilayer Graphene Quasicrystal. ACS Nano 2020, 14, 1656– 1664,  DOI: 10.1021/acsnano.9b07091
      Google Scholar
      33
      Interlayer decoupling in 30° twisted bilayer graphene quasicrystal
      Deng, Bing; Wang, Binbin; Li, Ning; Li, Rongtan; Wang, Yani; Tang, Jilin; Fu, Qiang; Tian, Zhen; Gao, Peng; Xue, Jiamin; Peng, Hailin
      ACS Nano (2020), 14 (2), 1656-1664CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Stacking order has a strong influence on the coupling between the two layers of twisted bilayer graphene (BLG), which in turn dets. its phys. properties. Here, we report the investigation of the interlayer coupling of the epitaxially grown single-crystal 30°-twisted BLG on Cu(111) at the at. scale. The stacking order and morphol. of BLG is controlled by a rationally designed two-step growth process, i.e., the thermodynamically controlled nucleation and kinetically controlled growth. The crystal structure of the 30°-twisted bilayer graphene (30°-tBLG) is detd. to have quasicrystal-like symmetry. The electronic properties and interlayer coupling of the 30°-tBLG are investigated using scanning tunneling microscopy and spectroscopy. The energy-dependent local d. of states with in situ electrostatic doping shows that the electronic states in two graphene layers are decoupled near the Dirac point. A linear dispersion originated from the constituent graphene monolayers is discovered with doubled degeneracy. This study contributes to controlled growth of twist-angle-defined BLG and provides insights on the electronic properties and interlayer coupling in this intriguing system.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhtlWhtb0%253D&md5=b9c97ccc9d9faf254183acfbae5ed32f
    34. 34
      Pezzini, S. 30°-Twisted Bilayer Graphene Quasicrystals From Chemical Vapor Deposition. Nano Lett. 2020, 20, 3313– 3319,  DOI: 10.1021/acs.nanolett.0c00172
      Google Scholar
      34
      30°-Twisted bilayer graphene quasicrystals from chemical vapor deposition
      Pezzini, Sergio; Miseikis, Vaidotas; Piccinini, Giulia; Forti, Stiven; Pace, Simona; Engelke, Rebecca; Rossella, Francesco; Watanabe, Kenji; Taniguchi, Takashi; Kim, Philip; Coletti, Camilla
      Nano Letters (2020), 20 (5), 3313-3319CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The artificial stacking of atomically thin crystals suffers from intrinsic limitations in terms of control and reproducibility of the relative orientation of exfoliated flakes. This drawback is particularly severe when the properties of the system critically depends on the twist angle, as in the case of the dodecagonal quasicrystal formed by two graphene layers rotated by 30°. Here we show that large-area 30°-rotated bilayer graphene can be grown deterministically by chem. vapor deposition on Cu, eliminating the need of artificial assembly. The quasicrystals are easily transferred to arbitrary substrates and integrated in high-quality hexagonal boron nitride-encapsulated heterostructures, which we process into dual-gated devices exhibiting carrier mobility up to 105 cm2/(V s). From low-temp. magnetotransport, we find that the graphene quasicrystals effectively behave as uncoupled graphene layers, showing 8-fold degenerate quantum Hall states. This result indicates that the Dirac cones replica detected by previous photoemission expts. do not contribute to the elec. transport.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXnt1Sqs7o%253D&md5=75952a93c94935d7385e199d2ba6f313
    35. 35
      Sun, Y.; Polani, S.; Luo, F.; Ott, S.; Strasser, P.; Dionigi, F. Hetero-site nucleation for growing twisted bilayer graphene with a wide range of twist angles. Nat. Commun. 2021, 12, 2391,  DOI: 10.1038/s41467-021-25911-x
      Google Scholar
      35
      Hetero-site nucleation for growing twisted bilayer graphene with a wide range of twist angles
      Sun, Luzhao; Wang, Zihao; Wang, Yuechen; Zhao, Liang; Li, Yanglizhi; Chen, Buhang; Huang, Shenghong; Zhang, Shishu; Wang, Wendong; Pei, Ding; Fang, Hongwei; Zhong, Shan; Liu, Haiyang; Zhang, Jincan; Tong, Lianming; Chen, Yulin; Li, Zhenyu; Rummeli, Mark H.; Novoselov, Kostya S.; Peng, Hailin; Lin, Li; Liu, Zhongfan
      Nature Communications (2021), 12 (1), 2391CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Twisted bilayer graphene (tBLG) has recently attracted growing interest due to its unique twist-angle-dependent electronic properties. The prepn. of high-quality large-area bilayer graphene with rich rotation angles would be important for the investigation of angle-dependent physics and applications, which, however, is still challenging. Here, we demonstrate a chem. vapor deposition (CVD) approach for growing high-quality tBLG using a hetero-site nucleation strategy, which enables the nucleation of the second layer at a different site from that of the first layer. The fraction of tBLGs in bilayer graphene domains with twist angles ranging from 0° to 30° was found to be improved to 88%, which is significantly higher than those reported previously. The hetero-site nucleation behavior was carefully investigated using an isotope-labeling technique. Furthermore, the clear Moire patterns and ultrahigh room-temp. carrier mobility of 68,000 cm2 V-1 s-1 confirmed the high cryst. quality of our tBLG. Our study opens an avenue for the controllable growth of tBLGs for both fundamental research and practical applications.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXpsVOgur0%253D&md5=a79ccfb5ea76a338671b92dceb0c3da5
    36. 36
      Lu, C.-C.; Lin, Y.-C.; Liu, Z.; Yeh, C.-H.; Suenaga, K.; Chiu, P.-W. Twisting Bilayer Graphene Superlattices. ACS Nano 2013, 7, 2587– 2594,  DOI: 10.1021/nn3059828
      Google Scholar
      36
      Twisting Bilayer Graphene Superlattices
      Lu, Chun-Chieh; Lin, Yung-Chang; Liu, Zheng; Yeh, Chao-Hui; Suenaga, Kazu; Chiu, Po-Wen
      ACS Nano (2013), 7 (3), 2587-2594CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Bilayer graphene is an intriguing material in that its electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of low-dimensional carbon system. Twisted bilayer graphene can be obtained by (i) thermal decompn. of SiC; (ii) CVD on metal catalysts; (iii) folding graphene; or (iv) stacking graphene layers one atop the other, the latter of which suffers from interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycryst. structures. The present study studies bilayer graphene grown by ambient pressure CVD on polycryst. Cu. Controlling the nucleation in early stage growth allows the constituent layers to form single hexagonal crystals. New Raman active modes result from the twist, with the angle detd. by TEM. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXjt1Wlsb8%253D&md5=fc7e5afd6cb530217d653070c5325582
    37. 37
      Ohta, T.; Robinson, J. T.; Feibelman, P. J.; Bostwick, A.; Rotenberg, E.; Beechem, T. E. Evidence for Interlayer Coupling and Moiré Periodic Potentials in Twisted Bilayer Graphene. Phys. Rev. Lett. 2012, 109, 186807,  DOI: 10.1103/PhysRevLett.109.186807
      Google Scholar
      37
      Evidence for interlayer coupling and Moire periodic potentials in twisted bilayer graphene
      Ohta, Taisuke; Robinson, Jeremy T.; Feibelman, Peter J.; Bostwick, Aaron; Rotenberg, Eli; Beechem, Thomas E.
      Physical Review Letters (2012), 109 (18), 186807/1-186807/6CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      We report a study of the valence band dispersion of twisted bilayer graphene using angle-resolved photoemission spectroscopy and ab initio calcns. We observe two noninteracting cones near the Dirac crossing energy and the emergence of van Hove singularities where the cones overlap for large twist angles (>5°). Besides the expected interaction between the Dirac cones, minigaps appeared at the Brillouin zone boundaries of the moire superlattice formed by the misorientation of the two graphene layers. We attribute the emergence of these minigaps to a periodic potential induced by the moire. These anticrossing features point to coupling between the two graphene sheets, mediated by moire periodic potentials.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslClsL%252FK&md5=8f47fe5f9da9daefcd72261fa016161e
    38. 38
      Tan, Z. Building Large-Domain Twisted Bilayer Graphene with van Hove Singularity. ACS Nano 2016, 10, 6725– 6730,  DOI: 10.1021/acsnano.6b02046
      Google Scholar
      38
      Building large-domain twisted bilayer graphene with van Hove singularity
      Tan, Zhenjun; Yin, Jianbo; Chen, Cheng; Wang, Huan; Lin, Li; Sun, Luzhao; Wu, Jinxiong; Sun, Xiao; Yang, Haifeng; Chen, Yulin; Peng, Hailin; Liu, Zhongfan
      ACS Nano (2016), 10 (7), 6725-6730CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Twisted bilayer graphene (tBLG) with van Hove Singularity (VHS) has exhibited novel twist-angle-dependent chem. and phys. phenomena. However, scalable prodn. of high-quality tBLG is still in its infancy, esp. lacking the angle controlled prepn. methods. Here, we report a facile approach to prep. tBLG with large domain sizes (>100 μm) and controlled twist angles by a clean layer-by-layer transfer of two constituent graphene monolayers. The whole process without interfacial polymer contamination in two monolayers guarantees the interlayer interaction of the π-bond electrons, which gives rise to the existence of minigaps in electronic structures and the consequent formation of VHSs in d. of state. Such perturbation on band structure was directly obsd. by angle-resolved photoemission spectroscopy with submicrometer spatial resoln. (micro-ARPES). The VHSs lead to a strong light-matter interaction and thus introduce ∼20-fold enhanced intensity of Raman G-band, which is a characteristic of high-quality tBLG. The as-prepd. tBLG with strong light-matter interaction was further fabricated into high-performance photodetectors with selectively enhanced photocurrent generation (up to ∼6 times compared with monolayer in our device).
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XnslCrtbg%253D&md5=3841b709e2fb1b87e5f71aa1dc0045fd
    39. 39
      Yang, S.-J. Wafer-Scale Programmed Assembly of One-Atom-Thick Crystals. Nano Lett. 2022, 22, 1518– 1524,  DOI: 10.1021/acs.nanolett.1c04139
      Google Scholar
      39
      Wafer-Scale Programmed Assembly of One-Atom-Thick Crystals
      Yang, Seong-Jun; Jung, Ju-Hyun; Lee, Eunsook; Han, Edmund; Choi, Min-Yeong; Jung, Daesung; Choi, Shinyoung; Park, Jun-Ho; Oh, Dongseok; Noh, Siwoo; Kim, Ki-Jeong; Huang, Pinshane Y.; Hwang, Chan-Cuk; Kim, Cheol-Joo
      Nano Letters (2022), 22 (4), 1518-1524CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Cryst. films offer various phys. properties based on the modulation of their thicknesses and at. structures. The layer-by-layer assembly of atomically thin crystals provides powerful means to arbitrarily design films at the at.-level, which are unattainable with existing growth technols. Atomically clean assembly of the materials with high scalability and reproducibility remains challenging. Programmed crystal assembly (PCA) of graphene and monolayer hexagonal BN (ML hBN), assisted by van der Waals interactions, to form wafer-scale films of pristine interfaces with near-unity yield are reported. The at. configurations of the films are tailored with layer-resolved compns. and in-plane cryst. orientations. Batch-fabricated tunnel device arrays are demonstrated with modulation of the resistance over orders of magnitude by thickness-control of the hBN barrier with single-atom precision, and large-scale, twisted multilayer graphene with programmable electronic band structures and crystal symmetries. The results constitute an important development in the artificial design of large-scale films.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XislertL8%253D&md5=3b9133ef25e943608637d185e0ee8ca3
    40. 40
      Wong, D. Local spectroscopy of moiré-induced electronic structure in gate-tunable twisted bilayer graphene. Phys. Rev. B 2015, 92, 155409,  DOI: 10.1103/PhysRevB.92.155409
      Google Scholar
      40
      Local spectroscopy of moir´e-induced electronic structure in gate-tunable twisted bilayer graphene
      Wong, Dillon; Wang, Yang; Jung, Jeil; Pezzini, Sergio; Da Silva, Ashley M.; Tsai, Hsin-Zon; Jung, Han Sae; Khajeh, Ramin; Kim, Youngkyou; Lee, Juwon; Kahn, Salman; Tollabimazraehno, Sajjad; Rasool, Haider; Watanabe, Kenji; Taniguchi, Takashi; Zettl, Alex; Adam, Shaffique; MacDonald, Allan H.; Crommie, Michael F.
      Physical Review B: Condensed Matter and Materials Physics (2015), 92 (15), 155409/1-155409/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Twisted bilayer graphene (tBLG) forms a quasicrystal whose structural and electronic properties depend on the angle of rotation between its layers. Here, we present a scanning tunneling microscopy study of gate-tunable tBLG devices supported by atomically smooth and chem. inert hexagonal boron nitride (BN). The high quality of these tBLG devices allows identification of coexisting moir´e patterns and moir´e super-superlattices produced by graphene-graphene and graphene-BN interlayer interactions. Furthermore, we examine addnl. tBLG spectroscopic features in the local d. of states beyond the first van Hove singularity. Our exptl. data are explained by a theory of moir´e bands that incorporates ab initio calcns. and confirms the strongly nonperturbative character of tBLG interlayer coupling in the small twist-angle regime.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xos1entLw%253D&md5=8f85f2ebde7e3a0f45a12a3aa8713284
    41. 41
      Ren, Y.-N. Spectroscopic Evidence for a Spin- and Valley-Polarized Metallic State in a Nonmagic-Angle Twisted Bilayer Graphene. ACS Nano 2020, 14, 13081– 13090,  DOI: 10.1021/acsnano.0c04631
      Google Scholar
      41
      Spectroscopic Evidence for a Spin- and Valley-Polarized Metallic State in a Nonmagic-Angle Twisted Bilayer Graphene
      Ren, Ya-Ning; Lu, Chen; Zhang, Yu; Li, Si-Yu; Liu, Yi-Wen; Yan, Chao; Guo, Zi-Han; Liu, Cheng-Cheng; Yang, Fan; He, Lin
      ACS Nano (2020), 14 (10), 13081-13090CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      In the magic-angle twisted bilayer graphene (MA-TBG), strong electron-electron (e-e) correlations caused by the band-flattening lead to many exotic quantum phases such as supercond., correlated insulator, ferromagnetism, and quantum anomalous Hall effects, when its low-energy van Hove singularities (VHSs) are partially filled. Here our high-resoln. scanning tunneling microscope and spectroscopy measurements demonstrate that the e-e correlation in a nonmagic-angle TBG with a twist angle θ = 1.49° still plays an important role in detg. its electronic properties. Our most interesting observation on that sample is when one of its VHSs is partially filled, the one assocd. peak in the spectrum splits into four peaks. Simultaneously, the spatial symmetry of electronic states around the split VHSs is broken by the e-e correlation. Our anal. based on the continuum model suggests that such a one-to-four split of the VHS originates from the formation of an interaction-driven spin-valley-polarized metallic state near the VHS, which is a symmetry-breaking phase that has not been identified in the MA-TBG or in other systems.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitVGlu7bK&md5=c8a6671b10b45130d2b99473048caf9e
    42. 42
      Shivayogimath, A. Do-It-Yourself Transfer of Large-Area Graphene Using an Office Laminator and Water. Chem. Mater. 2019, 31, 2328– 2336,  DOI: 10.1021/acs.chemmater.8b04196
      Google Scholar
      42
      Do-It-Yourself Transfer of Large-Area Graphene Using an Office Laminator and Water
      Shivayogimath, Abhay; Whelan, Patrick Rebsdorf; MacKenzie, David M. A.; Luo, Birong; Huang, Deping; Luo, Da; Wang, Meihui; Gammelgaard, Lene; Shi, Haofei; Ruoff, Rodney S.; Boeggild, Peter; Booth, Timothy J.
      Chemistry of Materials (2019), 31 (7), 2328-2336CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The authors demonstrate a simple method for transferring large areas (up to A4-size sheets) of CVD graphene from Cu foils onto a target substrate using a com. available polyvinyl alc. polymer foil as a carrier substrate and com. hot-roll office laminator. Through the use of terahertz time-domain spectroscopy and Raman spectroscopy, large-area quant. optical contrast mapping, and the fabrication and elec. characterization of ∼50 individual centimeter-scale van der Pauw field effect devices, the authors show a nondestructive technique to transfer large-area graphene with low residual doping that is scalable, economical, reproducible, and easy to use and that results in less doping and transfer-induced damage than etching or electrochem. delamination transfers. The Cu substrate can be used multiple times with minimal loss of material and no observable redn. in graphene quality. The authors have addnl. demonstrated the transfer of multilayer hexagonal B nitride from Cu and Fe foils. Finally, this approach allows graphene to be supplied on stand-alone polymer supports by CVD graphene manufacturers to end users, with the only equipment and consumables required to transfer graphene onto target substrates being a com. office laminator and H2O.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXktlOjtr4%253D&md5=e23f9158b101c9f2b99444c6e21bb67e
    43. 43
      Miseikis, V. Deterministic patterned growth of high-mobility large-crystal graphene: A path towards wafer scale integration. 2D Mater. 2017, 4, 021004  DOI: 10.1088/2053-1583/aa5481
      Google Scholar
      43
      Deterministic patterned growth of high-mobility large-crystal graphene: a path towards wafer scale integration
      Miseikis, Vaidotas; Bianco, Federica; David, Jeremy; Gemmi, Mauro; Pellegrini, Vittorio; Romagnoli, Marco; Coletti, Camilla
      2D Materials (2017), 4 (2), 021004/1-021004/8CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      We demonstrate rapid deterministic (seeded) growth of large single-crystals of graphene by chem. vapor deposition (CVD) utilizing pre-patterned copper substrates with chromium nucleation sites. Arrays of graphene single-crystals as large as several hundred microns are grown with a periodicity of up to 1 mm. The graphene is transferred to target substrates using aligned and contaminationfree semi-dry transfer. The high quality of the synthesized graphene is confirmed by Raman spectroscopy and transport measurements, demonstrating room-temp. carrier mobility of 21 000 cm2 V-1 s-1 when transferred on top of hexagonal boron nitride. By tailoring the nucleation of large single-crystals according to the desired device geometry, it will be possible to produce complex device architectures based on single-crystal graphene, thus paving the way to the adoption of CVD graphene in wafer-scale fabrication.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXnsVyisrs%253D&md5=98a0b57b5639191f352dc016d6983dbb
    44. 44
      Purdie, D. G.; Pugno, N. M.; Taniguchi, T.; Watanabe, K.; Ferrari, A. C.; Lombardo, A. Cleaning interfaces in layered materials heterostructures. Nat. Commun. 2018, 9, 5387,  DOI: 10.1038/s41467-018-07558-3
      Google Scholar
      44
      Cleaning interfaces in layered materials heterostructures
      Purdie, D. G.; Pugno, N. M.; Taniguchi, T.; Watanabe, K.; Ferrari, A. C.; Lombardo, A.
      Nature Communications (2018), 9 (1), 5387CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Heterostructures formed by stacking layered materials require atomically clean interfaces. However, contaminants are usually trapped between the layers, aggregating into randomly located blisters, incompatible with scalable fabrication processes. Here we report a process to remove blisters from fully formed heterostructures. Our method is over an order of magnitude faster than those previously reported and allows multiple interfaces to be cleaned simultaneously. We fabricate blister-free regions of graphene encapsulated in hexagonal boron nitride with an area ∼ 5000μm2, achieving mobilities up to 180,000 cm2 V-1 s-1 at room temp., and 1.8 × 106 cm2 V-1 s-1 at 9 K. We also assemble heterostructures using graphene intentionally exposed to polymers and solvents. After cleaning, these samples reach similar mobilities. This demonstrates that exposure of graphene to process-related contaminants is compatible with the realization of high mobility samples, paving the way to the development of wafer-scale processes for the integration of layered materials in (opto)electronic devices.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXisFKht7fI&md5=9218b0901bfcaa32530c6827d335fb83
    45. 45
      Giambra, M. A. Wafer-Scale Integration of Graphene-Based Photonic Devices. ACS Nano 2021, 15, 3171– 3187,  DOI: 10.1021/acsnano.0c09758
      Google Scholar
      45
      Wafer-Scale Integration of Graphene-Based Photonic Devices
      Giambra, Marco A.; Miseikis, Vaidotas; Pezzini, Sergio; Marconi, Simone; Montanaro, Alberto; Fabbri, Filippo; Sorianello, Vito; Ferrari, Andrea C.; Coletti, Camilla; Romagnoli, Marco
      ACS Nano (2021), 15 (2), 3171-3187CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Graphene and related materials can lead to disruptive advances in next-generation photonics and optoelectronics. The challenge is to devise growth, transfer and fabrication protocols providing high (≥5000 cm2 V-1 s-1) mobility devices with reliable performance at the wafer scale. Here, we present a flow for the integration of graphene in photonics circuits. This relies on chem. vapor deposition (CVD) of single layer graphene (SLG) matrixes comprising up to ~ 12000 individual single crystals, grown to match the geometrical configuration of the devices in the photonic circuit. This is followed by a transfer approach which guarantees coverage over ~ 80% of the device area, and integrity for up to 150 mm wafers, with room temp. mobility ~ 5000 cm2 V-1 s-1. We use this process flow to demonstrate double SLG electro-absorption modulators with modulation efficiency ~ 0.25, 0.45, 0.75, 1 dB V-1 for device lengths ~ 30, 60, 90, 120μm. The data rate is up to 20 Gbps. Encapsulation with single-layer hexagonal boron nitride (hBN) is used to protect SLG during plasma-enhanced CVD of Si3N4, ensuring reproducible device performance. The processes are compatible with full automation. This paves the way for large scale prodn. of graphene-based photonic devices.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXisVWgsLs%253D&md5=daefda6abc1764a849dd867736cc0836
    46. 46
      Gadelha, A. C.; Ohlberg, D. A. A.; Rabelo, C.; Neto, E. G. S.; Vasconcelos, T. L.; Campos, J. L.; Lemos, J. S.; Ornelas, V.; Miranda, D.; Nadas, R.; Santana, F. C.; Watanabe, K.; Taniguchi, T.; van Troeye, B.; Lamparski, M.; Meunier, V.; Nguyen, V.-H.; Paszko, D.; Charlier, J.-C.; Campos, L. C.; Cancado, L. G.; Medeiros-Ribeiro, G.; Jorio, A. Localization of lattice dynamics in low-angle twisted bilayer graphene. Nature 2021, 590, 405– 409,  DOI: 10.1038/s41586-021-03252-5
      Google Scholar
      46
      Localization of lattice dynamics in low-angle twisted bilayer graphene
      Gadelha, Andreij C.; Ohlberg, Douglas A. A.; Rabelo, Cassiano; Neto, Eliel G. S.; Vasconcelos, Thiago L.; Campos, Joao L.; Lemos, Jessica S.; Ornelas, Vinicius; Miranda, Daniel; Nadas, Rafael; Santana, Fabiano C.; Watanabe, Kenji; Taniguchi, Takashi; van Troeye, Benoit; Lamparski, Michael; Meunier, Vincent; Nguyen, Viet-Hung; Paszko, Dawid; Charlier, Jean-Christophe; Campos, Leonardo C.; Cancado, Luiz G.; Medeiros-Ribeiro, Gilberto; Jorio, Ado
      Nature (London, United Kingdom) (2021), 590 (7846), 405-409CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Abstr.: Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain1-3. The resulting superlattice modulates the vibrational3,4 and electronic5,6 structures within the material, leading to changes in the behavior of electron-phonon coupling7,8 and to the observation of strong correlations and supercond.9. However, accessing these modulations and understanding the related effects are challenging, because the modulations are too small for exptl. techniques to accurately resolve the relevant energy levels and too large for theor. models to properly describe the localized effects. Here we report hyperspectral optical images, generated by a nano-Raman spectroscope10, of the crystal superlattice in reconstructed (low-angle) twisted bilayer graphene. Observations of the crystallog. structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topol. points1 causing detectable spectral variations. The results are rationalized by an atomistic model that enables evaluation of the local d. of the electronic and vibrational states of the superlattice. This evaluation highlights the relevance of solitons and topol. points for the vibrational and electronic properties of the structures, particularly for small twist angles. Our results are an important step towards understanding phonon-related effects at at. and nanometric scales, such as Jahn-Teller effects11 and electronic Cooper pairing12-14, and may help to improve device characterization15 in the context of the rapidly developing field of twistronics16.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXktlGls7g%253D&md5=7440b55893e62ff8e9458bb37ffb0206
    47. 47
      Cao, Y. Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene. Phys. Rev. Lett. 2016, 117, 116804,  DOI: 10.1103/PhysRevLett.117.116804
      Google Scholar
      47
      Superlattice-Induced insulating states and valley-protected orbits in twisted bilayer Graphene
      Cao, Y.; Luo, J. Y.; Fatemi, V.; Fang, S.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P.
      Physical Review Letters (2016), 117 (11), 116804/1-116804/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moir´e physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theor. predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calcns. reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moir´e wave vector.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVWku7nE&md5=000d518b94f35535bbddfb29ca40c5b8
    48. 48
      Chung, T.-F.; Xu, Y.; Chen, Y. P. Transport measurements in twisted bilayer graphene: Electron-phonon coupling and Landau level crossing. Phys. Rev. B 2018, 98, 035425  DOI: 10.1103/PhysRevB.98.035425
      Google Scholar
      48
      Transport measurements in twisted bilayer graphene: Electron-phonon coupling and Landau level crossing
      Chung, Ting-Fung; Xu, Yang; Chen, Yong P.
      Physical Review B (2018), 98 (3), 035425CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      We investigate electronic transport in twisted bilayer graphene (tBLG) under variable temps. (T), carrier densities (n), and transverse magnetic fields, focusing on samples with small twist angles (θ). These samples show prominent signatures assocd. with the van Hove singularities (VHSs) and superlattice-induced minigaps (SMGs). Temp.-dependent field-effect measurement shows that the difference between temp.-dependent resistivity and residual resistivity, ρxx(T,n)-ρ0(n), follows ∼Tβ for n between the main Dirac point (DP) and SMG. The evolution of the temp. exponent β with n exhibits a W-shaped dependence, with min. of β∼0.9 near the VHSs and maxima of β∼1.7 toward the SMGs. This W-shaped behavior can be qual. understood with a theor. picture that considers both the Fermi surface smearing near the VHSs and flexural-acoustic phonon scattering. In the quantum Hall regime, we observe only Landau level crossings in the massless Dirac spectrum originating from the main DP but not in the parabolic band near the SMG. Such crossings enable the measurement of an enhanced interlayer dielec. const., attributed to a reduced Fermi velocity. Moreover, we measure the Fermi velocity, interlayer coupling strength, VHS energy relative to the DP, and gap size of SMG, four important parameters used to describe the peculiar band structure of the small-θ tBLG.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXltVGmsrs%253D&md5=2257f760d6fb5746b1fd991397426a8a
    49. 49
      Kim, Y. Charge Inversion and Topological Phase Transition at a Twist Angle Induced van Hove Singularity of Bilayer Graphene. Nano Lett. 2016, 16, 5053– 5059,  DOI: 10.1021/acs.nanolett.6b01906
      Google Scholar
      49
      Charge Inversion and Topological Phase Transition at a Twist Angle Induced van Hove Singularity of Bilayer Graphene
      Kim, Youngwook; Herlinger, Patrick; Moon, Pilkyung; Koshino, Mikito; Taniguchi, Takashi; Watanabe, Kenji; Smet, Jurgen H.
      Nano Letters (2016), 16 (8), 5053-5059CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Van Hove singularities (VHS's) in the d. of states play an outstanding and diverse role for the electronic and thermodn. properties of cryst. solids. At the crit. point the Fermi surface connectivity changes, and topol. properties undergo a transition. Opportunities to systematically pass a VHS at the turn of a voltage knob and study its diverse impact are however rare. With the advent of van der Waals heterostructures, control over the at. registry of neighboring graphene layers offers an unprecedented tool to generate a low energy VHS easily accessible with conventional gating. Here the authors have addressed magnetotransport when the chem. potential crosses the twist angle induced VHS in twisted bilayer graphene. A topol. phase transition is exptl. disclosed in the abrupt conversion of electrons to holes or vice versa, a loss of a nonzero Berry phase and distinct sequences of integer quantum Hall states above and below the singularity.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtFeku7bK&md5=dfd392a11beddb12980900909133f71a
    50. 50
      Berdyugin, A. I.; Tsim, B.; Kumaravadivel, P.; Xu, S. G.; Ceferino, A.; Knothe, A.; Kumar, R. K.; Taniguchi, T.; Watanabe, K.; Geim, A. K.; Grigorieva, I. V.; Fal’ko, V. I. Minibands in twisted bilayer graphene probed by magnetic focusing. Sci. Adv. 2020, 6, eaay7838  DOI: 10.1126/sciadv.aay7838
      Google Scholar
      There is no corresponding record for this reference.
    51. 51
      Kim, Y.; Moon, P.; Watanabe, K.; Taniguchi, T.; Smet, J. H. Odd Integer Quantum Hall States with Interlayer Coherence in Twisted Bilayer Graphene. Nano Lett. 2021, 21, 4249– 4254,  DOI: 10.1021/acs.nanolett.1c00360
      Google Scholar
      51
      Odd integer quantum hall states with interlayer coherence in twisted bilayer graphene
      Kim, Youngwook; Moon, Pilkyung; Watanabe, Kenji; Taniguchi, Takashi; Smet, Jurgen H.
      Nano Letters (2021), 21 (10), 4249-4254CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We report on the quantum Hall effect in two stacked graphene layers rotated by 2°. The tunneling strength among the layers can be varied from very weak to strong via the mechanism of magnetic breakdown when tuning the d. Odd-integer quantum Hall physics is not anticipated in the regime of suppressed tunneling for balanced layer densities, yet it is obsd. We interpret this as a signature of Coulomb interaction induced interlayer coherence and Bose-Einstein condensation of excitons that form at half filling of each layer. A d. imbalance gives rise to reentrant behavior due to a phase transition from the interlayer coherent state to incompressible behavior caused by simultaneous condensation of both layers in different quantum Hall states. With increasing overall d., magnetic breakdown gains the upper hand. As a consequence of the enhanced interlayer tunneling, the interlayer coherent state and the phase transition vanish.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhtVWrt7jL&md5=17b4780057a4b93db1b5fc17541d9085
    52. 52
      Yankowitz, M. Tuning superconductivity in twisted bilayer graphene. Science 2019, 363, 1059– 1064,  DOI: 10.1126/science.aav1910
      Google Scholar
      52
      Tuning superconductivity in twisted bilayer graphene
      Yankowitz, Matthew; Chen, Shaowen; Polshyn, Hryhoriy; Zhang, Yuxuan; Watanabe, K.; Taniguchi, T.; Graf, David; Young, Andrea F.; Dean, Cory R.
      Science (Washington, DC, United States) (2019), 363 (6431), 1059-1064CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Materials with flat electronic bands often exhibit exotic quantum phenomena owing to strong correlations. An isolated low-energy flat band can be induced in bilayer graphene by simply rotating the layers by 1.1°, resulting in the appearance of gate-tunable superconducting and correlated insulating phases. In this study, the authors demonstrate that in addn. to the twist angle, the interlayer coupling can be varied to precisely tune these phases. The authors induce supercond. at a twist angle larger than 1.1°-in which correlated phases are otherwise absent-by varying the interlayer spacing with hydrostatic pressure. Their low-disorder devices reveal details about the superconducting phase diagram and its relation to the nearby insulator. The results demonstrate twisted bilayer graphene to be a distinctively tunable platform for exploring correlated states.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXktFagt7c%253D&md5=c98ef0256319b9a3b8a1d2617c35ae87
    53. 53
      Piccinini, G.; Mišeikis, V.; Watanabe, K.; Taniguchi, T.; Coletti, C.; Pezzini, S. Parallel transport and layer-resolved thermodynamic measurements in twisted bilayer graphene. Phys. Rev. B 2021, 104, L241410,  DOI: 10.1103/PhysRevB.104.L241410
      Google Scholar
      53
      Parallel transport and layer-resolved thermodynamic measurements in twisted bilayer graphene
      Piccinini, G.; Miseikis, V.; Watanabe, K.; Taniguchi, T.; Coletti, C.; Pezzini, S.
      Physical Review B (2021), 104 (24), L241410CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      We employ dual-gated 30°-twisted bilayer graphene to demonstrate simultaneous ultrahigh mobility and cond. (up to 40 mS at room temp.), unattainable in a single layer of graphene. We find quant. agreement with a simple phenomenol. of parallel conduction between two pristine graphene sheets, with a gate-controlled carrier distribution. Based on the parallel transport mechanism, we then introduce a method for in situ measurements of the chem. potential of the two layers. This twist-enabled approach, neither requiring a dielec. spacer, nor sep. contacting, has the potential to greatly simplify the measurement of thermodn. quantities in graphene-based systems of high current interest.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhtV2jtr8%253D&md5=52373f01c13ff999c4427bc36f1e3e3d
    54. 54
      Novelli, P.; Torre, I.; Koppens, F. H. L.; Taddei, F.; Polini, M. Optical and plasmonic properties of twisted bilayer graphene: Impact of interlayer tunneling asymmetry and ground-state charge inhomogeneity. Phys. Rev. B 2020, 102, 125403,  DOI: 10.1103/PhysRevB.102.125403
      Google Scholar
      54
      Optical and plasmonic properties of twisted bilayer graphene: Impact of interlayer tunneling asymmetry and ground-state charge inhomogeneity
      Novelli, Pietro; Torre, Iacopo; Koppens, Frank H. L.; Taddei, Fabio; Polini, Marco
      Physical Review B (2020), 102 (12), 125403CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Twisted bilayer graphene (TBG) at twist angles θ≈1o has recently attracted a great deal of interest for its rich transport phenomenol. We present a theor. study of the local optical cond., plasmon spectra, and thermoelec. properties of TBG at different filling factors and twist angles θ. Our calcns. are based on the electronic band structures obtained from a continuum model that has two tunable parameters u0 and u1 which parametrize the intrasublattice interlayer and intersublattice interlayer tunneling rate, resp. In this article we focus on two key aspects: (i) we study the dependence of our results on the value of u0, exploring the whole range 0≤u0≤u1; and (ii) we take into account effects arising from the intrinsic charge d. inhomogeneity present in TBG, by calcg. the band structures within the self-consistent Hartree approxn. At zero filling factor, i.e., at the charge neutrality point, the optical cond. is quite sensitive to the value of u0 and twist angle, whereas the charge inhomogeneity brings about only modest corrections. On the other hand, away from zero filling, static screening dominates and the optical cond. is appreciably affected by the charge inhomogeneity, the largest effects being seen on the intraband contribution to it. These findings are also reflected by the plasmonic spectra. We compare our results with existing ones in the literature, where effects (i) and (ii) above have not been studied systematically. As natural byproducts of our calcns., we obtain the Drude wt. and Seebeck coeff. The former displays an enhanced particle-hole asymmetry stemming from the inhomogeneous ground-state charge distribution. The latter is shown to display a broad sign-changing feature even at low temps. (≈5K) due to the reduced slope of the bands, as compared to those of single-layer graphene.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitVOiu7rE&md5=301ca4d6806eb0952ca6771cb1415c05
    55. 55
      Hesp, N. C. H. Observation of interband collective excitations in twisted bilayer graphene. Nat. Phys. 2021, 17, 1162– 1168,  DOI: 10.1038/s41567-021-01327-8
      Google Scholar
      55
      Observation of interband collective excitations in twisted bilayer graphene
      Hesp, Niels C. H.; Torre, Iacopo; Rodan-Legrain, Daniel; Novelli, Pietro; Cao, Yuan; Carr, Stephen; Fang, Shiang; Stepanov, Petr; Barcons-Ruiz, David; Herzig Sheinfux, Hanan; Watanabe, Kenji; Taniguchi, Takashi; Efetov, Dmitri K.; Kaxiras, Efthimios; Jarillo-Herrero, Pablo; Polini, Marco; Koppens, Frank H. L.
      Nature Physics (2021), 17 (10), 1162-1168CODEN: NPAHAX; ISSN:1745-2473. (Nature Portfolio)
      The single-particle and many-body properties of twisted bilayer graphene (TBG) can be dramatically different from those of a single graphene layer, particularly when the two layers are rotated relative to each other by a small angle (θ ≈ 1°), owing to the moire potential induced by the twist. Here we probe the collective excitations of TBG with a spatial resoln. of 20 nm, by applying mid-IR near-field optical microscopy. We find a propagating plasmon mode in charge-neutral TBG for θ = 1.1-1.7°, which is different from the intraband plasmon in single-layer graphene. We interpret it as an interband plasmon assocd. with the optical transitions between minibands originating from the moire superlattice. The details of the plasmon dispersion are directly related to the motion of electrons in the moire superlattice and offer an insight into the phys. properties of TBG, such as band nesting between the flat band and remote band, local interlayer coupling, and losses. We find a strongly reduced interlayer coupling in the regions with AA stacking, pointing at screening due to electron-electron interactions. Optical nano-imaging of TBG allows the spatial probing of interaction effects at the nanoscale and potentially elucidates the contribution of collective excitations to many-body ground states.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXitFamtrvJ&md5=c30b2b8cf46afb06c451b8e1f04ca10e
    56. 56
      Giuliani, G. F.; Vignale, G. Quantum Theory of the Electron Liquid; Cambridge Univ. Press: Cambridge, England, 2005.
      Google Scholar
      There is no corresponding record for this reference.
    57. 57
      Berthod, C.; Zhang, H.; Morpurgo, A. F.; Giamarchi, T. Theory of cross quantum capacitance. Phys. Rev. Res. 2021, 3, 043036  DOI: 10.1103/PhysRevResearch.3.043036
      Google Scholar
      57
      Theory of cross quantum capacitance
      Berthod, Christophe; Zhang, Haijing; Morpurgo, Alberto F.; Giamarchi, Thierry
      Physical Review Research (2021), 3 (4), 043036CODEN: PRRHAI; ISSN:2643-1564. (American Physical Society)
      Impressive progress in the control of atomically thin crystals is now enabling the realization of gated structures in which two electrodes are sepd. by at. scale distances. The elec. capacitance of these structures is detd. by phenomena that are not relevant in capacitors with larger electrode sepn. With the aim to analyze these phenomena, we use linear-response theory to develop a systematic description of capacitance for two coupled electron liqs., accounting for the wave nature of electrons, as well as for the effect of both intra- and interlayer Coulomb interactions. Our theory leads to a general expression for the elec. capacitance in terms of both intra- and interlayer electronic polarizabilities. The intralayer polarizability is directly related to the conventional expression for the quantum capacitance, whereas the interlayer polarizability term accounts for interaction-induced correlations between charges hosted by opposite capacitor plates. We refer to this latter term-which has not been considered earlier-as to the cross quantum capacitance. We discuss the implications of the general expression for the capacitance, show that it leads to established results when the effect of interlayer correlations is negligible, and that the intra- and interlayer polarizabilities play a comparable role for capacitors with very small electrode sepn. (i.e., cross quantum capacitance effects can be large and cannot be neglected). Using two different approaches, we calc. the capacitance in specific cases, and find that the interlayer polarizability can be either pos. or neg., so that-depending on the regime considered-the cross quantum capacitance can either increase or decrease the total capacitance. We conclude by showing that the cross quantum capacitance term can lead to a nonmonotonic evolution of the total capacitance with increasing sepn. between the capacitor plates, which would represent an unambiguous manifestation of the cross quantum capacitance if obsd. exptl.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhtFShs7w%253D&md5=fe17d6c36655fd19a871cfd23126b8fc
    58. 58
      Slizovskiy, S. Out-of-Plane Dielectric Susceptibility of Graphene in Twistronic and Bernal Bilayers. Nano Lett. 2021, 21, 6678– 6683,  DOI: 10.1021/acs.nanolett.1c02211
      Google Scholar
      58
      Out-of-plane dielectric susceptibility of graphene in twistronic and bernal bilayers
      Slizovskiy, Sergey; Garcia-Ruiz, Aitor; Berdyugin, Alexey I.; Xin, Na; Taniguchi, Takashi; Watanabe, Kenji; Geim, Andre K.; Drummond, Neil D.; Fal'ko, Vladimir I.
      Nano Letters (2021), 21 (15), 6678-6683CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We describe how the out-of-plane dielec. polarizability of monolayer graphene influences the electrostatics of bilayer graphene-both Bernal (BLG) and twisted (tBLG). We compare the polarizability value computed using d. functional theory with the output from previously published exptl. data on the electrostatically controlled interlayer asymmetry potential in BLG and data on the on-layer d. distribution in tBLG. We show that monolayers in tBLG are described well by polarizability αexp = 10.8 Å3 and effective out-of-plane dielec. susceptibility εz = 2.5, including their on-layer electron d. distribution at zero magnetic field and the interlayer Landau level pinning at quantizing magnetic fields.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhs1SgsbvP&md5=f60d06525a8d1c542c0f25afb6ba1c0e
    59. 59
      Hejazi, K.; Liu, C.; Balents, L. Landau levels in twisted bilayer graphene and semiclassical orbits. Phys. Rev. B 2019, 100, 035115  DOI: 10.1103/PhysRevB.100.035115
      Google Scholar
      59
      Landau levels in twisted bilayer graphene and semiclassical orbits
      Hejazi, Kasra; Liu, Chunxiao; Balents, Leon
      Physical Review B (2019), 100 (3), 035115CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Twisted bilayer graphene has been argued theor. to host exceptionally flat bands when the angle between the two layers falls within a magic range near 1.1o. This is now strongly supported by expt., which furthermore reveals dramatic correlation effects in the magic range due to the relative dominance of interactions when the bandwidth is suppressed. Exptl., quantum oscillations exhibit different Landau level degeneracies when the angles fall in or outside the magic range; these observations can contain crucial information about the low-energy physics. In this paper, we report a thorough theor. study of the Landau level structure of the noninteracting continuum model for twisted bilayer graphene as the magnetic field and the twist angle are tuned. We first show that a discernible difference exists in the butterfly spectra when twist angle falls in and outside the magic range. Next, we carry out semiclassical anal. in detail, which quant. dets. the origin of the low-energy Landau levels from the zero field band structure. We find that the Landau level degeneracy predicted in the above analyses is capable of partially explaining features of the quantum oscillation expts. in a natural way. Finally, topol. aspects, validity, and other subtle points of the model are discussed.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhvFelsbnO&md5=38de8892eb1e86934688aabceafb5dac
    60. 60
      Krishna Kumar, R.; Chen, X.; Auton, G. H.; Mishchenko, A.; Bandurin, D. A.; Morozov, S. V.; Cao, Y.; Khestanova, E.; Ben Shalom, M.; Kretinin, A. V.; Novoselov, K. S.; Eaves, L.; Grigorieva, I. V.; Ponomarenko, L. A.; Fal’ko, V. I.; Geim, A. K. High-temperature quantum oscillations caused by recurring Bloch states in graphene superlattices. Science 2017, 357, 181– 184,  DOI: 10.1126/science.aal3357
      Google Scholar
      60
      High-temperature quantum oscillations caused by recurring Bloch states in graphene superlattices
      Krishna Kumar, R.; Chen, X.; Auton, G. H.; Mishchenko, A.; Bandurin, D. A.; Morozov, S. V.; Cao, Y.; Khestanova, E.; Ben Shalom, M.; Kretinin, A. V.; Novoselov, K. S.; Eaves, L.; Grigorieva, I. V.; Ponomarenko, L. A.; Fal'ko, V. I.; Geim, A. K.
      Science (Washington, DC, United States) (2017), 357 (6347), 181-184CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Cyclotron motion of charge carriers in metals and semiconductors leads to Landau quantization and magneto-oscillatory behavior in their properties. Cryogenic temps. are usually required to observe these oscillations. We show that graphene superlattices support a different type of quantum oscillation that does not rely on Landau quantization. The oscillations are extremely robust and persist well above room temp. in magnetic fields of only a few tesla. We attribute this phenomenon to repetitive changes in the electronic structure of superlattices such that charge carriers experience effectively no magnetic field at simple fractions of the flux quantum per superlattice unit cell. Our work hints at unexplored physics in Hofstadter butterfly systems at high temps.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtFOjs73J&md5=5a7b5d8db342f35e55d35295c8bfd289
    61. 61
      Kumar, R. K.; Mishchenko, A.; Chen, X.; Pezzini, S.; Auton, G. H.; Ponomarenko, L. A.; Zeitler, U.; Eaves, L.; Fal’ko, V. I.; Geim, A. K. High-order fractal states in graphene superlattices. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 5135– 5139,  DOI: 10.1073/pnas.1804572115
      Google Scholar
      61
      High-order fractal states in graphene superlattices
      Kumar, R. Krishna; Mishchenko, A.; Chen, X.; Pezzini, S.; Auton, G. H.; Ponomarenko, L. A.; Zeitler, U.; Eaves, L.; Fal'ko, V. I.; Geim, A. K.
      Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (20), 5135-5139CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Graphene superlattices were shown to exhibit high-temp. quantum oscillations due to periodic emergence of delocalized Bloch states in high magnetic fields such that unit fractions of the flux quantum pierce a superlattice unit cell. Under these conditions, semiclassical electron trajectories become straight again, similar to the case of zero magnetic field. Here, we report magnetotransport measurements that reveal second-, third-, and fourth-order magnetic Bloch states at high electron densities and temps. above 100 K. The recurrence of these states creates a fractal pattern intimately related to the origin of Hofstadter butterflies. The hierarchy of the fractal states is detd. by the width of magnetic minibands, in qual. agreement with our band-structure calcns.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvVGht7fF&md5=16e9fb066d124af6693837bc5c8d2297
    62. 62
      Barrier, J.; Kumaravadivel, P.; Krishna Kumar, R.; Ponomarenko, L. A.; Xin, N.; Holwill, M.; Mullan, C.; Kim, M.; Gorbachev, R. V.; Thompson, M. D.; Prance, J. R.; Taniguchi, T.; Watanabe, K.; Grigorieva, I. V.; Novoselov, K. S.; Mishchenko, A.; Fal’ko, V. I.; Geim, A. K.; Berdyugin, A. I. Long-range ballistic transport of Brown-Zak fermions in graphene superlattices. Nat. Commun. 2020, 11, 5756,  DOI: 10.1038/s41467-020-19604-0
      Google Scholar
      62
      Long-range ballistic transport of Brown-Zak fermions in graphene superlattices
      Barrier, Julien; Kumaravadivel, Piranavan; Krishna Kumar, Roshan; Ponomarenko, L. A.; Xin, Na; Holwill, Matthew; Mullan, Ciaran; Kim, Minsoo; Gorbachev, R. V.; Thompson, M. D.; Prance, J. R.; Taniguchi, T.; Watanabe, K.; Grigorieva, I. V.; Novoselov, K. S.; Mishchenko, A.; Fal'ko, V. I.; Geim, A. K.; Berdyugin, A. I.
      Nature Communications (2020), 11 (1), 5756CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      In quantizing magnetic fields, graphene superlattices exhibit a complex fractal spectrum often referred to as the Hofstadter butterfly. It can be viewed as a collection of Landau levels that arise from quantization of Brown-Zak minibands recurring at rational (p/q) fractions of the magnetic flux quantum per superlattice unit cell. Here we show that, in graphene-on-boron-nitride superlattices, Brown-Zak fermions can exhibit mobilities above 106 cm2 V-1 s-1 and the mean free path exceeding several micrometers. The exceptional quality of our devices allows us to show that Brown-Zak minibands are 4q times degenerate and all the degeneracies (spin, valley and mini-valley) can be lifted by exchange interactions below 1 K. We also found neg. bend resistance at 1/q fractions for elec. probes placed as far as several micrometers apart. The latter observation highlights the fact that Brown-Zak fermions are Bloch quasiparticles propagating in high fields along straight trajectories, just like electrons in zero field.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitlKltrfO&md5=882fe77d97079ff01a99d9c2be0b77c3
    63. 63
      Ponomarenko, L. A. Cloning of Dirac fermions in graphene superlattices. Nature 2013, 497, 594– 597,  DOI: 10.1038/nature12187
      Google Scholar
      63
      Cloning of Dirac fermions in graphene superlattices
      Ponomarenko, L. A.; Gorbachev, R. V.; Yu, G. L.; Elias, D. C.; Jalil, R.; Patel, A. A.; Mishchenko, A.; Mayorov, A. S.; Woods, C. R.; Wallbank, J. R.; Mucha-Kruczynski, M.; Piot, B. A.; Potemski, M.; Grigorieva, I. V.; Novoselov, K. S.; Guinea, F.; Fal'ko, V. I.; Geim, A. K.
      Nature (London, United Kingdom) (2013), 497 (7451), 594-597CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Superlattices have attracted great interest because their use may make it possible to modify the spectra of two-dimensional electron systems and, ultimately, create materials with tailored electronic properties. In previous studies (see, for example, refs 1, 2, 3, 4, 5, 6, 7, 8), it proved difficult to realize superlattices with short periodicities and weak disorder, and most of their obsd. features could be explained in terms of cyclotron orbits commensurate with the superlattice. Evidence for the formation of superlattice minibands (forming a fractal spectrum known as Hofstadter's butterfly) has been limited to the observation of new low-field oscillations and an internal structure within Landau levels. Here we report transport properties of graphene placed on a boron nitride substrate and accurately aligned along its crystallog. directions. The substrate's moire potential acts as a superlattice and leads to profound changes in the graphene's electronic spectrum. Second-generation Dirac points appear as pronounced peaks in resistivity, accompanied by reversal of the Hall effect. The latter indicates that the effective sign of the charge carriers changes within graphene's conduction and valence bands. Strong magnetic fields lead to Zak-type cloning of the third generation of Dirac points, which are obsd. as numerous neutrality points in fields where a unit fraction of the flux quantum pierces the superlattice unit cell. Graphene superlattices such as this one provide a way of studying the rich physics expected in incommensurable quantum systems and illustrate the possibility of controllably modifying the electronic spectra of two-dimensional at. crystals by varying their crystallog. alignment within van der Waals heterostuctures.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXotFSgtbo%253D&md5=3a155ffd5b544c2b4f870a6931e23072
    64. 64
      Dean, C. R. Hofstadter’s butterfly and the fractal quantum Hall effect in moiré superlattices. Nature 2013, 497, 598– 602,  DOI: 10.1038/nature12186
      Google Scholar
      64
      Hofstadter's butterfly and the fractal quantum Hall effect in moire superlattices
      Dean, C. R.; Wang, L.; Maher, P.; Forsythe, C.; Ghahari, F.; Gao, Y.; Katoch, J.; Ishigami, M.; Moon, P.; Koshino, M.; Taniguchi, T.; Watanabe, K.; Shepard, K. L.; Hone, J.; Kim, P.
      Nature (London, United Kingdom) (2013), 497 (7451), 598-602CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Electrons moving through a spatially periodic lattice potential develop a quantized energy spectrum consisting of discrete Bloch bands. In 2 dimensions, electrons moving through a magnetic field also develop a quantized energy spectrum, consisting of highly degenerate Landau energy levels. When subject to both a magnetic field and a periodic electrostatic potential, 2D systems of electrons exhibit a self-similar recursive energy spectrum. Known as Hofstadter's butterfly, this complex spectrum results from an interplay between the characteristic lengths assocd. with the 2 quantizing fields, and is one of the first quantum fractals discovered in physics. In the decades since its prediction, exptl. attempts to study this effect were limited by difficulties in reconciling the 2 length scales. Typical at. lattices (with periodicities of less than one nanometer) require unfeasibly large magnetic fields to reach the commensurability condition, and in artificially engineered structures (with periodicities greater than about 100 nm) the corresponding fields are too small to overcome disorder completely. Moire superlattices arising in bilayer graphene coupled to hexagonal BN provide a periodic modulation with ideal length scales of the order of ten nanometers, enabling unprecedented exptl. access to the fractal spectrum. Quantum Hall features assocd. with the fractal gaps are described by 2 integer topol. quantum nos., and report evidence of their recursive structure. Observation of a Hofstadter spectrum in bilayer graphene means that it is possible to investigate emergent behavior within a fractal energy landscape in a system with tunable internal degrees of freedom.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXotFSntb8%253D&md5=507cf9b9e6769653489870f59f6c1a17
    65. 65
      Hunt, B. Massive Dirac Fermions and Hofstadter Butterfly in a van der Waals Heterostructure. Science 2013, 340, 1427– 1430,  DOI: 10.1126/science.1237240
      Google Scholar
      65
      Massive Dirac Fermions and Hofstadter Butterfly in a van der Waals Heterostructure
      Hunt, B.; Sanchez-Yamagishi, J. D.; Young, A. F.; Yankowitz, M.; Le Roy, B. J.; Watanabe, K.; Taniguchi, T.; Moon, P.; Koshino, M.; Jarillo-Herrero, P.; Ashoori, R. C.
      Science (Washington, DC, United States) (2013), 340 (6139), 1427-1430CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Van der Waals heterostructures constitute a new class of artificial materials formed by stacking atomically thin planar crystals. We demonstrated band structure engineering in a van der Waals heterostructure composed of a monolayer graphene flake coupled to a rotationally aligned hexagonal boron nitride substrate. The spatially varying interlayer at. registry results in both a local breaking of the carbon sublattice symmetry and a long-range moire superlattice potential in the graphene. In our samples, this interplay between short- and long-wavelength effects resulted in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality. This picture is confirmed by our observation of fractional quantum Hall states at ±53 filling and features assocd. with the Hofstadter butterfly at ultrahigh magnetic fields.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXpsFKlt70%253D&md5=96c441306af443349b95d6f4cb8df576
    66. 66
      Phinney, I. Y. Strong interminivalley scattering in twisted bilayer graphene revealed by high-temperature magneto-oscillations. Phys. Rev. Lett. 2021, 127, 056802  DOI: 10.1103/PhysRevLett.127.056802
      Google Scholar
      66
      Strong Interminivalley Scattering in Twisted Bilayer Graphene Revealed by High-Temperature Magneto-Oscillations
      Phinney, I. Y.; Bandurin, D. A.; Collignon, C.; Dmitriev, I. A.; Taniguchi, T.; Watanabe, K.; Jarillo-Herrero, P.
      Physical Review Letters (2021), 127 (5), 056802CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
      Twisted bilayer graphene (TBG) provides an example of a system in which the interplay of interlayer interactions and superlattice structure impacts electron transport in a variety of nontrivial ways and gives rise to a plethora of interesting effects. Understanding the mechanisms of electron scattering in TBG has, however, proven challenging, raising many questions about the origins of resistivity in this system. Here we show that TBG exhibits high-temp. magneto-oscillations originating from the scattering of charge carriers between TBG minivalleys. The amplitude of these oscillations reveals that interminivalley scattering is strong, and its characteristic timescale is comparable to that of its intraminivalley counterpart. Furthermore, by exploring the temp. dependence of these oscillations, we est. the electron-electron collision rate in TBG and find that it exceeds that of monolayer graphene. Our study demonstrates the consequences of the relatively small size of the superlattice Brillouin zone and Fermi velocity redn. on lateral transport in TBG.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhvVamtbrJ&md5=c83f2230f9ca4787dc51a8b6aabbded2
    67. 67
      Yin, J.; Wang, H.; Peng, H.; Tan, Z.; Liao, L.; Lin, L.; Sun, X.; Koh, A. L.; Chen, Y.; Peng, H.; Liu, Z. Selectively enhanced photocurrent generation in twisted bilayer graphene with van Hove singularity. Nat. Commun. 2016, 7, 10699,  DOI: 10.1038/ncomms10699
      Google Scholar
      67
      Selectively enhanced photocurrent generation in twisted bilayer graphene with van Hove singularity
      Yin, Jianbo; Wang, Huan; Peng, Han; Tan, Zhenjun; Liao, Lei; Lin, Li; Sun, Xiao; Koh, Ai Leen; Chen, Yulin; Peng, Hailin; Liu, Zhongfan
      Nature Communications (2016), 7 (), 10699CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Graphene with ultra-high carrier mobility and ultra-short photoresponse time has shown remarkable potential in ultrafast photodetection. However, the broad and weak optical absorption (∼2.3%) of monolayer graphene hinders its practical application in photodetectors with high responsivity and selectivity. Here we demonstrate that twisted bilayer graphene, a stack of two graphene monolayers with an interlayer twist angle, exhibits a strong light-matter interaction and selectively enhanced photocurrent generation. Such enhancement is attributed to the emergence of unique twist-angle-dependent van Hove singularities, which are directly revealed by spatially resolved angle-resolved photoemission spectroscopy. When the energy interval between the van Hove singularities of the conduction and valance bands matches the energy of incident photons, the photocurrent generated can be significantly enhanced (up to ∼80 times with the integration of plasmonic structures in our devices). These results provide valuable insight for designing graphene photodetectors with enhanced sensitivity for variable wavelength.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XjslGntr4%253D&md5=e04571669b7fcd0ca9aa62ae2c739ed7
    68. 68
      Sunku, S. S. Photonic crystals for nano-light in moiré graphene superlattices. Science 2018, 362, 1153– 1156,  DOI: 10.1126/science.aau5144
      Google Scholar
      68
      Photonic crystals for nano-light in Moire graphene superlattices
      Sunku, S. S.; Ni, G. X.; Jiang, B. Y.; Yoo, H.; Sternbach, A.; McLeod, A. S.; Stauber, T.; Xiong, L.; Taniguchi, T.; Watanabe, K.; Kim, P.; Fogler, M. M.; Basov, D. N.
      Science (Washington, DC, United States) (2018), 362 (6419), 1153-1156CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Graphene is an atomically thin plasmonic medium that supports highly confined plasmon polaritons, or nano-light, with very low loss. Electronic properties of graphene can be drastically altered when it is laid upon another graphene layer, resulting in a moire superlattice. The relative twist angle between the 2 layers is a key tuning parameter of the interlayer coupling in thus-obtained twisted bilayer graphene (TBG). The authors studied the propagation of plasmon polaritons in TBG by IR nano-imaging. The at. reconstruction occurring at small twist angles transforms the TBG into a natural plasmon photonic crystal for propagating nano-light. This discovery points to a pathway for controlling nano-light by exploiting quantum properties of graphene and other atomically layered van der Waals materials, eliminating the need for arduous top-down nanofabrication.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXisVegu7zP&md5=52e76b884a18585e17b5c2727e3f3c54
    69. 69
      Tomadin, A.; Guinea, F.; Polini, M. Generation and morphing of plasmons in graphene superlattices. Phys. Rev. B 2014, 90, 161406,  DOI: 10.1103/PhysRevB.90.161406
      Google Scholar
      69
      Generation and morphing of plasmons in graphene superlattices
      Tomadin, Andrea; Guinea, Francisco; Polini, Marco
      Physical Review B: Condensed Matter and Materials Physics (2014), 90 (16), 161406/1-161406/5, 5 pp.CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Recent exptl. studies on graphene on hexagonal boron nitride (hBN) have demonstrated that hBN is not only a passive substrate that ensures superb electronic properties of graphene's carriers, but that it actively modifies their massless Dirac fermion character through a periodic moir´e potential. Here we present a theory of the plasmon excitation spectrum of massless Dirac fermions in a moir´e superlattice. We demonstrate that graphene-hBN stacks offer a rich platform for plasmonics in which control of plasmon modes can occur not only via electrostatic gating but also by adjusting, e.g., the relative crystallog. alignment.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhvVKjt7Y%253D&md5=0e9d2f9b9de300c9b955919dd012846f
    70. 70
      Zhang, Y.; et al. Ascendance of Superconductivity in Magic-Angle Graphene Multilayers. arXiv 2112.09270, 2021, accessed 2022-06-27.
      Google Scholar
      There is no corresponding record for this reference.
    71. 71
      Park, J. M.; et al. Magic-Angle Multilayer Graphene: A Robust Family of Moiré Superconductors. arXiv 2112.10760, 2021, accessed 2022-06-27.
      Google Scholar
      There is no corresponding record for this reference.

  • Supporting Information

    Supporting Information

    ARTICLE SECTIONS
    Jump To

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.2c01114.

    • Details on the experimental methods for CVD growth, Raman spectroscopy, device fabrication, and low-temperature magnetotransport; assembly of a second SA-TBG sample; Raman and transport data on CVD-based bilayer graphene with sub-MA twisting; dependence of the CNPs splitting on Fermi velocity and interlayer capacitance; Figures S1–S4 (PDF)


    Terms & Conditions

    Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

PDF [ 5MB]

Pair your accounts.

Export articles to Mendeley

Get article recommendations from ACS based on references in your Mendeley library.

Pair your accounts.

Export articles to Mendeley

Get article recommendations from ACS based on references in your Mendeley library.

You’ve supercharged your research process with ACS and Mendeley!

STEP 1:
Click to create an ACS ID

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

MENDELEY PAIRING EXPIRED
Your Mendeley pairing has expired. Please reconnect