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Ultrafast Exciton Dynamics in the Atomically Thin van der Waals Magnet CrSBr
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  • Christian Meineke
    Christian Meineke
    Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
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    Orcidhttps://orcid.org/0000-0003-0328-1816
  • Jakob Schlosser
    Jakob Schlosser
    Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
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  • Martin Zizlsperger
    Martin Zizlsperger
    Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
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  • Marlene Liebich
    Marlene Liebich
    Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
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  • Niloufar Nilforoushan
    Niloufar Nilforoushan
    Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
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  • Kseniia Mosina
    Kseniia Mosina
    Department of Inorganic Chemistry, University of Chemistry and Technology Prague, 166 28 Prague 6, Czech Republic
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  • Sophia Terres
    Sophia Terres
    Institute of Applied Physics and Würzburg-Dresden Cluster of Excellence, Dresden University of Technology, 01187 Dresden, Germany
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  • Alexey Chernikov
    Alexey Chernikov
    Institute of Applied Physics and Würzburg-Dresden Cluster of Excellence, Dresden University of Technology, 01187 Dresden, Germany
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    Orcidhttps://orcid.org/0000-0002-9213-2777
  • Zdenek Sofer
    Zdenek Sofer
    Department of Inorganic Chemistry, University of Chemistry and Technology Prague, 166 28 Prague 6, Czech Republic
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    Orcidhttps://orcid.org/0000-0002-1391-4448
  • Markus A. Huber
    Markus A. Huber
    Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
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  • Matthias Florian
    Matthias Florian
    Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States
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  • Mackillo Kira
    Mackillo Kira
    Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States
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  • Florian Dirnberger*
    Florian Dirnberger
    Institute of Applied Physics and Würzburg-Dresden Cluster of Excellence, Dresden University of Technology, 01187 Dresden, Germany
    *Email for F.D.: [email protected]
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  • Rupert Huber*
    Rupert Huber
    Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
    *Email for R.H.: [email protected]
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Nano Letters

Cite this: Nano Lett. 2024, 24, 14, 4101–4107
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https://doi.org/10.1021/acs.nanolett.3c05010
Published March 20, 2024

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Among atomically thin semiconductors, CrSBr stands out as both its bulk and monolayer forms host tightly bound, quasi-one-dimensional excitons in a magnetic environment. Despite its pivotal importance for solid-state research, the exciton lifetime has remained unknown. While terahertz polarization probing can directly trace all excitons, independently of interband selection rules, the corresponding large far-field foci substantially exceed the lateral sample dimensions. Here, we combine terahertz polarization spectroscopy with near-field microscopy to reveal a femtosecond decay of paramagnetic excitons in a monolayer of CrSBr, which is 30 times shorter than the bulk lifetime. We unveil low-energy fingerprints of bound and unbound electron–hole pairs in bulk CrSBr and extract the nonequilibrium dielectric function of the monolayer in a model-free manner. Our results demonstrate the first direct access to the ultrafast dielectric response of quasi-one-dimensional excitons in CrSBr, potentially advancing the development of quantum devices based on ultrathin van der Waals magnets.

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The advent of magnetic two-dimensional crystals hosting strongly bound excitons has initiated an unparalleled development in the realm of quantum materials. (1−6) In these systems, magnetic order fundamentally influences excitonic properties, such as effective mass, eigenenergies, and polarization, spawning a variety of emergent quantum phenomena. (7−10) The van der Waals layered magnet CrSBr stands out particularly for its intricate interplay between magnetic order and quasi-one-dimensional electronic and lattice structure, (11−22) establishing CrSBr as a unique platform for fundamental research and future quantum devices. In CrSBr, electron–hole pairs form highly anisotropic excitons whose real-space wave function extends along the in-plane crystallographic b axis. Below its Néel temperature of TN = 132 K, bulk CrSBr features in-plane ferromagnetic order among layers, which are antiferromagnetically coupled along the stacking direction, while quasi-one-dimensional excitons are strongly localized within individual layers. The breakdown of magnetic order in the paramagnetic phase, above TN, should result in a more 2D character of excitons tunneling between adjacent layers. In the monolayer limit, the out-of-plane confinement causes the quasi-one-dimensional nature of excitons to persist even beyond room temperature.

The combination of tight spatial confinement of excitons in the Cr–S chains and the separation of the van der Waals layers by the Br atoms has been reported to result in high exciton binding energies of ∼0.7 and ∼0.1 eV in monolayers and bulk, respectively. (22) These robust excitons pave the way for applications in optoelectronics and quantum information processing at room temperature. While clear photoluminescence signatures of exciton recombination were identified at a photon energy of 1.37 eV, (22) the actual size of the single-particle bandgap in CrSBr is still under debate. (23) Scanning tunneling spectroscopy (STS) measurements have estimated the bulk bandgap to amount to 1.5 ± 0.2 eV, (7,22) whereas the bandgap of monolayer CrSBr has only been predicted by theory. To selectively prepare and study unbound electron–hole pairs above the bandgap or excitons, pump–probe measurements with tunable excitation wavelengths are hence desirable.

Recent studies of CrSBr have primarily focused on controlling exciton-coupled magnons at radio frequencies using magnetic fields and cavity photons. (9−11) Meanwhile, the ultrafast dynamics of nonequilibrium electron–hole pairs has remained unexplored, to the best of our knowledge. Particularly, the potential impact of short-lived magnetic fluctuations, so-called paramagnons, (24,25) on exciton dynamics has not been considered yet. Terahertz (THz) time-domain spectroscopy (26) provides a unique tool to trace the ultrafast dielectric response of photoexcited electron–hole pairs. However, as the micron-scale lateral dimensions of typical exfoliated CrSBr monolayers are much smaller than the diffraction limit of THz pulses, subwavelength spatial resolution is a prerequisite for conclusive studies.

Here, we employ ultrafast polarization nanoscopy, (27,28) a technique based on time-resolved near-field microscopy (29−33) at THz frequencies, (34−37) to explore the dynamics of electron–hole pairs in paramagnetic CrSBr. Following optical excitation by tunable femtosecond laser pulses, THz probe fields directly trace the dynamics of continuum states and excitons regardless of interband selection rules. Our nanoscopic subcycle approach allows us to resolve the femtosecond decay and the dielectric response of unbound electron–hole pairs and quasi-one-dimensional excitons in bulk and atomically thin CrSBr, for the first time. Furthermore, we observe ultrafast relaxation of excitons in bulk CrSBr in which scattering with paramagnons could occur.

Figure 1a shows an optical micrograph of a typical CrSBr sample exfoliated on a SiO2 layer (thickness, 285 nm) fabricated on a p++-doped silicon substrate. The depicted area contains both bulk (yellow/blue) and monolayer (ML, faint blue) flakes, which, owing to the structural anisotropy of CrSBr, extend along the crystallographic a axis. The micrometer-sized lateral dimensions of the monolayers are much smaller than the diffraction limit of THz pulses. To overcome this mismatch, we couple phase-locked THz waveforms (blue) to the apex of a metallic tip of an atomic force microscope (Figure 1b and section 1 in the Supporting Information). The confined evanescent near field interacts with the sample in a nanoscopic area on the order of the tip’s radius of curvature. Therefore, the nanoscale dielectric response of the sample is imprinted in the scattered THz electric field, which we retrieve by electro-optic sampling (EOS) and demodulation of the signal at the oscillation frequency of the tip.

Figure 1

Figure 1. Probing ultrafast electron–hole pair dynamics in bulk and monolayer CrSBr by THz polarization nanoscopy. (a) Optical micrograph of a typical CrSBr sample including bulk (yellow, blue) and monolayer (ML, faint blue) flakes. The crystallographic a- and b-axes are indicated. (b) Schematic of the THz near-field spectroscopy technique. Optical pump pulses (red) tunable in photon energy generate electron–hole pairs in CrSBr. After a variable delay time, tp, a phase-locked THz probe transient, ETHz (blue, left), is coupled into the evanescent near field of a metallic tip. By phase-resolved detection of the scattered THz waveform (blue, right) information about the nanoscale dielectric function of the sample is obtained. (c) Polarization nanoscopy. Tunable pump pulses excite either excitons (red, hνp,X) or continuum states (green, hνp,c). The THz electric field (blue arrow) polarizes the electron–hole pairs with polarizability αX and αc, respectively. (d) Electro-optically detected steady-state scattered THz waveform, E (black line), and pump-induced change, ΔE (red line), of a bulk CrSBr flake at a pump delay, tp = 0.5 ps, as a function of the EOS time, tEOS.

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Moreover, the sample can be photoexcited with ultrashort optical pulses (pulse duration <100 fs) from a noncollinear optical parametric amplifier, allowing us to tune the pump photon energy, hνp, between 1.30 and 2.41 eV (section 2 in the Supporting Information). This enables us to selectively inject electron–hole pairs in excitonic or continuum states (Figure 1c, left). The THz electric near field features components along the highly polarizable b-axis (blue arrow), (27) inducing each electron–hole pair to carry a dipole moment, p, which is proportional to the polarizability, α. By analyzing the pump-induced change in the scattered THz field, ΔE, we can discriminate between highly polarizable unbound electron–hole pairs and excitons, whose polarizability is reduced by Coulomb binding (Figure 1c, right). Importantly, owing to the small pump spot size (∼5 μm), signal contributions from regions far away from the tip are excluded from ΔE. Figure 1d displays the electro-optically detected steady-state near-field response of a CrSBr bulk flake, E (black curve), together with a typical pump-induced change (red curve), recorded at a pump delay time tp = 0.5 ps after photoexcitation with hνp = 1.39 eV. ΔE roughly traces E with a phase shift of about π/3. The spectra of E and ΔE will be discussed later in the text.

To gain first insights into the lifetime of electron–hole pairs in bulk and monolayer CrSBr, we record the maximum pump-induced change of the scattered THz transient, which is proportional to the density, neh, and polarizability, α, of photoexcited electron–hole pairs, ΔEpeak ∝ nehα, as a function of tp. We set the pump polarization along the b-axis because the corresponding optical interband transitions are dipole-allowed in this direction, while they are forbidden along the a-axis. (14) Figure 2a shows the pump–probe dynamics of the THz near-field response (electro-optic delay time, tEOS = 0.15 ps) in bulk CrSBr (thickness, 400 nm) for a pump fluence Φp = 5 mJ cm–2 and photon energies 1.30 eV ≤ hνp ≤ 1.81 eV, spanning both the 1s exciton resonance at 1.37 eV and the reported bandgap of 1.5 ± 0.2 eV. To gauge the contribution per electron–hole pair, we divide ΔEpeak by the respective electron–hole pair density, neh, weighted with the finite probing depth of the THz near field (section 3 in the Supporting Information). Irrespective of hνp, the pump-induced signal grows abruptly upon photoexcitation, reaches its maximum at tp = 0.5 ps, and subsequently decays with biexponential behavior.

Figure 2

Figure 2. Femtosecond electron–hole pair dynamics in bulk and monolayer CrSBr. (a) Maximal pump-induced change of the near-field response at tEOS = 0.15 ps, ΔEpeak, of bulk CrSBr as a function of pump delay time, tp, for various excitation photon energies (solid lines). The decay of ΔEpeak is fitted with a biexponential function (dashed lines). The gray shaded region in the top panel shows the excess of the pump-induced signal at early delays with respect to lower excitation photon energies. The data are offset for clarity. (b) Analogous to (a) for the monolayer (ML) limit. Solid lines are calculated with a rate equation model. (c, d) Polarizability of the photoexcited electron–hole pairs as a function of the pump photon energy for bulk (purple circles) and monolayer (teal circles). For context, the low-temperature photoluminescence spectra (PL, solid lines) as well as bandgaps measured with STS (bulk, purple ribbon) (7) and predicted by GW calculations (ML, teal ribbon) (22) are shown.

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While we observe a slow decay time (1/e), τslow = 15 ± 3 ps common to all data sets, the fast initial decay strongly depends on hνp. For hνp = 1.39, 1.46, and 1.65 eV, we extract an initial decay time, τfast = 1.6 ± 0.3 ps. However, at hνp = 1.81 eV, this decay takes place faster within τfast = 1.0 ± 0.1 ps. Moreover, the amplitude of ΔEpeak is significantly higher (gray area) than that for lower hνp, suggesting that different species of electron–hole pairs are involved throughout the decay. For excitation below the bandgap, we expect to initially prepare hot excitons, which relax into the 1s ground state within about 1.6 ps. The thermalization may result from scattering with phonons or short-range spin correlations, that is, paramagnons. (24,25) We attribute the slow decay common to all photon energies to the recombination of 1s excitons with lifetimes of ∼15 ps. In contrast, the fast decay of the polarization signal at hνp = 1.81 eV is indicative of the formation of 1s excitons; when excitons from energetically more distant bands or unbound electron–hole pairs bind into 1s states, their polarizability is quenched by Coulomb attraction. We will investigate the different excitation scenarios in more detail with complementary measurements presented further below.

Proceeding from bulk to monolayer CrSBr, we estimate that depending on the excitation energy only a few tens to hundreds of electron–hole pairs are probed in the near-field volume of the tip. The excellent sensitivity of our setup enables us to still detect their femtosecond dynamics. We trace ΔEpeak as a function of tp for hνp = 1.30 eV (below the 1s state), 1.39, 1.46, 1.91 eV (above the 1s state), and 2.41 eV (above the calculated bandgap), as shown in Figure 2b. The applied pump fluences were restricted to Φp ≤ 2.5 mJ cm–2, which is safely below the damage threshold of the monolayer. At hνp = 1.30 eV, below the exciton resonance, no pump-induced change in the THz signal is detectable. In contrast, when the pump photon energy is sufficient to excite excitons, we see an abrupt increase of ΔEpeak followed by an ultrafast, subpicosecond decay. The observed dynamics can thus be assigned to the excitation and decay of excitons in monolayer CrSBr.

As ΔEpeak changes on time scales comparable to the duration of our pump and gate pulses (∼100 fs), for a quantitative investigation, we simulate the time evolution of ΔEpeak with a rate equation model comprising a source term given by the pump pulse and a subsequent exponential decay (section 4 in the Supporting Information). The model accurately reproduces the observed onset and decay (solid lines) and thus allows us to directly gauge the exciton lifetime in monolayer CrSBr, for the first time. The best agreement with the experimental data is achieved with an exciton lifetime of 0.5 ps, which is 30 times faster than the decay observed in the bulk. Due to the large oscillator strength of quasi-one-dimensional excitons, we expect an ultrashort radiative lifetime in the monolayer, which we estimate to be of the order of 1 ps (section 5 in the Supporting Information). Yet the decay is unaffected by the pump photon energy, suggesting an important contribution also from nonradiative recombination. As the dynamics are independent of the pump fluence, we can rule out Auger processes, leaving radiative recombination and recombination at defects and surface impurities as the most important decay channels.

Polarization excitation spectroscopy can reveal how the polarizability of the initially photoexcited electron–hole pairs depends on their binding state. To this end, the maximum of ΔEpeak at tp = 0.5 ps (bulk) and tp = 0.4 ps (monolayer), respectively, is traced as a function of hνp (Figure 2c,d). For comparison, the data are overlaid with the measured low-temperature photoluminescence spectrum of bulk and monolayer CrSBr as well as the bandgaps obtained from STS (bulk) and calculations (monolayer), respectively. The polarizability of the photoexcited bulk sample (Figure 2c, purple circles) is constant for 1.30 eV ≤ hνp ≤ 1.65 eV, whereas the polarizability dramatically increases for hνp = 1.81 eV, indicating a dominant contribution of electron–hole pairs exhibiting weak or no Coulomb binding. In the monolayer, the polarizability increases upon photoexcitation above the 1s ground state at a photon energy of 1.37 eV. After a plateau at hνp = 1.46 and 1.91 eV, the polarizability decreases by a factor of 3, when hνp is tuned to 2.41 eV. This reduction contrasts with the bulk polarizability spectrum and suggests the observation of a less polarizable, more strongly bound exciton originating from a lower valence band with higher effective masses, which has been theoretically predicted. (22)

While recording ΔEpeak provides helpful insights into the polarizability and the decay dynamics of photoexcited electron–hole pairs, a quantitative analysis of the binding states of the photoexcited electron–hole pairs calls for complete THz near-field spectroscopy. To this end, the scattered THz waveform is electro-optically sampled for various hνp and tp. We focus on excitation close to the 1s exciton resonance (hνp = 1.39 eV) and above the bandgap (1.81 eV). Based on the dynamics shown in Figure 2a, the observed ultrafast initial decay of ΔEpeak for hνp = 1.81 eV has been associated with the dynamics of electron–hole pairs binding into 1s excitons. To test this hypothesis, we compare the pump-induced near-field responses at tp = 0.5 and 2.5 ps, which would originate from unbound electron–hole pairs or excitons from energetically lower valence bands and 1s excitons, respectively. Figure 3a depicts the steady-state scattered near-field waveform, E (gray), and the pump-induced change, ΔE (Φp = 4 mJ cm–2), for hνp = 1.81 eV at tp = 0.5 ps (blue) and hνp = 1.39 eV at tp = 0.5 ps (red) as well as hνp = 1.81 eV at tp = 2.5 ps (dark blue). The first minimum and second maximum of all ΔE waveforms are located at zero crossings of the steady-state response. However, the transients clearly differ at positive tEOS, as highlighted in Figure 3b: both the minimum at tEOS = 0.45 ps and the maximum at tEOS = 0.70 ps are more strongly pronounced for hνp = 1.81 eV and tp = 0.5 ps compared to the other two waveforms, which share similar amplitudes.

Figure 3

Figure 3. Identifying species of photoexcited electron–hole pairs in bulk CrSBr by THz near-field spectroscopy. (a) Experimental pump-induced changes of the scattered THz waveform, ΔE, on bulk CrSBr as a function of the EOS time, tEOS, for different pump photon energies and pump delay times, tp. The steady-state near-field response is shown in gray. (b) Zoom-in to the EOS time window between 0.4 and 1 ps. (c) Time-domain near-field responses modeled with the finite-dipole model. (d–f) Relative spectral amplitude, ΔẼ/Ẽ (top left panels), and phase, Δϕ – ϕ (bottom left panels), of the near-field response. The modeled data (dashed lines) calculated with the dielectric functions either comprising a Drude term (d) or two Lorentzians (e, f), shown in the right panels, excellently reproduce the measurements (circles).

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This difference is characteristic of distinct changes in the dielectric function. To quantitatively connect the microscopic spectral response with the time-domain signatures observed in Figure 3a, we Fourier transform the waveforms ΔE and E and consider the corresponding relative spectral amplitude, ΔẼ/Ẽ (Figure 3d–f, top left panels), and phase, Δϕ – ϕ (Figure 3d–f, bottom left panels), in the spectral range of our THz probe pulse. The relative spectral amplitude for hνp = 1.81 eV and tp = 0.5 ps features a minimum around 1.6 THz and increases toward the spectral edges, while Δϕ – ϕ (Figure 3d, bottom left) monotonically increases with frequency. This response is markedly different from the two other cases (Figure 3e,f), where ΔẼ/Ẽ decreases monotonically with frequency, while Δϕ – ϕ shows no observable feature and stays around 0.3π.

We analyze how the dielectric response of the sample relates to the observed spectral characteristics by modeling near-field scattering off the photoexcited bulk CrSBr sample with the finite-dipole model (section 6 in the Supporting Information). The terahertz response for hνp = 1.81 eV and tp = 0.5 ps can be reproduced best (Figure 3d, dashed line) by a Drude dielectric function shown in Figure 3d (right panel). The modeled spectrum agrees well with the experimental data, indicating the dominant contribution of unbound electron–hole pairs after photoexcitation. In contrast, the Drude response fails to explain the pump-induced change for hνp = 1.81 eV at tp = 2.5 ps and hνp = 1.39 eV at tp = 0.5 ps (Figure 3e,f, left panels). Therefore, we model the corresponding dielectric function with two Lorentzians, where one oscillator represents the strong, off-resonantly probed 1s–2p transition expected at ∼14 THz. The second, low-energy resonator at 1 THz covers all intraexcitonic transitions from states with large principal quantum numbers (see Figure 3e,f, right panel). The pump-induced response yields excellent agreement with the experiment (Figure 3e,f, left panels). Moreover, in the modeled time-domain data (Figure 3c), the contrast between the Drude and the excitonic near-field response is evident and matches the relative amplitudes of the peaks around tEOS = 0.45 and 0.70 ps seen in the experiment (Figure 3b).

In the monolayer sample, effects of finite probing depths and interlayer tunneling are negligible, allowing us to reliably extract the complex nonequilibrium dielectric function in a model-free manner. We record E and ΔE on a monolayer flake at tp = 0.4 ps after resonant excitation of the exciton ground state (hνp = 1.39 eV) with a fluence Φp = 1.6 mJ cm–2 (Figure 4a). The pump-induced change ΔE is significantly delayed with respect to E. While the first minimum of ΔE is enhanced, the second minimum is suppressed. The relative spectral amplitude (Figure 4b, red circles) decreases monotonically, while Δϕ – ϕ (black circles) is mostly flat around 0.3π. From the spectral response of the photoexcited monolayer, we can directly retrieve its complex dielectric function, ε, for the first time, by inverting the finite-dipole model (section 6 in the Supporting Information). The only required assumption is the steady-state dielectric function, εeq, which we found to be constant within our probe spectrum (section 7 in the Supporting Information). For εeq = 10, (38) we obtain the nonequilibrium dielectric function depicted in Figure 4c. For frequencies below 1.2 THz, the real part of ε, εreal (teal circles), is increased by ∼30% compared to the steady state (gray dashed line). Above 1.2 THz, εreal is only slightly larger than εeq. The imaginary part, εimag (purple circles), considerably decreases toward higher frequencies.

Figure 4

Figure 4. Extracting the complex-valued dielectric function of a photoexcited CrSBr monolayer. (a) Experimental pump-induced change of the scattered THz waveform, ΔE (red), for a pump photon energy of 1.39 eV at tp = 0.4 ps. The steady-state near-field response, E, is shown in gray. (b) Relative spectral amplitude, ΔẼ/Ẽ (red circles), and phase, Δϕ – ϕ (black circles), of the near-field response. (c) Dielectric function obtained for an anisotropic Rytova–Keldysh confinement potential (solid lines) used to calculate the near-field response shown in (b) (solid lines). By numerically inverting the finite-dipole model, we retrieve the complex dielectric function, ε, of the photoexcited monolayer (circles). The assumed equilibrium dielectric function, εeq = 10, is shown as a gray dashed line.

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The increase of εreal and εimag toward smaller frequencies is reminiscent of a Lorentz oscillator near the low-frequency edge of our probe spectrum, which indicates transitions between highly excited excitons. To corroborate this hypothesis, we model the dielectric function of the nonequilibrium system by calculating the excitonic eigenstates and transition energies for a Rytova–Keldysh potential, considering the anisotropy of the effective mass (section 8 in the Supporting Information). Figure 4c depicts the real (teal line) and imaginary (purple line) parts of the modeled dielectric function. The 1s–2p transition manifests as a peak in εimag around 27 THz, while the transitions between the more narrowly spaced, higher-energy excitonic states are imprinted in the dielectric function as a steep increase of both εreal and εimag for decreasing frequency, reliably capturing the shape of the retrieved dielectric function. Lastly, calculating the spectral near-field response of the photoexcited monolayer using the modeled dielectric function (Figure 4b, solid lines) yields excellent agreement with the experimental spectra. These findings provide strong evidence that, at room temperature, the terahertz dielectric response of monolayer CrSBr is dominated by transitions between highly excited, quasi-one-dimensional exciton states.

In conclusion, we explored the ultrafast dynamics of tightly bound electron–hole pairs in the van der Waals magnet CrSBr. In the bulk, we observe an ultrafast relaxation of hot excitons, which may be related to scattering with phonons, defects, or paramagnons and an exciton lifetime of 15 ps. An ultrashort recombination on the time scale of 0.5 ps is revealed in the monolayer, representing the first direct access to the femtosecond dynamics of quasi-one-dimensional excitons in an atomically thin van der Waals magnet. Analyzing the near-field response of bulk CrSBr, we can distinguish the signatures of Coulomb-bound and unbound electron–hole pairs. Furthermore, the nonequilibrium dielectric response of a photoexcited monolayer features the spectral fingerprint of internal transitions between exciton states with high principal quantum numbers. In the future, our near-field spectroscopy approach may be harnessed to investigate the temporal and spectral signatures of coupling of excitons to the various magnetic phases of CrSBr, ultimately even allowing one to image magnetic domains and observe magnetic phase transitions on the nanoscale. Moreover, polarization nanoscopy poses an ideal probe for strain-induced modulations of electronic and magnetic order (39,40) as well as moiré-twisted van der Waals magnets, (41) which are promising candidates for next-generation spintronic devices.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.3c05010.

  • Ultrafast THz near-field spectroscopy setup, tunable optical pump pulses from a noncollinear optical parametric amplifier (NOPA), estimation of the electron–hole pair density, rate-equation model for the ultrafast dynamics of electron–hole pairs, estimating the radiative exciton lifetime in monolayer CrSBr, modeling the spectral near-field response, steady-state nanospectroscopy of monolayer CrSBr, modeling the nonequilibrium dielectric function of monolayer CrSBr, and dependence of the pump–probe dynamics on the pump polarization (PDF)

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Author Information

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  • Corresponding Authors
    • Florian Dirnberger - Institute of Applied Physics and Würzburg-Dresden Cluster of Excellence, Dresden University of Technology, 01187 Dresden, Germany;  Email: [email protected]
    • Rupert Huber - Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany;  Email: [email protected]
  • Authors
    • Christian Meineke - Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany;  Orcidhttps://orcid.org/0000-0003-0328-1816
    • Jakob Schlosser - Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
    • Martin Zizlsperger - Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
    • Marlene Liebich - Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
    • Niloufar Nilforoushan - Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
    • Kseniia Mosina - Department of Inorganic Chemistry, University of Chemistry and Technology Prague, 166 28 Prague 6, Czech Republic
    • Sophia Terres - Institute of Applied Physics and Würzburg-Dresden Cluster of Excellence, Dresden University of Technology, 01187 Dresden, Germany
    • Alexey Chernikov - Institute of Applied Physics and Würzburg-Dresden Cluster of Excellence, Dresden University of Technology, 01187 Dresden, Germany;  Orcidhttps://orcid.org/0000-0002-9213-2777
    • Zdenek Sofer - Department of Inorganic Chemistry, University of Chemistry and Technology Prague, 166 28 Prague 6, Czech Republic;  Orcidhttps://orcid.org/0000-0002-1391-4448
    • Markus A. Huber - Department of Physics and Regensburg Center for Ultrafast Nanoscopy (RUN), University of Regensburg, 93040 Regensburg, Germany
    • Matthias Florian - Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States
    • Mackillo Kira - Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States
  • Author Contributions

    C.M., F.D., and R.H. conceived the study. C.M., J.S., M.Z., and M.A.H. performed the ultrafast near-field experiments, analyzed the data, and calculated the finite-dipole scattering model. C.M., J.S., M.L., and N.N. calculated the exciton wave functions. S.T., A.C., and F.D. performed the low-temperature photoluminescence measurements. K.M. and Z.S. synthesized high-quality CrSBr crystals. All authors contributed to the discussions of the results. C.M., J.S., N.N., F.D., and R.H. wrote the manuscript with input from all authors.

  • Funding

    This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through Project ID 422 314695032-SFB 1277 (Subprojects A05 and B05) as well as project HU1598/8. N.N. acknowledges financial support by the Alexander von Humboldt Foundation. F.D., S.T., and A.C. gratefully acknowledge financial support from the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter ct.qmat (EXC 2147, Project-ID 390858490). M.F. and M.K. were supported by NSF DMREF award No. 2118809. Z.S. and K.M. were supported by ERC-CZ program (project LL2101) from Ministry of Education Youth and Sports (MEYS) and used large infrastructure from project reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

Click to copy section linkSection link copied!

We thank J. Hayes for helpful discussions and M. Furthmeier as well as I. Gronwald for technical assistance.

References

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This article references 41 other publications.

  1. 1
    Huang, B. Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit. Nature 2017, 546, 270– 273,  DOI: 10.1038/nature22391
    Google Scholar
    1
    Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit
    Huang, Bevin; Clark, Genevieve; Navarro-Moratalla, Efren; Klein, Dahlia R.; Cheng, Ran; Seyler, Kyle L.; Zhong, Ding; Schmidgall, Emma; McGuire, Michael A.; Cobden, David H.; Yao, Wang; Xiao, Di; Jarillo-Herrero, Pablo; Xu, Xiaodong
    Nature (London, United Kingdom) (2017), 546 (7657), 270-273CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Since the discovery of graphene, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent addns. include semiconductors with spin-valley coupling, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-d. waves, and topol. semimetals with edge transport. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered; such a crystal would be useful in many technologies from sensing to data storage. Theor., magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temps. by the Mermin-Wagner theorem. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temp. of 45 K is only slightly lower than that of the bulk crystal, 61 K, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent phys. properties typical of van der Waals crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect, whereas in trilayer CrI3 the interlayer ferromagnetism obsd. in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as elec. control for realizing magnetoelectronics, and van der Waals engineering to produce interface phenomena.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXps1KhsrY%253D&md5=7d05b09e4895aca19b6db4a9a157fb29
  2. 2
    Seyler, K. L. Ligand-field helical luminescence in a 2D ferromagnetic insulator. Nat. Phys. 2018, 14, 277– 281,  DOI: 10.1038/s41567-017-0006-7
    Google Scholar
    2
    Ligand-field helical luminescence in a 2D ferromagnetic insulator
    Seyler, Kyle L.; Zhong, Ding; Klein, Dahlia R.; Gao, Shiyuan; Zhang, Xiaoou; Huang, Bevin; Navarro-Moratalla, Efren; Yang, Li; Cobden, David H.; McGuire, Michael A.; Yao, Wang; Xiao, Di; Jarillo-Herrero, Pablo; Xu, Xiaodong
    Nature Physics (2018), 14 (3), 277-281CODEN: NPAHAX; ISSN:1745-2473. (Nature Research)
    Bulk chromium tri-iodide (CrI3) has long been known as a layered van der Waals ferromagnet1. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet2, providing a new platform for investigating light-matter interactions and magneto-optical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI3 under linearly polarized excitation, with helicity detd. by the monolayer magnetization direction. In contrast, the bilayer CrI3 photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI3 bilayers2. Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors3, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI3. We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr3+ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized MO nature.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhtFGktrzP&md5=f2df3c03459177d97ebb6013bcdfc1e7
  3. 3
    Burch, K. S.; Mandrus, D.; Park, J. G. Magnetism in two-dimensional van der Waals materials. Nature 2018, 563, 47– 52,  DOI: 10.1038/s41586-018-0631-z
    Google Scholar
    3
    Magnetism in two-dimensional van der Waals materials
    Burch, Kenneth S.; Mandrus, David; Park, Je-Geun
    Nature (London, United Kingdom) (2018), 563 (7729), 47-52CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    The discovery of materials has often introduced new phys. paradigms and enabled the development of novel devices. Two-dimensional magnetism, which is assocd. with strong intrinsic spin fluctuations, has long been the focus of fundamental questions in condensed matter physics regarding our understanding and control of new phases. Here we discuss magnetic van der Waals materials: two-dimensional at. crystals that contain magnetic elements and thus exhibit intrinsic magnetic properties. These cleavable materials provide the ideal platform for exploring magnetism in the two-dimensional limit, where new phys. phenomena are expected, and represent a substantial shift in our ability to control and investigate nanoscale phases. We present the theor. background and motivation for investigating this class of crystals, describe the material landscape and the current exptl. status of measurement techniques as well as devices, and discuss promising future directions for the study of magnetic van der Waals materials.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXitVKgs7fI&md5=35ec22875eb95985358c132661aa891f
  4. 4
    Kang, S. Coherent many-body exciton in van der Waals antiferromagnet NiPS3. Nature 2020, 583, 785– 789,  DOI: 10.1038/s41586-020-2520-5
    Google Scholar
    4
    Coherent many-body exciton in van der Waals antiferromagnet NiPS3
    Kang, Soonmin; Kim, Kangwon; Kim, Beom Hyun; Kim, Jonghyeon; Sim, Kyung Ik; Lee, Jae-Ung; Lee, Sungmin; Park, Kisoo; Yun, Seokhwan; Kim, Taehun; Nag, Abhishek; Walters, Andrew; Garcia-Fernandez, Mirian; Li, Jiemin; Chapon, Laurent; Zhou, Ke-Jin; Son, Young-Woo; Kim, Jae Hoon; Cheong, Hyeonsik; Park, Je-Geun
    Nature (London, United Kingdom) (2020), 583 (7818), 785-789CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Abstr.: An exciton is the bosonic quasiparticle of electron-hole pairs bound by the Coulomb interaction. Bose-Einstein condensation of this exciton state has long been the subject of speculation in various model systems2,3, and examples have been found more recently in optical lattices and two-dimensional materials4-9. Unlike these conventional excitons formed from extended Bloch states4-9, excitonic bound states from intrinsically many-body localized states are rare. Here we show that a spin-orbit-entangled exciton state appears below the Neel temp. of 150 K in NiPS3, an antiferromagnetic van der Waals material. It arises intrinsically from the archetypal many-body states of the Zhang-Rice singlet10,11, and reaches a coherent state assisted by the antiferromagnetic order. Using configuration-interaction theory, we det. the origin of the coherent excitonic excitation to be a transition from a Zhang-Rice triplet to a Zhang-Rice singlet. We combine three spectroscopic tools-resonant inelastic X-ray scattering, photoluminescence and optical absorption-to characterize the exciton and to demonstrate an extremely narrow excitonic linewidth below 50 K. The discovery of the spin-orbit-entangled exciton in antiferromagnetic NiPS3 introduces van der Waals magnets as a platform to study coherent many-body excitons.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhsVWgt7vJ&md5=94d6f3b7df1d3550e6810fea9b5d30da
  5. 5
    Hwangbo, K. Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator. Nat. Nanotechnol. 2021, 16, 655– 660,  DOI: 10.1038/s41565-021-00873-9
    Google Scholar
    5
    Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator
    Hwangbo, Kyle; Zhang, Qi; Jiang, Qianni; Wang, Yong; Fonseca, Jordan; Wang, Chong; Diederich, Geoffrey M.; Gamelin, Daniel R.; Xiao, Di; Chu, Jiun-Haw; Yao, Wang; Xu, Xiaodong
    Nature Nanotechnology (2021), 16 (6), 655-660CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
    Two-dimensional (2D) semiconductors enable the investigation of light-matter interactions in low dimensions1,2. Yet, the study of elementary photoexcitations in 2D semiconductors with intrinsic magnetic order remains a challenge due to the lack of suitable materials3,4. Here, we report the observation of excitons coupled to zigzag antiferromagnetic order in the layered antiferromagnetic insulator NiPS3. The exciton exhibits a narrow photoluminescence linewidth of roughly 350μeV with near-unity linear polarization. When we reduce the sample thickness from five to two layers, the photoluminescence is suppressed and eventually vanishes for the monolayer. This suppression is consistent with the calcd. bandgap of NiPS3, which is highly indirect for both the bilayer and the monolayer5. Furthermore, we observe strong linear dichroism (LD) over a broad spectral range. The optical anisotropy axes of LD and of photoluminescence are locked to the zigzag direction. Furthermore, their temp. dependence is reminiscent of the in-plane magnetic susceptibility anisotropy. Hence, our results indicate that LD and photoluminescence could probe the symmetry breaking magnetic order parameter of 2D magnetic materials. In addn., we observe over ten exciton-A1g-phonon bound states on the high-energy side of the exciton resonance, which we interpret as signs of a strong modulation of the ligand-to-metal charge-transfer energy by electron-lattice interactions. Our work establishes NiPS3 as a 2D platform for exploring magneto-exciton physics with strong correlations.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXmtlGgtrk%253D&md5=28e27572a00330ff85bdcbd63a947f7c
  6. 6
    Wang, Q. H. The Magnetic Genome of Two-Dimensional van der Waals Materials. ACS Nano 2022, 16, 6960– 7079,  DOI: 10.1021/acsnano.1c09150
    Google Scholar
    6
    The Magnetic Genome of Two-Dimensional van der Waals Materials
    Wang, Qing Hua; Bedoya-Pinto, Amilcar; Blei, Mark; Dismukes, Avalon H.; Hamo, Assaf; Jenkins, Sarah; Koperski, Maciej; Liu, Yu; Sun, Qi-Chao; Telford, Evan J.; Kim, Hyun Ho; Augustin, Mathias; Vool, Uri; Yin, Jia-Xin; Li, Lu Hua; Falin, Alexey; Dean, Cory R.; Casanova, Felix; Evans, Richard F. L.; Chshiev, Mairbek; Mishchenko, Artem; Petrovic, Cedomir; He, Rui; Zhao, Liuyan; Tsen, Adam W.; Gerardot, Brian D.; Brotons-Gisbert, Mauro; Guguchia, Zurab; Roy, Xavier; Tongay, Sefaattin; Wang, Ziwei; Hasan, M. Zahid; Wrachtrup, Joerg; Yacoby, Amir; Fert, Albert; Parkin, Stuart; Novoselov, Kostya S.; Dai, Pengcheng; Balicas, Luis; Santos, Elton J. G.
    ACS Nano (2022), 16 (5), 6960-7079CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A review. Magnetism in two-dimensional (2D) van der Waals (vdW) materials has recently emerged as one of the most promising areas in condensed matter research, with many exciting emerging properties and significant potential for applications ranging from topol. magnonics to low-power spintronics, quantum computing and optical communications. In the short time since their discovery, 2D magnets have blossomed into a rich area for investigation, where fundamental concepts in magnetism are challenged by the behavior that spins can develop at the single layer limit. However, much effort is still needed in different fronts before 2D magnets can be routinely used for practical implementations. In this comprehensive review, prominent authors with expertise in complementary fields (i.e., synthesis, device engineering, magneto-optics, imaging, transport, mechanics, spin excitations, and theory and simulations) of 2D magnetism have joined together to provide a genome of current knowledge and a guideline for developments in 2D magnetic materials research.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhtVentLvP&md5=2963373a52d7c0a4baea02ba334d7065
  7. 7
    Telford, E. J. Layered Antiferromagnetism Induces Large Negative Magnetoresistance in the van der Waals Semiconductor CrSBr. Adv. Mater. 2020, 32, 2003240  DOI: 10.1002/adma.202003240
    Google Scholar
    7
    Layered Antiferromagnetism Induces Large Negative Magnetoresistance in the van der Waals Semiconductor CrSBr
    Telford, Evan J.; Dismukes, Avalon H.; Lee, Kihong; Cheng, Minghao; Wieteska, Andrew; Bartholomew, Amymarie K.; Chen, Yu-Sheng; Xu, Xiaodong; Pasupathy, Abhay N.; Zhu, Xiaoyang; Dean, Cory R.; Roy, Xavier
    Advanced Materials (Weinheim, Germany) (2020), 32 (37), 2003240CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The recent discovery of magnetism within the family of exfoliatable van der Waals (vdW) compds. has attracted considerable interest in these materials for both fundamental research and technol. applications. However, current vdW magnets are limited by their extreme sensitivity to air, low ordering temps., and poor charge transport properties. Here the magnetic and electronic properties of CrSBr are reported, an air-stable vdW antiferromagnetic semiconductor that readily cleaves perpendicular to the stacking axis. Below its Neel temp., TN = 132 ± 1 K, CrSBr adopts an A-type antiferromagnetic structure with each individual layer ferromagnetically ordered internally and the layers coupled antiferromagnetically along the stacking direction. Scanning tunneling spectroscopy and photoluminescence (PL) reveal that the electronic gap is ΔE = 1.5 ± 0.2 eV with a corresponding PL peak centered at 1.25 ± 0.07 eV. Using magnetotransport measurements, strong coupling between magnetic order and transport properties in CrSBr is demonstrated, leading to a large neg. magnetoresistance response that is unique among vdW materials. These findings establish CrSBr as a promising material platform for increasing the applicability of vdW magnets to the field of spin-based electronics.
    >> More from SciFinder ®
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  8. 8
    Dirnberger, F. Spin-correlated exciton–polaritons in a van der Waals magnet. Nat. Nanotechnol. 2022, 17, 1060– 1064,  DOI: 10.1038/s41565-022-01204-2
    Google Scholar
    8
    Spin-correlated exciton-polaritons in a van der Waals magnet
    Dirnberger, Florian; Bushati, Rezlind; Datta, Biswajit; Kumar, Ajesh; MacDonald, Allan H.; Baldini, Edoardo; Menon, Vinod M.
    Nature Nanotechnology (2022), 17 (10), 1060-1064CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
    Strong coupling between light and elementary excitations is emerging as a powerful tool to engineer the properties of solid-state systems. Spin-correlated excitations that couple strongly to optical cavities promise control over collective quantum phenomena such as magnetic phase transitions, but their suitable electronic resonances are yet to be found. Here, we report strong light-matter coupling in NiPS3, a van der Waals antiferromagnet with highly correlated electronic degrees of freedom. A previously unobserved class of polaritonic quasiparticles emerges from the strong coupling between its spin-correlated excitons and the photons inside a microcavity. Detailed spectroscopic anal. in conjunction with a microscopic theory provides unique insights into the origin and interactions of these exotic magnetically coupled excitations. Our work introduces van der Waals magnets to the field of strong light-matter physics and provides a path towards the design and control of correlated electron systems via cavity quantum electrodynamics.
    >> More from SciFinder ®
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  9. 9
    Bae, Y. J. Exciton-coupled coherent magnons in a 2D semiconductor. Nature 2022, 609, 282– 286,  DOI: 10.1038/s41586-022-05024-1
    Google Scholar
    9
    Exciton-coupled coherent magnons in a 2D semiconductor
    Bae, Youn Jue; Wang, Jue; Scheie, Allen; Xu, Junwen; Chica, Daniel G.; Diederich, Geoffrey M.; Cenker, John; Ziebel, Michael E.; Bai, Yusong; Ren, Haowen; Dean, Cory R.; Delor, Milan; Xu, Xiaodong; Roy, Xavier; Kent, Andrew D.; Zhu, Xiaoyang
    Nature (London, United Kingdom) (2022), 609 (7926), 282-286CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
    The recent discoveries of two-dimensional (2D) magnets1-6 and their stacking into van der Waals structures7-11 have expanded the horizon of 2D phenomena. One exciting application is to exploit coherent magnons12 as energy-efficient information carriers in spintronics and magnonics13,14 or as interconnects in hybrid quantum systems15-17. A particular opportunity arises when a 2D magnet is also a semiconductor, as reported recently for CrSBr (refs. 18-20) and NiPS3 (refs. 21-23) that feature both tightly bound excitons with a large oscillator strength and potentially long-lived coherent magnons owing to the bandgap and spatial confinement. Although magnons and excitons are energetically mismatched by orders of magnitude, their coupling can lead to efficient optical access to spin information. Here we report strong magnon-exciton coupling in the 2D A-type antiferromagnetic semiconductor CrSBr. Coherent magnons launched by above-gap excitation modulate the exciton energies. Time-resolved exciton sensing reveals magnons that can coherently travel beyond seven micrometres, with a coherence time of above five nanoseconds. We observe these exciton-coupled coherent magnons in both even and odd nos. of layers, with and without compensated magnetization, down to the bilayer limit. Given the versatility of van der Waals heterostructures, these coherent 2D magnons may be a basis for optically accessible spintronics, magnonics and quantum interconnects.
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  10. 10
    Diederich, G. M. Tunable interaction between excitons and hybridized magnons in a layered semiconductor. Nat. Nanotechnol. 2023, 18, 23– 28,  DOI: 10.1038/s41565-022-01259-1
    Google Scholar
    10
    Tunable interaction between excitons and hybridized magnons in a layered semiconductor
    Diederich, Geoffrey M.; Cenker, John; Ren, Yafei; Fonseca, Jordan; Chica, Daniel G.; Bae, Youn Jue; Zhu, Xiaoyang; Roy, Xavier; Cao, Ting; Xiao, Di; Xu, Xiaodong
    Nature Nanotechnology (2023), 18 (1), 23-28CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
    The interaction between distinct excitations in solids is of both fundamental interest and technol. importance. One such interaction is the coupling between an exciton, a Coulomb bound electron-hole pair, and a magnon, a collective spin excitation. The recent emergence of van der Waals magnetic semiconductors1 provides a platform to explore these exciton-magnon interactions and their fundamental properties, such as strong correlation2, as well as their photospintronic and quantum transduction3 applications. Here we demonstrate the precise control of coherent exciton-magnon interactions in the layered magnetic semiconductor CrSBr. We varied the direction of an applied magnetic field relative to the crystal axes, and thus the rotational symmetry of the magnetic system4. Thereby, we tuned not only the exciton coupling to the bright magnon, but also to an optically dark mode via magnon-magnon hybridization. We further modulated the exciton-magnon coupling and the assocd. magnon dispersion curves through the application of uniaxial strain. At a crit. strain, a dispersionless dark magnon band emerged. Our results demonstrate an unprecedented level of control of the opto-mech.-magnonic coupling, and a step towards the predictable and controllable implementation of hybrid quantum magnonics5-11.
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  11. 11
    Dirnberger, F. Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons. Nature 2023, 620, 533– 538,  DOI: 10.1038/s41586-023-06275-2
    Google Scholar
    11
    Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons
    Dirnberger, Florian; Quan, Jiamin; Bushati, Rezlind; Diederich, Geoffrey M.; Florian, Matthias; Klein, Julian; Mosina, Kseniia; Sofer, Zdenek; Xu, Xiaodong; Kamra, Akashdeep; Garcia-Vidal, Francisco J.; Alu, Andrea; Menon, Vinod M.
    Nature (London, United Kingdom) (2023), 620 (7974), 533-537CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
    The magnetooptical properties of a van der Waals magnet that supports strong coupling of photons and excitons even in the absence of external cavity mirrors was studied. In this material-the layered magnetic semiconductor CrSBr-emergent light-matter hybrids called polaritons substantially increase the spectral bandwidth of correlations between the magnetic, electronic and optical properties, enabling largely tunable optical responses to applied magnetic fields and magnons. The results highlight the importance of exciton-photon self-hybridization in van der Waals magnets and motivate novel directions for the manipulation of quantum material properties by strong light-matter coupling.
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  12. 12
    Lee, K. Magnetic Order and Symmetry in the 2D Semiconductor CrSBr. Nano Lett. 2021, 21, 3511– 3517,  DOI: 10.1021/acs.nanolett.1c00219
    Google Scholar
    12
    Magnetic Order and Symmetry in the 2D Semiconductor CrSBr
    Lee, Kihong; Dismukes, Avalon H.; Telford, Evan J.; Wiscons, Ren A.; Wang, Jue; Xu, Xiaodong; Nuckolls, Colin; Dean, Cory R.; Roy, Xavier; Zhu, Xiaoyang
    Nano Letters (2021), 21 (8), 3511-3517CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The advent of two-dimensional (2D) magnets offers unprecedented control over electrons and spins. A key factor in detg. exchange coupling and magnetic order is symmetry. Here, we apply second harmonic generation (SHG) to probe a 2D magnetic semiconductor CrSBr. We find that monolayers are ferromagnetically ordered below 146 K, an observation enabled by the discovery of a large magnetic dipole SHG effect in the centrosym. structure. In multilayers, the ferromagnetic monolayers are coupled antiferromagnetically, and in contrast to other 2D magnets, the Neel temp. of CrSBr increases with decreasing layer no. We identify magnetic dipole and magnetic toroidal moments as order parameters of the ferromagnetic monolayer and antiferromagnetic bilayer, resp. These findings establish CrSBr as an exciting 2D magnetic semiconductor and extend the SHG probe of magnetic symmetry to the monolayer limit, opening the door to exploring the applications of magnetic-electronic coupling and the magnetic toroidal moment.
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  13. 13
    Rizzo, D. J. Visualizing Atomically Layered Magnetism in CrSBr. Adv. Mater. 2022, 34, 2201000  DOI: 10.1002/adma.202201000
    Google Scholar
    13
    Visualizing Atomically Layered Magnetism in CrSBr
    Rizzo, Daniel J.; McLeod, Alexander S.; Carnahan, Caitlin; Telford, Evan J.; Dismukes, Avalon H.; Wiscons, Ren A.; Dong, Yinan; Nuckolls, Colin; Dean, Cory R.; Pasupathy, Abhay N.; Roy, Xavier; Xiao, Di; Basov, D. N.
    Advanced Materials (Weinheim, Germany) (2022), 34 (27), 2201000CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    2D materials can host long-range magnetic order in the presence of underlying magnetic anisotropy. The ability to realize the full potential of 2D magnets necessitates systematic investigation of the role of individual at. layers and nanoscale inhomogeneity (i.e., strain) on the emergence of stable magnetic phases. Here, spatially dependent magnetism in few-layer CrSBr is revealed using magnetic force microscopy (MFM) and Monte Carlo-based simulations. Nanoscale visualization of the magnetic sheet susceptibility is extd. from MFM data and force-distance curves, revealing a characteristic onset of both intra- and interlayer magnetic correlations as a function of temp. and layer-thickness. These results demonstrate that the presence of a single uncompensated layer in odd-layer terraces significantly reduces the stability of the low-temp. antiferromagnetic (AFM) phase and gives rise to multiple coexisting magnetic ground states at temps. close to the bulk Neel temp. (TN). Furthermore, the AFM phase can be reliably suppressed using modest fields (∼16 mT) from the MFM probe, behaving as a nanoscale magnetic switch. This prototypical study of few-layer CrSBr demonstrates the crit. role of layer parity on field-tunable 2D magnetism and validates MFM for use in nanomagnetometry of 2D materials (despite the ubiquitous absence of bulk zero-field magnetism in magnetized sheets).
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  14. 14
    Wilson, N. P. Interlayer electronic coupling on demand in a 2D magnetic semiconductor. Nat. Mater. 2021, 20, 1657– 1662,  DOI: 10.1038/s41563-021-01070-8
    Google Scholar
    14
    Interlayer electronic coupling on demand in a 2D magnetic semiconductor
    Wilson, Nathan P.; Lee, Kihong; Cenker, John; Xie, Kaichen; Dismukes, Avalon H.; Telford, Evan J.; Fonseca, Jordan; Sivakumar, Shivesh; Dean, Cory; Cao, Ting; Roy, Xavier; Xu, Xiaodong; Zhu, Xiaoyang
    Nature Materials (2021), 20 (12), 1657-1662CODEN: NMAACR; ISSN:1476-1122. (Nature Portfolio)
    When monolayers of two-dimensional (2D) materials are stacked into van der Waals structures, interlayer electronic coupling can introduce entirely new properties, as exemplified by recent discoveries of moire bands that host highly correlated electronic states and quantum dot-like interlayer exciton lattices. Here we show the magnetic control of interlayer electronic coupling, as manifested in tunable excitonic transitions, in an A-type antiferromagnetic 2D semiconductor CrSBr. Excitonic transitions in bilayers and above can be drastically changed when the magnetic order is switched from the layered antiferromagnetic ground state to a field-induced ferromagnetic state, an effect attributed to the spin-allowed interlayer hybridization of electron and hole orbitals in the latter, as revealed by Green's function-Bethe-Salpeter equation (GW-BSE) calcns. Our work uncovers a magnetic approach to engineer electronic and excitonic effects in layered magnetic semiconductors.
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  15. 15
    Klein, J. Control of structure and spin texture in the van der Waals layered magnet CrSBr. Nat. Commun. 2022, 13, 5420,  DOI: 10.1038/s41467-022-32737-8
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    15
    Control of structure and spin texture in the van der Waals layered magnet CrSBr
    Klein, J.; Pham, T.; Thomsen, J. D.; Curtis, J. B.; Denneulin, T.; Lorke, M.; Florian, M.; Steinhoff, A.; Wiscons, R. A.; Luxa, J.; Sofer, Z.; Jahnke, F.; Narang, P.; Ross, F. M.
    Nature Communications (2022), 13 (1), 5420CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
    Abstr.: Controlling magnetism at nanometer length scales is essential for realizing high-performance spintronic, magneto-elec. and topol. devices and creating on-demand spin Hamiltonians probing fundamental concepts in physics. Van der Waals (vdW)-bonded layered magnets offer exceptional opportunities for such spin texture engineering. Here, we demonstrate nanoscale structural control in the layered magnet CrSBr with the potential to create spin patterns without the environmental sensitivity that has hindered such manipulations in other vdW magnets. We drive a local phase transformation using an electron beam that moves atoms and exchanges bond directions, effectively creating regions that have vertical vdW layers embedded within the initial horizontally vdW bonded exfoliated flakes. We calc. that the newly formed two-dimensional structure is ferromagnetically ordered in-plane with an energy gap in the visible spectrum, and weak antiferromagnetism between the planes, suggesting possibilities for creating spin textures and quantum magnetic phases.
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  16. 16
    Liu, W. A Three-Stage Magnetic Phase Transition Revealed in Ultrahigh-Quality van der Waals Bulk Magnet CrSBr. ACS Nano 2022, 16, 15917– 15926,  DOI: 10.1021/acsnano.2c02896
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    16
    A Three-Stage Magnetic Phase Transition Revealed in Ultrahigh-Quality van der Waals Bulk Magnet CrSBr
    Liu, Wenhao; Guo, Xiaoyu; Schwartz, Jonathan; Xie, Hongchao; Dhale, Nikhil Uday; Sung, Suk Hyun; Kondusamy, Aswin Lakshmi Narayanan; Wang, Xiqu; Zhao, Haonan; Berman, Diana; Hovden, Robert; Zhao, Liuyan; Lv, Bing
    ACS Nano (2022), 16 (10), 15917-15926CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Van der Waals (vdW) magnets are receiving ever-growing attention nowadays due to their significance in both fundamental research on low-dimensional magnetism and potential applications in spintronic devices. The high cryst. quality of vdW magnets is the key to maintaining intrinsic magnetic and electronic properties, esp. when exfoliated down to the two-dimensional limit. Here, ultrahigh-quality air-stable vdW CrSBr crystals are synthesized using the direct solid-vapor synthesis method. The high single crystallinity and spatial homogeneity have been thoroughly evidenced at length scales from submm to at. resoln. by X-ray diffraction, second harmonic generation, and scanning transmission electron microscopy. More importantly, sp. heat measurements of ultrahigh-quality CrSBr crystals show three thermodn. anomalies at 185, 156, and 132 K, revealing a stage-by-stage development of the magnetic order upon cooling, which is also corroborated with the magnetization and transport results. Our ultrahigh-quality CrSBr can further be exfoliated down to monolayers and bilayers easily, providing the building blocks of heterostructures for spintronic and magneto-optoelectronic applications.
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  17. 17
    Xu, X. Strong Spin-Phonon Coupling in Two-Dimensional Magnetic Semiconductor CrSBr. J. Phys. Chem. C 2022, 126, 10574– 10583,  DOI: 10.1021/acs.jpcc.2c02742
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    17
    Strong Spin-Phonon Coupling in Two-Dimensional Magnetic Semiconductor CrSBr
    Xu, Xiaomin; Wang, Xiaohu; Chang, Pu; Chen, Xiaoyu; Guan, Lixiu; Tao, Junguang
    Journal of Physical Chemistry C (2022), 126 (25), 10574-10583CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Recently, spin-phonon coupling (SPC) has gained considerable attention esp. in two-dimensional (2D) materials. Herein, d.-functional theory is used to investigate the SPC effect in CrSBr, a recently fabricated 2D magnetic semiconductor. It is found that the phonon vibrations are strongly dependent on the spin ordering. The breaking of magnetic symmetry changes phonon spectrum dependency obviously. In particular, the SPC const. is found to be 20.2 cm-1, which is one order of magnitude larger than that of most other 2D materials. The group velocity and Gr.ovrddot.uneisen const. in the ferromagnetic (FM) state are increased by ~ 23 and ~ 16% than that in the antiferromagnetic state. Furthermore, the thermal cond. is enhanced by ~ 43% for FM spin ordering because of stronger lattice anharmonicity. The Curie temp. of the system can be tuned ~ 30% by lattice deformation because of the strong SPC. Our work provides fundamental insights into the SPC effect on the CrSBr monolayer and sheds light on its potential for novel spintronic application.
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  18. 18
    Wu, F. Quasi-1D Electronic Transport in a 2D Magnetic Semiconductor. Adv. Mater. 2022, 34, 2109759  DOI: 10.1002/adma.202109759
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    18
    Quasi-1D Electronic Transport in a 2D Magnetic Semiconductor
    Wu, Fan; Gutierrez-Lezama, Ignacio; Lopez-Paz, Sara A.; Gibertini, Marco; Watanabe, Kenji; Taniguchi, Takashi; von Rohr, Fabian O.; Ubrig, Nicolas; Morpurgo, Alberto F.
    Advanced Materials (Weinheim, Germany) (2022), 34 (16), 2109759CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Electronic transport through exfoliated multilayers of CrSBr, a 2D semiconductor of interest because of its magnetic properties, is investigated. An extremely pronounced anisotropy manifesting itself in qual. and quant. differences of all quantities measured along the in-plane a and b crystallog. directions is found. In particular, a qual. different dependence of the conductivities σa and σb on temp. and gate voltage, accompanied by orders of magnitude differences in their values (σb/σa 3 x 102 to 105 at low temp. and neg. gate voltage) are obsd., together with a different behavior of the longitudinal magnetoresistance in the two directions and the complete absence of the Hall effect in transverse resistance measurements. These observations appear not to be compatible with a description in terms of conventional band transport of a 2D doped semiconductor. The obsd. phenomenol.-and unambiguous signatures of a 1D van Hove singularity detected in energy-resolved photocurrent measurements-indicate that electronic transport through CrSBr multilayers is better interpreted by considering the system as formed by weakly and incoherently coupled 1D wires, than by conventional 2D band transport. It is concluded that CrSBr is the first 2D semiconductor to show distinctly quasi-1D electronic transport properties.
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  19. 19
    Klein, J. Sensing the Local Magnetic Environment through Optically Active Defects in a Layered Magnetic Semiconductor. ACS Nano 2023, 17, 288– 299,  DOI: 10.1021/acsnano.2c07655
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    19
    Sensing the Local Magnetic Environment through Optically Active Defects in a Layered Magnetic Semiconductor
    Klein, Julian; Song, Zhigang; Pingault, Benjamin; Dirnberger, Florian; Chi, Hang; Curtis, Jonathan B.; Dana, Rami; Bushati, Rezlind; Quan, Jiamin; Dekanovsky, Lukas; Sofer, Zdenek; Alu, Andrea; Menon, Vinod M.; Moodera, Jagadeesh S.; Loncar, Marko; Narang, Prineha; Ross, Frances M.
    ACS Nano (2023), 17 (1), 288-299CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Atomic-level defects in van der Waals (vdW) materials are essential building blocks for quantum technologies and quantum sensing applications. The layered magnetic semiconductor CrSBr is an outstanding candidate for exploring optically active defects because of a direct gap, in addn. to a rich magnetic phase diagram, including a recently hypothesized defect-induced magnetic order at low temp. Here, we show optically active defects in CrSBr that are probes of the local magnetic environment. We observe a spectrally narrow (1 meV) defect emission in CrSBr that is correlated with both the bulk magnetic order and an addnl. low-temp., defect-induced magnetic order. We elucidate the origin of this magnetic order in the context of local and nonlocal exchange coupling effects. Our work establishes vdW magnets like CrSBr as an exceptional platform to optically study defects that are correlated with the magnetic lattice. We anticipate that controlled defect creation allows for tailor-made complex magnetic textures and phases with direct optical access.
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  20. 20
    Torres, K. Probing Defects and Spin-Phonon Coupling in CrSBr via Resonant Raman Scattering. Adv. Funct. Mater. 2023, 33, 2211366  DOI: 10.1002/adfm.202211366
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    20
    Probing Defects and Spin-Phonon Coupling in CrSBr via Resonant Raman Scattering
    Torres, Kierstin; Kuc, Agnieszka; Maschio, Lorenzo; Pham, Thang; Reidy, Kate; Dekanovsky, Lukas; Sofer, Zdenek; Ross, Frances M.; Klein, Julian
    Advanced Functional Materials (2023), 33 (12), 2211366CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Understanding the stability limitations and defect formation mechanisms in 2D magnets is essential for their utilization in spintronic and memory technologies. Here, defects in mono- to multilayer CrSBr are correlated with structural, vibrational, and magnetic properties. Resonant Raman scattering is used to reveal distinct vibrational defect signatures. In pristine CrSBr, it is shown that bromine atoms mediate vibrational interlayer coupling, allowing for distinguishing between surface and bulk defect modes. Environmental exposure is shown to cause drastic degrdn. in monolayers, with the formation of intralayer defects. This is in contrast to multilayers that predominantly show bromine surface defects. Through deliberate ion irradn., the formation of defect modes is tuned: these are strongly polarized and resonantly enhanced, reflecting the quasi--1D electronic character of CrSBr. Strikingly, pronounced signatures of spin-phonon coupling of the intrinsic phonon modes and the ion beam-induced defect modes are obsd. throughout the magnetic transition temp. Overall, defect engineering of magnetic properties is possible, with resonant Raman spectroscopy serving as a direct fingerprint of magnetic phases and defects in CrSBr.
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  21. 21
    Pawbake, A. Raman scattering signatures of strong spin-phonon coupling in the bulk magnetic van der Waals material CrSBr. Phys. Rev. B 2023, 107, 075421  DOI: 10.1103/PhysRevB.107.075421
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    21
    Raman scattering signatures of strong spin-phonon coupling in the bulk magnetic van der Waals material CrSBr
    Pawbake, Amit; Pelini, Thomas; Wilson, Nathan P.; Mosina, Kseniia; Sofer, Zdenek; Heid, Rolf; Faugeras, Clement
    Physical Review B (2023), 107 (7), 075421CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Magnetic excitations in layered magnetic materials that can be thinned down to the two-dimensional (2D) monolayer limit are of great interest from a fundamental point of view and for applications. Raman scattering has played a crucial role in exploring the properties of magnetic layered materials and, even though it is essentially a probe of lattice vibrations, it can reflect magnetic ordering in solids through the spin-phonon interaction or through the observation of magnon excitations. In bulk CrSBr, a layered A-type antiferromagnet (AF), we show that the magnetic ordering can be directly obsd. in the temp. dependence of the Raman scattering response (i) through the variations of the scattered intensities, (ii) through the activation of new phonon lines reflecting the change of symmetry with the appearance of the addnl. magnetic periodicity, and (iii) through the observation below the Neel temp. (TN) of second-order Raman scattering processes. We addnl. show that the three different magnetic phases encountered in CrSBr, including the recently identified low-temp. phase, have a particular Raman scattering signature. This work demonstrates that magnetic ordering can be obsd. directly in the Raman scattering response of bulk CrSBr with in-plane magnetization and that it can provide unique insight into the magnetic phases encountered in magnetic layered materials.
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  22. 22
    Klein, J. The Bulk van der Waals Layered Magnet CrSBr is a Quasi-1D Material. ACS Nano 2023, 17, 5316– 5328,  DOI: 10.1021/acsnano.2c07316
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    22
    The Bulk van der Waals Layered Magnet CrSBr is a Quasi-1D Material
    Klein, Julian; Pingault, Benjamin; Florian, Matthias; Heissenbuttel, Marie-Christin; Steinhoff, Alexander; Song, Zhigang; Torres, Kierstin; Dirnberger, Florian; Curtis, Jonathan B.; Weile, Mads; Penn, Aubrey; Deilmann, Thorsten; Dana, Rami; Bushati, Rezlind; Quan, Jiamin; Luxa, Jan; Sofer, Zdenek; Alu, Andrea; Menon, Vinod M.; Wurstbauer, Ursula; Rohlfing, Michael; Narang, Prineha; Loncar, Marko; Ross, Frances M.
    ACS Nano (2023), 17 (6), 5316-5328CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for the study of fundamental interactions and excitations, such as Tomonaga-Luttinger liqs. and topol. orders and defects with properties completely different from the quasiparticles expected in their higher-dimensional counterparts. However, clean 1D electronic systems are difficult to realize exptl., particularly for magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr-S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielec. screening, with an effective electron mass ratio of mXe/mYe ~ 50. This extreme anisotropy exptl. manifests in strong electron-phonon and exciton-phonon interactions, a Peierls-like structural instability, and a Fano resonance from a van Hove singularity of similar strength to that of metallic carbon nanotubes. Moreover, because of the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an exptl. attractive candidate for the study of exotic exciton and 1D-correlated many-body physics in the presence of magnetic order.
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  23. 23
    Bianchi, M. Paramagnetic Electronic Structure of CrSBr: Comparison between Ab Initio GW Theory and Angle-Resolved Photoemission Spectroscopy. Phys. Rev. B 2023, 107, 235107  DOI: 10.1103/PhysRevB.107.235107
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    There is no corresponding record for this reference.
  24. 24
    Zheng, Y. Paramagnon drag in high thermoelectric figure of merit Li-doped MnTe. Sci. Adv. 2019, 5, eaat9461  DOI: 10.1126/sciadv.aat9461
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    There is no corresponding record for this reference.
  25. 25
    Wang, L. Paramagnons and high-temperature superconductivity in a model family of cuprates. Nat. Commun. 2022, 13, 3163,  DOI: 10.1038/s41467-022-30918-z
    Google Scholar
    25
    Paramagnons and high-temperature superconductivity in a model family of cuprates
    Wang, Lichen; He, Guanhong; Yang, Zichen; Garcia-Fernandez, Mirian; Nag, Abhishek; Zhou, Kejin; Minola, Matteo; Tacon, Matthieu Le; Keimer, Bernhard; Peng, Yingying; Li, Yuan
    Nature Communications (2022), 13 (1), 3163CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
    Cuprate superconductors have the highest crit. temps. (Tc) at ambient pressure, yet a consensus on the superconducting mechanism remains to be established. Finding an empirical parameter that limits the highest reachable Tc can provide crucial insight into this outstanding problem. Here, in the first two Ruddlesden-Popper members of the model Hg-family of cuprates, which are chem. nearly identical and have the highest Tc among all cuprate families, we use inelastic photon scattering to reveal that the energy of magnetic fluctuations may play such a role. In particular, we observe the single-paramagnon spectra to be nearly identical between the two compds., apart from an energy scale difference of ∼30% which matches their difference in Tc. The empirical correlation between paramagnon energy and maximal Tc is further found to extend to other cuprate families with relatively high Tc's, hinting at a fundamental connection between them.
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  26. 26
    Leitenstorfer, A. The 2023 terahertz science and technology roadmap. J. Phys. D. Appl. Phys. 2023, 56, 223001  DOI: 10.1088/1361-6463/acbe4c
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    There is no corresponding record for this reference.
  27. 27
    Plankl, M. Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures. Nat. Photonics 2021, 15, 594– 600,  DOI: 10.1038/s41566-021-00813-y
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    27
    Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures
    Plankl, M.; Faria Junior, P. E.; Mooshammer, F.; Siday, T.; Zizlsperger, M.; Sandner, F.; Schiegl, F.; Maier, S.; Huber, M. A.; Gmitra, M.; Fabian, J.; Boland, J. L.; Cocker, T. L.; Huber, R.
    Nature Photonics (2021), 15 (8), 594-600CODEN: NPAHBY; ISSN:1749-4885. (Nature Portfolio)
    Abstr Tunnelling is one of the most fundamental manifestations of quantum mechanics. The recent advent of lightwave-driven scanning tunnelling microscopy has revolutionized ultrafast nanoscience by directly resolving electron tunnelling in elec. conducting samples on the relevant ultrashort length- and timescales. Here, we introduce a complementary approach based on terahertz near-field microscopy to perform ultrafast nano-videog. of tunnelling processes even in insulators. The central idea is to probe the evolution of the local polarizability of electron-hole pairs with evanescent terahertz fields, which we detect with subcycle temporal resoln. In a proof of concept, we resolve femtosecond interlayer transport in van der Waals heterobilayers and reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometer scales. Such contact-free nanoscopy of tunnelling-induced dynamics should be universally applicable to conducting and non-conducting samples and reveal how ultrafast transport processes shape functionalities in a wide range of condensed matter systems.
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  28. 28
    Siday, T. Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2. Nano Lett. 2022, 22, 2561– 2568,  DOI: 10.1021/acs.nanolett.1c04741
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    28
    Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2
    Siday, Thomas; Sandner, Fabian; Brem, Samuel; Zizlsperger, Martin; Perea-Causin, Raul; Schiegl, Felix; Nerreter, Svenja; Plankl, Markus; Merkl, Philipp; Mooshammer, Fabian; Huber, Markus A.; Malic, Ermin; Huber, Rupert
    Nano Letters (2022), 22 (6), 2561-2568CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The d.-driven transition of an exciton gas into an electron-hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron-hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quant. investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the d.-dependent recombination dynamics of electron-hole pairs within a WSe2 homobilayer. For increasing carrier d., an initial monomol. recombination of optically dark excitons transitions continuously into a bimol. recombination of an unbound electron-hole plasma above 7 x 1012 cm-2. We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted two-dimensional materials.
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  29. 29
    Eisele, M. Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution. Nat. Photonics 2014, 8, 841– 845,  DOI: 10.1038/nphoton.2014.225
    Google Scholar
    29
    Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution
    Eisele, M.; Cocker, T. L.; Huber, M. A.; Plankl, M.; Viti, L.; Ercolani, D.; Sorba, L.; Vitiello, M. S.; Huber, R.
    Nature Photonics (2014), 8 (11), 841-845CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    A review. Phase-locked ultrashort pulses in the rich terahertz spectral range have provided key insights into phenomena as diverse as quantum confinement, first-order phase transitions, high-temp. supercond. and carrier transport in nanomaterials. Ultrabroadband electro-optic sampling of few-cycle field transients can even reveal novel dynamics that occur faster than a single oscillation cycle of light. However, conventional terahertz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielec. functions averaged over the size, structure, orientation and d. of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved terahertz spectroscopy to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM). We trace the time-dependent dielec. function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (<50 fs) formation of a local carrier depletion layer.
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  30. 30
    Wagner, M. Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump–Probe Nanoscopy. Nano Lett. 2014, 14, 894– 900,  DOI: 10.1021/nl4042577
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    30
    Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump-Probe Nanoscopy
    Wagner, Martin; Fei, Zhe; McLeod, Alexander S.; Rodin, Aleksandr S.; Bao, Wenzhong; Iwinski, Eric G.; Zhao, Zeng; Goldflam, Michael; Liu, Mengkun; Dominguez, Gerardo; Thiemens, Mark; Fogler, Michael M.; Castro Neto, Antonio H.; Lau, Chun Ning; Amarie, Sergiu; Keilmann, Fritz; Basov, D. N.
    Nano Letters (2014), 14 (2), 894-900CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe expts. inherently av. over local chem., compositional, and electronic inhomogeneities. This deficiency is circumvented and pump-probe IR spectroscopy with ∼20 nm spatial resoln., far below the diffraction limit, is introduced, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows study of exfoliated graphene single-layers on SiO2 at technol. significant mid-IR (MIR) frequencies where the local optical cond. becomes exptl. accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-IR (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 fs time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temp. up to several thousand Kelvin, as deduced directly from the Drude wt. assocd. with the plasmonic resonances.
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  31. 31
    Ni, G. X. Ultrafast optical switching of infrared plasmon polaritons in high-mobility graphene. Nat. Photonics 2016, 10, 244– 247,  DOI: 10.1038/nphoton.2016.45
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    31
    Ultrafast optical switching of infrared plasmon polaritons in high-mobility graphene
    Ni, G. X.; Wang, L.; Goldflam, M. D.; Wagner, M.; Fei, Z.; McLeod, A. S.; Liu, M. K.; Keilmann, F.; Ozyilmaz, B.; Castro Neto, A. H.; Hone, J.; Fogler, M. M.; Basov, D. N.
    Nature Photonics (2016), 10 (4), 244-247CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    The success of metal-based plasmonics for manipulating light at the nanoscale has been empowered by imaginative designs and advanced nano-fabrication. However, the fundamental optical and electronic properties of elemental metals, the prevailing plasmonic media, are difficult to alter using external stimuli. This limitation is particularly restrictive in applications that require modification of the plasmonic response at sub-picosecond timescales. This handicap has prompted the search for alternative plasmonic media, with graphene emerging as one of the most capable candidates for IR wavelengths. Here we visualize and elucidate the properties of non-equil. photo-induced plasmons in a high-mobility graphene monolayer. We activate plasmons with femtosecond optical pulses in a specimen of graphene that otherwise lacks IR plasmonic response at equil. In combination with static nano-imaging results on plasmon propagation, our IR pump-probe nano-spectroscopy investigation reveals new aspects of carrier relaxation in heterostructures based on high-purity graphene.
    >> More from SciFinder ®
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  32. 32
    Huber, M. A. Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures. Nat. Nanotechnol. 2017, 12, 207– 211,  DOI: 10.1038/nnano.2016.261
    Google Scholar
    32
    Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures
    Huber, Markus A.; Mooshammer, Fabian; Plankl, Markus; Viti, Leonardo; Sandner, Fabian; Kastner, Lukas Z.; Frank, Tobias; Fabian, Jaroslav; Vitiello, Miriam S.; Cocker, Tyler L.; Huber, Rupert
    Nature Nanotechnology (2017), 12 (3), 207-211CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    The possibility of hybridizing collective electronic motion with mid-IR light to form surface polaritons has made van der Waals layered materials a versatile platform for extreme light confinement and tailored nanophotonics. Graphene and its heterostructures have attracted particular attention because the absence of an energy gap allows plasmon polaritons to be tuned continuously. Here, we introduce black phosphorus as a promising new material in surface polaritonics that features key advantages for ultrafast switching. Unlike graphene, black phosphorus is a van der Waals bonded semiconductor, which enables high-contrast interband excitation of electron-hole pairs by ultrashort near-IR pulses. Here, we design a SiO2/black phosphorus/SiO2 heterostructure in which the surface phonon modes of the SiO2 layers hybridize with surface plasmon modes in black phosphorus that can be activated by photo-induced interband excitation. Within the Reststrahlen band of SiO2, the hybrid interface polariton assumes surface-phonon-like properties, with a well-defined frequency and momentum and excellent coherence. During the lifetime of the photogenerated electron-hole plasma, coherent hybrid polariton waves can be launched by a broadband mid-IR pulse coupled to the tip of a scattering-type scanning near-field optical microscopy set-up. The scattered radiation allows us to trace the new hybrid mode in time, energy and space. We find that the surface mode can be activated within ∼50 fs and disappears within 5 ps, as the electron-hole pairs in black phosphorus recombine. The excellent switching contrast and switching speed, the coherence properties and the const. wavelength of this transient mode make it a promising candidate for ultrafast nanophotonic devices.
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  33. 33
    Sternbach, A. J. Programmable hyperbolic polaritons in van der Waals semiconductors. Science 2021, 371, 617– 620,  DOI: 10.1126/science.abe9163
    Google Scholar
    33
    Programmable hyperbolic polaritons in van der Waals semiconductors
    Sternbach, A. J.; Chae, S. H.; Latini, S.; Rikhter, A. A.; Shao, Y.; Li, B.; Rhodes, D.; Kim, B.; Schuck, P. J.; Xu, X.; Zhu, X.-Y.; Averitt, R. D.; Hone, J.; Fogler, M. M.; Rubio, A.; Basov, D. N.
    Science (Washington, DC, United States) (2021), 371 (6529), 617-620CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    Collective electronic modes or lattice vibrations usually prohibit propagation of electromagnetic radiation through the bulk of common materials over a frequency range assocd. with these oscillations. However, this textbook tenet does not necessarily apply to layered crystals. Highly anisotropic materials often display nonintuitive optical properties and can permit propagation of sub-diffractional waveguide modes, with hyperbolic dispersion, throughout their bulk. Here, we report on the observation of optically induced electronic hyperbolicity in the layered transition metal dichalcogenide tungsten diselenide (WSe2). We used photoexcitation to inject electron-hole pairs in WSe2 and then visualized, by transient nanoimaging, the hyperbolic rays that traveled along conical trajectories inside of the crystal. We establish here the signatures of programmable hyperbolic electrodynamics and assess the role of quantum transitions of excitons within the Rydberg series in the obsd. polaritonic response.
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  34. 34
    Kim, R. H. J. Terahertz Nanoimaging of Perovskite Solar Cell Materials. ACS Photonics 2022, 9, 3550– 3556,  DOI: 10.1021/acsphotonics.2c00861
    Google Scholar
    34
    Terahertz Nanoimaging of Perovskite Solar Cell Materials
    Kim, Richard H. J.; Liu, Zhaoyu; Huang, Chuankun; Park, Joong-Mok; Haeuser, Samuel J.; Song, Zhaoning; Yan, Yanfa; Yao, Yongxin; Luo, Liang; Wang, Jigang
    ACS Photonics (2022), 9 (11), 3550-3556CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Direct visualization and quant. evaluation of charge filling in grain boundary (GB) traps of hybrid metal halide perovskites require dynamic cond. imaging simultaneously at the terahertz (THz) frequency and nanometer (nm) spatial scales not accessible by conventional transport and imaging methods used thus far. Here, we apply a THz near-field nanocond. mapping to the archetypal metal halide perovskite photovoltaic films and demonstrate that it is a powerful tool to reveal distinct dielec. heterogeneity due to charge trapping and degrdn. at the single GB level. Our approach visualizes the filled defect ion traps by local THz charge cond. and allows for extg. a quant. profile of trapping d. in the vicinity of GBs with sub-20 nm resoln. Furthermore, imaging material degrdn. by tracking local nanodefect distributions overtime identifies a distinct degrdn. pathway that starts from the GBs and propagates inside the grains over time. The single GB, nano-THz cond. imaging demonstrated here can be extended to benchmark various perovskite materials and devices for their global photoenergy conversion performance and local charge transfer proprieties of absorbers and interfaces.
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  35. 35
    Klarskov, P. Nanoscale Laser Terahertz Emission Microscopy. ACS Photonics 2017, 4, 2676– 2680,  DOI: 10.1021/acsphotonics.7b00870
    Google Scholar
    35
    Nanoscale Laser Terahertz Emission Microscopy
    Klarskov, Pernille; Kim, Hyewon; Colvin, Vicki L.; Mittleman, Daniel M.
    ACS Photonics (2017), 4 (11), 2676-2680CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Laser terahertz emission microscopy (LTEM) has become a powerful tool for studying ultrafast dynamics and local fields in many different types of materials. This technique, which relies on acceleration of charge carriers in a material upon femtosecond excitation, can provide insight into the physics of charge transport, built-in fields, grain boundaries or surface states. We describe a new implementation of LTEM with a spatial resoln. in the nanoscale regime based on a scattering-type near-field tip-based approach. We observe a spectral reshaping of the signal compared to conventional LTEM, which is analyzed using a resonant antenna model. Our exptl. and computational results clarify the importance of the mechanisms for both the plasmonic in-coupling of the near-IR pulses into the near field and the out-coupling of the generated terahertz pulses. We demonstrate a tip-size-limited spatial resoln. of ∼20 nm by imaging a gold nanorod using terahertz emission from the underlying substrate. This work enables for the first time the possibility of performing LTEM measurements on individual nanostructures.
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  36. 36
    Zhang, J. Terahertz Nanoimaging of Graphene. ACS Photonics 2018, 5, 2645– 2651,  DOI: 10.1021/acsphotonics.8b00190
    Google Scholar
    36
    Terahertz nanoimaging of graphene
    Zhang, Jiawei; Chen, Xinzhong; Mills, Scott; Ciavatti, Thomas; Yao, Ziheng; Mescall, Ryan; Hu, Hai; Semenenko, Vyacheslav; Fei, Zhe; Li, Hua; Perebeinos, Vasili; Tao, Hu; Dai, Qing; Du, Xu; Liu, Mengkun
    ACS Photonics (2018), 5 (7), 2645-2651CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Accessing the nonradiative near-field electromagnetic interactions with high in-plane momentum (q) is the key to achieve super resoln. imaging far beyond the diffraction limit. At far-IR and terahertz (THz) wavelengths (e.g., 300 μm = 1 THz = 4 meV), the study of high q response and nanoscale near-field imaging is still a nascent research field. In this work, we report on THz nanoimaging of exfoliated single and multilayer graphene flakes by using a state-of-the-art scattering-type near-field optical microscope (s-SNOM). We exptl. demonstrated that the single layer graphene is close to a perfect near-field reflector at ambient environment, comparable to that of the noble metal films at the same frequency range. Further modeling and anal. considering the nonlocal graphene cond. indicate that the high near-field reflectivity of graphene is a rather universal behavior: graphene operates as a perfect high-q reflector at room temp. Our work uncovers the unique high-q THz response of graphene, which is essential for future applications of graphene in nano-optics or tip-enhanced technologies.
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  37. 37
    Pizzuto, A. Nonlocal Time-Resolved Terahertz Spectroscopy in the Near Field. ACS Photonics 2021, 8, 2904– 2911,  DOI: 10.1021/acsphotonics.1c01367
    Google Scholar
    37
    Nonlocal Time-Resolved Terahertz Spectroscopy in the Near Field
    Pizzuto, Angela; Castro-Camus, Enrique; Wilson, William; Choi, Wonsik; Li, Xiuling; Mittleman, Daniel M.
    ACS Photonics (2021), 8 (10), 2904-2911CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Scattering-type near-field optical microscopy (s-SNOM) has enabled subwavelength spectroscopy measurements on a wide variety of materials and over a large spectral range. These tip-based measurements are of particular interest in the long wavelength regimes, where the study of individual nanoscale samples is very challenging. The combination of s-SNOM techniques with short pulse durations has opened a new realm of possibilities in which nanosystems can be characterized with both high spatial and temporal resoln., for example via optical-pump, terahertz-probe measurements. Here, we demonstrate the first "nonlocal" pump-probe measurement using a scattering-type scanning near-field microscopy technique, in which the pump spot is laterally displaced from the probe location. We observe nonlocal effects corresponding to this pump-probe offset, assocd. with carrier drift into the s-SNOM near-field probe region.
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  38. 38
    Wang, T. Magnetically-dressed CrSBr exciton-polaritons in ultrastrong coupling regime. Nat. Commun. 2023, 14, 5966,  DOI: 10.1038/s41467-023-41688-7
    Google Scholar
    38
    Magnetically-dressed CrSBr exciton-polaritons in ultrastrong coupling regime
    Wang, Tingting; Zhang, Dingyang; Yang, Shiqi; Lin, Zhongchong; Chen, Quan; Yang, Jinbo; Gong, Qihuang; Chen, Zuxin; Ye, Yu; Liu, Wenjing
    Nature Communications (2023), 14 (1), 5966CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
    Over the past few decades, exciton-polaritons have attracted substantial research interest due to their half-light-half-matter bosonic nature. Coupling exciton-polaritons with magnetic orders grants access to rich many-body phenomena, but has been limited by the availability of material systems that exhibit simultaneous exciton resonances and magnetic ordering. Here we report magnetically-dressed microcavity exciton-polaritons in the van der Waals antiferromagnetic (AFM) semiconductor CrSBr coupled to a Tamm plasmon microcavity. Using angle-resolved spectroscopy, we reveal an exceptionally high exciton-photon coupling strength, up to 169 meV, demonstrating ultrastrong coupling that persists up to room temp. By performing temp.-dependent spectroscopy, we show the magnetic nature of the exciton-polaritons in CrSBr microcavity as the magnetic order changes from AFM to paramagnetic. By applying an out-of-plane magnetic field, we achieve effective tuning of the polariton energy while maintaining the ultrastrong exciton-photon coupling strength. We attribute this to the spin canting process that modulates the interlayer exciton interaction.
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  39. 39
    Esteras, D. L. Straintronics in the 2D van der Waals Ferromagnet CrSBr from First-Principles. Nano Lett. 2022, 22, 8771– 8778,  DOI: 10.1021/acs.nanolett.2c02863
    Google Scholar
    There is no corresponding record for this reference.
  40. 40
    Cenker, J. Reversible strain-induced magnetic phase transition in a van der Waals magnet. Nat. Nanotechnol. 2022, 17, 256– 261,  DOI: 10.1038/s41565-021-01052-6
    Google Scholar
    40
    Reversible strain-induced magnetic phase transition in a van der Waals magnet
    Cenker, John; Sivakumar, Shivesh; Xie, Kaichen; Miller, Aaron; Thijssen, Pearl; Liu, Zhaoyu; Dismukes, Avalon; Fonseca, Jordan; Anderson, Eric; Zhu, Xiaoyang; Roy, Xavier; Xiao, Di; Chu, Jiun-Haw; Cao, Ting; Xu, Xiaodong
    Nature Nanotechnology (2022), 17 (3), 256-261CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
    Abstr.: Mech. deformation of a crystal can have a profound effect on its phys. properties. Notably, even small modifications of bond geometry can completely change the size and sign of magnetic exchange interactions and thus the magnetic ground state. Here we report the strain tuning of the magnetic properties of the A-type layered antiferromagnetic semiconductor CrSBr achieved by designing a strain device that can apply continuous, in situ uniaxial tensile strain to two-dimensional materials, reaching several percent at cryogenic temps. Using this app., we realize a reversible strain-induced antiferromagnetic-to-ferromagnetic phase transition at zero magnetic field and strain control of the out-of-plane spin-canting process. First-principles calcns. reveal that the tuning of the in-plane lattice const. strongly modifies the interlayer magnetic exchange interaction, which changes sign at the crit. strain. Our work creates new opportunities for harnessing the strain control of magnetism and other electronic states in low-dimensional materials and heterostructures.
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  41. 41
    Boix-Constant, C. Multistep magnetization switching in orthogonally twisted ferromagnetic monolayers. Nat. Mater. 2024, 23, 212– 218,  DOI: 10.1038/s41563-023-01735-6
    Google Scholar
    There is no corresponding record for this reference.

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  1. Rundi Yang, Runxuan Li, Brian W. Blankenship, Jingang Li, Costas P. Grigoropoulos. Decoupling Carrier Dynamics and Energy Transport in Ultrafast Near-Field Nanoscopy. Nano Letters 2025, 25 (6) , 2242-2247. https://doi.org/10.1021/acs.nanolett.4c05419
  2. Pengzuo Jiang, Yaolong Li, Xiaying Lyu, Jingying Xiao, Xiaofang Li, Tingting Wang, Jinglin Tang, Yang Wang, Linfeng Zhang, Yu Liu, Hong Yang, Xiaoyong Hu, Yu Ye, Zuxin Chen, Yunan Gao, Chengyin Wu, Qihuang Gong. Ultrafast Electron Dynamics Dominated by Electron–Phonon Coupling in CrSBr Revealed by Photoemission Electron Microscopy. The Journal of Physical Chemistry C 2024, 128 (51) , 21855-21860. https://doi.org/10.1021/acs.jpcc.4c07231
  3. Priyanka Mondal, Daria I. Markina, Lennard Hopf, Lukas Krelle, Sai Shradha, Julian Klein, Mikhail M. Glazov, Iann Gerber, Kevin Hagmann, Regine von Klitzing, Kseniia Mosina, Zdenek Sofer, Bernhard Urbaszek. Raman polarization switching in CrSBr. npj 2D Materials and Applications 2025, 9 (1) https://doi.org/10.1038/s41699-025-00542-8
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  • Abstract

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    Figure 1

    Figure 1. Probing ultrafast electron–hole pair dynamics in bulk and monolayer CrSBr by THz polarization nanoscopy. (a) Optical micrograph of a typical CrSBr sample including bulk (yellow, blue) and monolayer (ML, faint blue) flakes. The crystallographic a- and b-axes are indicated. (b) Schematic of the THz near-field spectroscopy technique. Optical pump pulses (red) tunable in photon energy generate electron–hole pairs in CrSBr. After a variable delay time, tp, a phase-locked THz probe transient, ETHz (blue, left), is coupled into the evanescent near field of a metallic tip. By phase-resolved detection of the scattered THz waveform (blue, right) information about the nanoscale dielectric function of the sample is obtained. (c) Polarization nanoscopy. Tunable pump pulses excite either excitons (red, hνp,X) or continuum states (green, hνp,c). The THz electric field (blue arrow) polarizes the electron–hole pairs with polarizability αX and αc, respectively. (d) Electro-optically detected steady-state scattered THz waveform, E (black line), and pump-induced change, ΔE (red line), of a bulk CrSBr flake at a pump delay, tp = 0.5 ps, as a function of the EOS time, tEOS.

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    Figure 2

    Figure 2. Femtosecond electron–hole pair dynamics in bulk and monolayer CrSBr. (a) Maximal pump-induced change of the near-field response at tEOS = 0.15 ps, ΔEpeak, of bulk CrSBr as a function of pump delay time, tp, for various excitation photon energies (solid lines). The decay of ΔEpeak is fitted with a biexponential function (dashed lines). The gray shaded region in the top panel shows the excess of the pump-induced signal at early delays with respect to lower excitation photon energies. The data are offset for clarity. (b) Analogous to (a) for the monolayer (ML) limit. Solid lines are calculated with a rate equation model. (c, d) Polarizability of the photoexcited electron–hole pairs as a function of the pump photon energy for bulk (purple circles) and monolayer (teal circles). For context, the low-temperature photoluminescence spectra (PL, solid lines) as well as bandgaps measured with STS (bulk, purple ribbon) (7) and predicted by GW calculations (ML, teal ribbon) (22) are shown.

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    Figure 3

    Figure 3. Identifying species of photoexcited electron–hole pairs in bulk CrSBr by THz near-field spectroscopy. (a) Experimental pump-induced changes of the scattered THz waveform, ΔE, on bulk CrSBr as a function of the EOS time, tEOS, for different pump photon energies and pump delay times, tp. The steady-state near-field response is shown in gray. (b) Zoom-in to the EOS time window between 0.4 and 1 ps. (c) Time-domain near-field responses modeled with the finite-dipole model. (d–f) Relative spectral amplitude, ΔẼ/Ẽ (top left panels), and phase, Δϕ – ϕ (bottom left panels), of the near-field response. The modeled data (dashed lines) calculated with the dielectric functions either comprising a Drude term (d) or two Lorentzians (e, f), shown in the right panels, excellently reproduce the measurements (circles).

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    Figure 4

    Figure 4. Extracting the complex-valued dielectric function of a photoexcited CrSBr monolayer. (a) Experimental pump-induced change of the scattered THz waveform, ΔE (red), for a pump photon energy of 1.39 eV at tp = 0.4 ps. The steady-state near-field response, E, is shown in gray. (b) Relative spectral amplitude, ΔẼ/Ẽ (red circles), and phase, Δϕ – ϕ (black circles), of the near-field response. (c) Dielectric function obtained for an anisotropic Rytova–Keldysh confinement potential (solid lines) used to calculate the near-field response shown in (b) (solid lines). By numerically inverting the finite-dipole model, we retrieve the complex dielectric function, ε, of the photoexcited monolayer (circles). The assumed equilibrium dielectric function, εeq = 10, is shown as a gray dashed line.

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  • References

    This article references 41 other publications.

    1. 1
      Huang, B. Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit. Nature 2017, 546, 270– 273,  DOI: 10.1038/nature22391
      1
      Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit
      Huang, Bevin; Clark, Genevieve; Navarro-Moratalla, Efren; Klein, Dahlia R.; Cheng, Ran; Seyler, Kyle L.; Zhong, Ding; Schmidgall, Emma; McGuire, Michael A.; Cobden, David H.; Yao, Wang; Xiao, Di; Jarillo-Herrero, Pablo; Xu, Xiaodong
      Nature (London, United Kingdom) (2017), 546 (7657), 270-273CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Since the discovery of graphene, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent addns. include semiconductors with spin-valley coupling, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-d. waves, and topol. semimetals with edge transport. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered; such a crystal would be useful in many technologies from sensing to data storage. Theor., magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temps. by the Mermin-Wagner theorem. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temp. of 45 K is only slightly lower than that of the bulk crystal, 61 K, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent phys. properties typical of van der Waals crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect, whereas in trilayer CrI3 the interlayer ferromagnetism obsd. in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as elec. control for realizing magnetoelectronics, and van der Waals engineering to produce interface phenomena.
      >> More from SciFinder ®
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    2. 2
      Seyler, K. L. Ligand-field helical luminescence in a 2D ferromagnetic insulator. Nat. Phys. 2018, 14, 277– 281,  DOI: 10.1038/s41567-017-0006-7
      2
      Ligand-field helical luminescence in a 2D ferromagnetic insulator
      Seyler, Kyle L.; Zhong, Ding; Klein, Dahlia R.; Gao, Shiyuan; Zhang, Xiaoou; Huang, Bevin; Navarro-Moratalla, Efren; Yang, Li; Cobden, David H.; McGuire, Michael A.; Yao, Wang; Xiao, Di; Jarillo-Herrero, Pablo; Xu, Xiaodong
      Nature Physics (2018), 14 (3), 277-281CODEN: NPAHAX; ISSN:1745-2473. (Nature Research)
      Bulk chromium tri-iodide (CrI3) has long been known as a layered van der Waals ferromagnet1. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet2, providing a new platform for investigating light-matter interactions and magneto-optical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI3 under linearly polarized excitation, with helicity detd. by the monolayer magnetization direction. In contrast, the bilayer CrI3 photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI3 bilayers2. Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors3, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI3. We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr3+ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized MO nature.
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    3. 3
      Burch, K. S.; Mandrus, D.; Park, J. G. Magnetism in two-dimensional van der Waals materials. Nature 2018, 563, 47– 52,  DOI: 10.1038/s41586-018-0631-z
      3
      Magnetism in two-dimensional van der Waals materials
      Burch, Kenneth S.; Mandrus, David; Park, Je-Geun
      Nature (London, United Kingdom) (2018), 563 (7729), 47-52CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      The discovery of materials has often introduced new phys. paradigms and enabled the development of novel devices. Two-dimensional magnetism, which is assocd. with strong intrinsic spin fluctuations, has long been the focus of fundamental questions in condensed matter physics regarding our understanding and control of new phases. Here we discuss magnetic van der Waals materials: two-dimensional at. crystals that contain magnetic elements and thus exhibit intrinsic magnetic properties. These cleavable materials provide the ideal platform for exploring magnetism in the two-dimensional limit, where new phys. phenomena are expected, and represent a substantial shift in our ability to control and investigate nanoscale phases. We present the theor. background and motivation for investigating this class of crystals, describe the material landscape and the current exptl. status of measurement techniques as well as devices, and discuss promising future directions for the study of magnetic van der Waals materials.
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    4. 4
      Kang, S. Coherent many-body exciton in van der Waals antiferromagnet NiPS3. Nature 2020, 583, 785– 789,  DOI: 10.1038/s41586-020-2520-5
      4
      Coherent many-body exciton in van der Waals antiferromagnet NiPS3
      Kang, Soonmin; Kim, Kangwon; Kim, Beom Hyun; Kim, Jonghyeon; Sim, Kyung Ik; Lee, Jae-Ung; Lee, Sungmin; Park, Kisoo; Yun, Seokhwan; Kim, Taehun; Nag, Abhishek; Walters, Andrew; Garcia-Fernandez, Mirian; Li, Jiemin; Chapon, Laurent; Zhou, Ke-Jin; Son, Young-Woo; Kim, Jae Hoon; Cheong, Hyeonsik; Park, Je-Geun
      Nature (London, United Kingdom) (2020), 583 (7818), 785-789CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Abstr.: An exciton is the bosonic quasiparticle of electron-hole pairs bound by the Coulomb interaction. Bose-Einstein condensation of this exciton state has long been the subject of speculation in various model systems2,3, and examples have been found more recently in optical lattices and two-dimensional materials4-9. Unlike these conventional excitons formed from extended Bloch states4-9, excitonic bound states from intrinsically many-body localized states are rare. Here we show that a spin-orbit-entangled exciton state appears below the Neel temp. of 150 K in NiPS3, an antiferromagnetic van der Waals material. It arises intrinsically from the archetypal many-body states of the Zhang-Rice singlet10,11, and reaches a coherent state assisted by the antiferromagnetic order. Using configuration-interaction theory, we det. the origin of the coherent excitonic excitation to be a transition from a Zhang-Rice triplet to a Zhang-Rice singlet. We combine three spectroscopic tools-resonant inelastic X-ray scattering, photoluminescence and optical absorption-to characterize the exciton and to demonstrate an extremely narrow excitonic linewidth below 50 K. The discovery of the spin-orbit-entangled exciton in antiferromagnetic NiPS3 introduces van der Waals magnets as a platform to study coherent many-body excitons.
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    5. 5
      Hwangbo, K. Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator. Nat. Nanotechnol. 2021, 16, 655– 660,  DOI: 10.1038/s41565-021-00873-9
      5
      Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator
      Hwangbo, Kyle; Zhang, Qi; Jiang, Qianni; Wang, Yong; Fonseca, Jordan; Wang, Chong; Diederich, Geoffrey M.; Gamelin, Daniel R.; Xiao, Di; Chu, Jiun-Haw; Yao, Wang; Xu, Xiaodong
      Nature Nanotechnology (2021), 16 (6), 655-660CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
      Two-dimensional (2D) semiconductors enable the investigation of light-matter interactions in low dimensions1,2. Yet, the study of elementary photoexcitations in 2D semiconductors with intrinsic magnetic order remains a challenge due to the lack of suitable materials3,4. Here, we report the observation of excitons coupled to zigzag antiferromagnetic order in the layered antiferromagnetic insulator NiPS3. The exciton exhibits a narrow photoluminescence linewidth of roughly 350μeV with near-unity linear polarization. When we reduce the sample thickness from five to two layers, the photoluminescence is suppressed and eventually vanishes for the monolayer. This suppression is consistent with the calcd. bandgap of NiPS3, which is highly indirect for both the bilayer and the monolayer5. Furthermore, we observe strong linear dichroism (LD) over a broad spectral range. The optical anisotropy axes of LD and of photoluminescence are locked to the zigzag direction. Furthermore, their temp. dependence is reminiscent of the in-plane magnetic susceptibility anisotropy. Hence, our results indicate that LD and photoluminescence could probe the symmetry breaking magnetic order parameter of 2D magnetic materials. In addn., we observe over ten exciton-A1g-phonon bound states on the high-energy side of the exciton resonance, which we interpret as signs of a strong modulation of the ligand-to-metal charge-transfer energy by electron-lattice interactions. Our work establishes NiPS3 as a 2D platform for exploring magneto-exciton physics with strong correlations.
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    6. 6
      Wang, Q. H. The Magnetic Genome of Two-Dimensional van der Waals Materials. ACS Nano 2022, 16, 6960– 7079,  DOI: 10.1021/acsnano.1c09150
      6
      The Magnetic Genome of Two-Dimensional van der Waals Materials
      Wang, Qing Hua; Bedoya-Pinto, Amilcar; Blei, Mark; Dismukes, Avalon H.; Hamo, Assaf; Jenkins, Sarah; Koperski, Maciej; Liu, Yu; Sun, Qi-Chao; Telford, Evan J.; Kim, Hyun Ho; Augustin, Mathias; Vool, Uri; Yin, Jia-Xin; Li, Lu Hua; Falin, Alexey; Dean, Cory R.; Casanova, Felix; Evans, Richard F. L.; Chshiev, Mairbek; Mishchenko, Artem; Petrovic, Cedomir; He, Rui; Zhao, Liuyan; Tsen, Adam W.; Gerardot, Brian D.; Brotons-Gisbert, Mauro; Guguchia, Zurab; Roy, Xavier; Tongay, Sefaattin; Wang, Ziwei; Hasan, M. Zahid; Wrachtrup, Joerg; Yacoby, Amir; Fert, Albert; Parkin, Stuart; Novoselov, Kostya S.; Dai, Pengcheng; Balicas, Luis; Santos, Elton J. G.
      ACS Nano (2022), 16 (5), 6960-7079CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A review. Magnetism in two-dimensional (2D) van der Waals (vdW) materials has recently emerged as one of the most promising areas in condensed matter research, with many exciting emerging properties and significant potential for applications ranging from topol. magnonics to low-power spintronics, quantum computing and optical communications. In the short time since their discovery, 2D magnets have blossomed into a rich area for investigation, where fundamental concepts in magnetism are challenged by the behavior that spins can develop at the single layer limit. However, much effort is still needed in different fronts before 2D magnets can be routinely used for practical implementations. In this comprehensive review, prominent authors with expertise in complementary fields (i.e., synthesis, device engineering, magneto-optics, imaging, transport, mechanics, spin excitations, and theory and simulations) of 2D magnetism have joined together to provide a genome of current knowledge and a guideline for developments in 2D magnetic materials research.
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    7. 7
      Telford, E. J. Layered Antiferromagnetism Induces Large Negative Magnetoresistance in the van der Waals Semiconductor CrSBr. Adv. Mater. 2020, 32, 2003240  DOI: 10.1002/adma.202003240
      7
      Layered Antiferromagnetism Induces Large Negative Magnetoresistance in the van der Waals Semiconductor CrSBr
      Telford, Evan J.; Dismukes, Avalon H.; Lee, Kihong; Cheng, Minghao; Wieteska, Andrew; Bartholomew, Amymarie K.; Chen, Yu-Sheng; Xu, Xiaodong; Pasupathy, Abhay N.; Zhu, Xiaoyang; Dean, Cory R.; Roy, Xavier
      Advanced Materials (Weinheim, Germany) (2020), 32 (37), 2003240CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The recent discovery of magnetism within the family of exfoliatable van der Waals (vdW) compds. has attracted considerable interest in these materials for both fundamental research and technol. applications. However, current vdW magnets are limited by their extreme sensitivity to air, low ordering temps., and poor charge transport properties. Here the magnetic and electronic properties of CrSBr are reported, an air-stable vdW antiferromagnetic semiconductor that readily cleaves perpendicular to the stacking axis. Below its Neel temp., TN = 132 ± 1 K, CrSBr adopts an A-type antiferromagnetic structure with each individual layer ferromagnetically ordered internally and the layers coupled antiferromagnetically along the stacking direction. Scanning tunneling spectroscopy and photoluminescence (PL) reveal that the electronic gap is ΔE = 1.5 ± 0.2 eV with a corresponding PL peak centered at 1.25 ± 0.07 eV. Using magnetotransport measurements, strong coupling between magnetic order and transport properties in CrSBr is demonstrated, leading to a large neg. magnetoresistance response that is unique among vdW materials. These findings establish CrSBr as a promising material platform for increasing the applicability of vdW magnets to the field of spin-based electronics.
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    8. 8
      Dirnberger, F. Spin-correlated exciton–polaritons in a van der Waals magnet. Nat. Nanotechnol. 2022, 17, 1060– 1064,  DOI: 10.1038/s41565-022-01204-2
      8
      Spin-correlated exciton-polaritons in a van der Waals magnet
      Dirnberger, Florian; Bushati, Rezlind; Datta, Biswajit; Kumar, Ajesh; MacDonald, Allan H.; Baldini, Edoardo; Menon, Vinod M.
      Nature Nanotechnology (2022), 17 (10), 1060-1064CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
      Strong coupling between light and elementary excitations is emerging as a powerful tool to engineer the properties of solid-state systems. Spin-correlated excitations that couple strongly to optical cavities promise control over collective quantum phenomena such as magnetic phase transitions, but their suitable electronic resonances are yet to be found. Here, we report strong light-matter coupling in NiPS3, a van der Waals antiferromagnet with highly correlated electronic degrees of freedom. A previously unobserved class of polaritonic quasiparticles emerges from the strong coupling between its spin-correlated excitons and the photons inside a microcavity. Detailed spectroscopic anal. in conjunction with a microscopic theory provides unique insights into the origin and interactions of these exotic magnetically coupled excitations. Our work introduces van der Waals magnets to the field of strong light-matter physics and provides a path towards the design and control of correlated electron systems via cavity quantum electrodynamics.
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    9. 9
      Bae, Y. J. Exciton-coupled coherent magnons in a 2D semiconductor. Nature 2022, 609, 282– 286,  DOI: 10.1038/s41586-022-05024-1
      9
      Exciton-coupled coherent magnons in a 2D semiconductor
      Bae, Youn Jue; Wang, Jue; Scheie, Allen; Xu, Junwen; Chica, Daniel G.; Diederich, Geoffrey M.; Cenker, John; Ziebel, Michael E.; Bai, Yusong; Ren, Haowen; Dean, Cory R.; Delor, Milan; Xu, Xiaodong; Roy, Xavier; Kent, Andrew D.; Zhu, Xiaoyang
      Nature (London, United Kingdom) (2022), 609 (7926), 282-286CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
      The recent discoveries of two-dimensional (2D) magnets1-6 and their stacking into van der Waals structures7-11 have expanded the horizon of 2D phenomena. One exciting application is to exploit coherent magnons12 as energy-efficient information carriers in spintronics and magnonics13,14 or as interconnects in hybrid quantum systems15-17. A particular opportunity arises when a 2D magnet is also a semiconductor, as reported recently for CrSBr (refs. 18-20) and NiPS3 (refs. 21-23) that feature both tightly bound excitons with a large oscillator strength and potentially long-lived coherent magnons owing to the bandgap and spatial confinement. Although magnons and excitons are energetically mismatched by orders of magnitude, their coupling can lead to efficient optical access to spin information. Here we report strong magnon-exciton coupling in the 2D A-type antiferromagnetic semiconductor CrSBr. Coherent magnons launched by above-gap excitation modulate the exciton energies. Time-resolved exciton sensing reveals magnons that can coherently travel beyond seven micrometres, with a coherence time of above five nanoseconds. We observe these exciton-coupled coherent magnons in both even and odd nos. of layers, with and without compensated magnetization, down to the bilayer limit. Given the versatility of van der Waals heterostructures, these coherent 2D magnons may be a basis for optically accessible spintronics, magnonics and quantum interconnects.
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    10. 10
      Diederich, G. M. Tunable interaction between excitons and hybridized magnons in a layered semiconductor. Nat. Nanotechnol. 2023, 18, 23– 28,  DOI: 10.1038/s41565-022-01259-1
      10
      Tunable interaction between excitons and hybridized magnons in a layered semiconductor
      Diederich, Geoffrey M.; Cenker, John; Ren, Yafei; Fonseca, Jordan; Chica, Daniel G.; Bae, Youn Jue; Zhu, Xiaoyang; Roy, Xavier; Cao, Ting; Xiao, Di; Xu, Xiaodong
      Nature Nanotechnology (2023), 18 (1), 23-28CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
      The interaction between distinct excitations in solids is of both fundamental interest and technol. importance. One such interaction is the coupling between an exciton, a Coulomb bound electron-hole pair, and a magnon, a collective spin excitation. The recent emergence of van der Waals magnetic semiconductors1 provides a platform to explore these exciton-magnon interactions and their fundamental properties, such as strong correlation2, as well as their photospintronic and quantum transduction3 applications. Here we demonstrate the precise control of coherent exciton-magnon interactions in the layered magnetic semiconductor CrSBr. We varied the direction of an applied magnetic field relative to the crystal axes, and thus the rotational symmetry of the magnetic system4. Thereby, we tuned not only the exciton coupling to the bright magnon, but also to an optically dark mode via magnon-magnon hybridization. We further modulated the exciton-magnon coupling and the assocd. magnon dispersion curves through the application of uniaxial strain. At a crit. strain, a dispersionless dark magnon band emerged. Our results demonstrate an unprecedented level of control of the opto-mech.-magnonic coupling, and a step towards the predictable and controllable implementation of hybrid quantum magnonics5-11.
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    11. 11
      Dirnberger, F. Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons. Nature 2023, 620, 533– 538,  DOI: 10.1038/s41586-023-06275-2
      11
      Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons
      Dirnberger, Florian; Quan, Jiamin; Bushati, Rezlind; Diederich, Geoffrey M.; Florian, Matthias; Klein, Julian; Mosina, Kseniia; Sofer, Zdenek; Xu, Xiaodong; Kamra, Akashdeep; Garcia-Vidal, Francisco J.; Alu, Andrea; Menon, Vinod M.
      Nature (London, United Kingdom) (2023), 620 (7974), 533-537CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
      The magnetooptical properties of a van der Waals magnet that supports strong coupling of photons and excitons even in the absence of external cavity mirrors was studied. In this material-the layered magnetic semiconductor CrSBr-emergent light-matter hybrids called polaritons substantially increase the spectral bandwidth of correlations between the magnetic, electronic and optical properties, enabling largely tunable optical responses to applied magnetic fields and magnons. The results highlight the importance of exciton-photon self-hybridization in van der Waals magnets and motivate novel directions for the manipulation of quantum material properties by strong light-matter coupling.
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    12. 12
      Lee, K. Magnetic Order and Symmetry in the 2D Semiconductor CrSBr. Nano Lett. 2021, 21, 3511– 3517,  DOI: 10.1021/acs.nanolett.1c00219
      12
      Magnetic Order and Symmetry in the 2D Semiconductor CrSBr
      Lee, Kihong; Dismukes, Avalon H.; Telford, Evan J.; Wiscons, Ren A.; Wang, Jue; Xu, Xiaodong; Nuckolls, Colin; Dean, Cory R.; Roy, Xavier; Zhu, Xiaoyang
      Nano Letters (2021), 21 (8), 3511-3517CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The advent of two-dimensional (2D) magnets offers unprecedented control over electrons and spins. A key factor in detg. exchange coupling and magnetic order is symmetry. Here, we apply second harmonic generation (SHG) to probe a 2D magnetic semiconductor CrSBr. We find that monolayers are ferromagnetically ordered below 146 K, an observation enabled by the discovery of a large magnetic dipole SHG effect in the centrosym. structure. In multilayers, the ferromagnetic monolayers are coupled antiferromagnetically, and in contrast to other 2D magnets, the Neel temp. of CrSBr increases with decreasing layer no. We identify magnetic dipole and magnetic toroidal moments as order parameters of the ferromagnetic monolayer and antiferromagnetic bilayer, resp. These findings establish CrSBr as an exciting 2D magnetic semiconductor and extend the SHG probe of magnetic symmetry to the monolayer limit, opening the door to exploring the applications of magnetic-electronic coupling and the magnetic toroidal moment.
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    13. 13
      Rizzo, D. J. Visualizing Atomically Layered Magnetism in CrSBr. Adv. Mater. 2022, 34, 2201000  DOI: 10.1002/adma.202201000
      13
      Visualizing Atomically Layered Magnetism in CrSBr
      Rizzo, Daniel J.; McLeod, Alexander S.; Carnahan, Caitlin; Telford, Evan J.; Dismukes, Avalon H.; Wiscons, Ren A.; Dong, Yinan; Nuckolls, Colin; Dean, Cory R.; Pasupathy, Abhay N.; Roy, Xavier; Xiao, Di; Basov, D. N.
      Advanced Materials (Weinheim, Germany) (2022), 34 (27), 2201000CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      2D materials can host long-range magnetic order in the presence of underlying magnetic anisotropy. The ability to realize the full potential of 2D magnets necessitates systematic investigation of the role of individual at. layers and nanoscale inhomogeneity (i.e., strain) on the emergence of stable magnetic phases. Here, spatially dependent magnetism in few-layer CrSBr is revealed using magnetic force microscopy (MFM) and Monte Carlo-based simulations. Nanoscale visualization of the magnetic sheet susceptibility is extd. from MFM data and force-distance curves, revealing a characteristic onset of both intra- and interlayer magnetic correlations as a function of temp. and layer-thickness. These results demonstrate that the presence of a single uncompensated layer in odd-layer terraces significantly reduces the stability of the low-temp. antiferromagnetic (AFM) phase and gives rise to multiple coexisting magnetic ground states at temps. close to the bulk Neel temp. (TN). Furthermore, the AFM phase can be reliably suppressed using modest fields (∼16 mT) from the MFM probe, behaving as a nanoscale magnetic switch. This prototypical study of few-layer CrSBr demonstrates the crit. role of layer parity on field-tunable 2D magnetism and validates MFM for use in nanomagnetometry of 2D materials (despite the ubiquitous absence of bulk zero-field magnetism in magnetized sheets).
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    14. 14
      Wilson, N. P. Interlayer electronic coupling on demand in a 2D magnetic semiconductor. Nat. Mater. 2021, 20, 1657– 1662,  DOI: 10.1038/s41563-021-01070-8
      14
      Interlayer electronic coupling on demand in a 2D magnetic semiconductor
      Wilson, Nathan P.; Lee, Kihong; Cenker, John; Xie, Kaichen; Dismukes, Avalon H.; Telford, Evan J.; Fonseca, Jordan; Sivakumar, Shivesh; Dean, Cory; Cao, Ting; Roy, Xavier; Xu, Xiaodong; Zhu, Xiaoyang
      Nature Materials (2021), 20 (12), 1657-1662CODEN: NMAACR; ISSN:1476-1122. (Nature Portfolio)
      When monolayers of two-dimensional (2D) materials are stacked into van der Waals structures, interlayer electronic coupling can introduce entirely new properties, as exemplified by recent discoveries of moire bands that host highly correlated electronic states and quantum dot-like interlayer exciton lattices. Here we show the magnetic control of interlayer electronic coupling, as manifested in tunable excitonic transitions, in an A-type antiferromagnetic 2D semiconductor CrSBr. Excitonic transitions in bilayers and above can be drastically changed when the magnetic order is switched from the layered antiferromagnetic ground state to a field-induced ferromagnetic state, an effect attributed to the spin-allowed interlayer hybridization of electron and hole orbitals in the latter, as revealed by Green's function-Bethe-Salpeter equation (GW-BSE) calcns. Our work uncovers a magnetic approach to engineer electronic and excitonic effects in layered magnetic semiconductors.
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    15. 15
      Klein, J. Control of structure and spin texture in the van der Waals layered magnet CrSBr. Nat. Commun. 2022, 13, 5420,  DOI: 10.1038/s41467-022-32737-8
      15
      Control of structure and spin texture in the van der Waals layered magnet CrSBr
      Klein, J.; Pham, T.; Thomsen, J. D.; Curtis, J. B.; Denneulin, T.; Lorke, M.; Florian, M.; Steinhoff, A.; Wiscons, R. A.; Luxa, J.; Sofer, Z.; Jahnke, F.; Narang, P.; Ross, F. M.
      Nature Communications (2022), 13 (1), 5420CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
      Abstr.: Controlling magnetism at nanometer length scales is essential for realizing high-performance spintronic, magneto-elec. and topol. devices and creating on-demand spin Hamiltonians probing fundamental concepts in physics. Van der Waals (vdW)-bonded layered magnets offer exceptional opportunities for such spin texture engineering. Here, we demonstrate nanoscale structural control in the layered magnet CrSBr with the potential to create spin patterns without the environmental sensitivity that has hindered such manipulations in other vdW magnets. We drive a local phase transformation using an electron beam that moves atoms and exchanges bond directions, effectively creating regions that have vertical vdW layers embedded within the initial horizontally vdW bonded exfoliated flakes. We calc. that the newly formed two-dimensional structure is ferromagnetically ordered in-plane with an energy gap in the visible spectrum, and weak antiferromagnetism between the planes, suggesting possibilities for creating spin textures and quantum magnetic phases.
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    16. 16
      Liu, W. A Three-Stage Magnetic Phase Transition Revealed in Ultrahigh-Quality van der Waals Bulk Magnet CrSBr. ACS Nano 2022, 16, 15917– 15926,  DOI: 10.1021/acsnano.2c02896
      16
      A Three-Stage Magnetic Phase Transition Revealed in Ultrahigh-Quality van der Waals Bulk Magnet CrSBr
      Liu, Wenhao; Guo, Xiaoyu; Schwartz, Jonathan; Xie, Hongchao; Dhale, Nikhil Uday; Sung, Suk Hyun; Kondusamy, Aswin Lakshmi Narayanan; Wang, Xiqu; Zhao, Haonan; Berman, Diana; Hovden, Robert; Zhao, Liuyan; Lv, Bing
      ACS Nano (2022), 16 (10), 15917-15926CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Van der Waals (vdW) magnets are receiving ever-growing attention nowadays due to their significance in both fundamental research on low-dimensional magnetism and potential applications in spintronic devices. The high cryst. quality of vdW magnets is the key to maintaining intrinsic magnetic and electronic properties, esp. when exfoliated down to the two-dimensional limit. Here, ultrahigh-quality air-stable vdW CrSBr crystals are synthesized using the direct solid-vapor synthesis method. The high single crystallinity and spatial homogeneity have been thoroughly evidenced at length scales from submm to at. resoln. by X-ray diffraction, second harmonic generation, and scanning transmission electron microscopy. More importantly, sp. heat measurements of ultrahigh-quality CrSBr crystals show three thermodn. anomalies at 185, 156, and 132 K, revealing a stage-by-stage development of the magnetic order upon cooling, which is also corroborated with the magnetization and transport results. Our ultrahigh-quality CrSBr can further be exfoliated down to monolayers and bilayers easily, providing the building blocks of heterostructures for spintronic and magneto-optoelectronic applications.
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    17. 17
      Xu, X. Strong Spin-Phonon Coupling in Two-Dimensional Magnetic Semiconductor CrSBr. J. Phys. Chem. C 2022, 126, 10574– 10583,  DOI: 10.1021/acs.jpcc.2c02742
      17
      Strong Spin-Phonon Coupling in Two-Dimensional Magnetic Semiconductor CrSBr
      Xu, Xiaomin; Wang, Xiaohu; Chang, Pu; Chen, Xiaoyu; Guan, Lixiu; Tao, Junguang
      Journal of Physical Chemistry C (2022), 126 (25), 10574-10583CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Recently, spin-phonon coupling (SPC) has gained considerable attention esp. in two-dimensional (2D) materials. Herein, d.-functional theory is used to investigate the SPC effect in CrSBr, a recently fabricated 2D magnetic semiconductor. It is found that the phonon vibrations are strongly dependent on the spin ordering. The breaking of magnetic symmetry changes phonon spectrum dependency obviously. In particular, the SPC const. is found to be 20.2 cm-1, which is one order of magnitude larger than that of most other 2D materials. The group velocity and Gr.ovrddot.uneisen const. in the ferromagnetic (FM) state are increased by ~ 23 and ~ 16% than that in the antiferromagnetic state. Furthermore, the thermal cond. is enhanced by ~ 43% for FM spin ordering because of stronger lattice anharmonicity. The Curie temp. of the system can be tuned ~ 30% by lattice deformation because of the strong SPC. Our work provides fundamental insights into the SPC effect on the CrSBr monolayer and sheds light on its potential for novel spintronic application.
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    18. 18
      Wu, F. Quasi-1D Electronic Transport in a 2D Magnetic Semiconductor. Adv. Mater. 2022, 34, 2109759  DOI: 10.1002/adma.202109759
      18
      Quasi-1D Electronic Transport in a 2D Magnetic Semiconductor
      Wu, Fan; Gutierrez-Lezama, Ignacio; Lopez-Paz, Sara A.; Gibertini, Marco; Watanabe, Kenji; Taniguchi, Takashi; von Rohr, Fabian O.; Ubrig, Nicolas; Morpurgo, Alberto F.
      Advanced Materials (Weinheim, Germany) (2022), 34 (16), 2109759CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Electronic transport through exfoliated multilayers of CrSBr, a 2D semiconductor of interest because of its magnetic properties, is investigated. An extremely pronounced anisotropy manifesting itself in qual. and quant. differences of all quantities measured along the in-plane a and b crystallog. directions is found. In particular, a qual. different dependence of the conductivities σa and σb on temp. and gate voltage, accompanied by orders of magnitude differences in their values (σb/σa 3 x 102 to 105 at low temp. and neg. gate voltage) are obsd., together with a different behavior of the longitudinal magnetoresistance in the two directions and the complete absence of the Hall effect in transverse resistance measurements. These observations appear not to be compatible with a description in terms of conventional band transport of a 2D doped semiconductor. The obsd. phenomenol.-and unambiguous signatures of a 1D van Hove singularity detected in energy-resolved photocurrent measurements-indicate that electronic transport through CrSBr multilayers is better interpreted by considering the system as formed by weakly and incoherently coupled 1D wires, than by conventional 2D band transport. It is concluded that CrSBr is the first 2D semiconductor to show distinctly quasi-1D electronic transport properties.
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    19. 19
      Klein, J. Sensing the Local Magnetic Environment through Optically Active Defects in a Layered Magnetic Semiconductor. ACS Nano 2023, 17, 288– 299,  DOI: 10.1021/acsnano.2c07655
      19
      Sensing the Local Magnetic Environment through Optically Active Defects in a Layered Magnetic Semiconductor
      Klein, Julian; Song, Zhigang; Pingault, Benjamin; Dirnberger, Florian; Chi, Hang; Curtis, Jonathan B.; Dana, Rami; Bushati, Rezlind; Quan, Jiamin; Dekanovsky, Lukas; Sofer, Zdenek; Alu, Andrea; Menon, Vinod M.; Moodera, Jagadeesh S.; Loncar, Marko; Narang, Prineha; Ross, Frances M.
      ACS Nano (2023), 17 (1), 288-299CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Atomic-level defects in van der Waals (vdW) materials are essential building blocks for quantum technologies and quantum sensing applications. The layered magnetic semiconductor CrSBr is an outstanding candidate for exploring optically active defects because of a direct gap, in addn. to a rich magnetic phase diagram, including a recently hypothesized defect-induced magnetic order at low temp. Here, we show optically active defects in CrSBr that are probes of the local magnetic environment. We observe a spectrally narrow (1 meV) defect emission in CrSBr that is correlated with both the bulk magnetic order and an addnl. low-temp., defect-induced magnetic order. We elucidate the origin of this magnetic order in the context of local and nonlocal exchange coupling effects. Our work establishes vdW magnets like CrSBr as an exceptional platform to optically study defects that are correlated with the magnetic lattice. We anticipate that controlled defect creation allows for tailor-made complex magnetic textures and phases with direct optical access.
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    20. 20
      Torres, K. Probing Defects and Spin-Phonon Coupling in CrSBr via Resonant Raman Scattering. Adv. Funct. Mater. 2023, 33, 2211366  DOI: 10.1002/adfm.202211366
      20
      Probing Defects and Spin-Phonon Coupling in CrSBr via Resonant Raman Scattering
      Torres, Kierstin; Kuc, Agnieszka; Maschio, Lorenzo; Pham, Thang; Reidy, Kate; Dekanovsky, Lukas; Sofer, Zdenek; Ross, Frances M.; Klein, Julian
      Advanced Functional Materials (2023), 33 (12), 2211366CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Understanding the stability limitations and defect formation mechanisms in 2D magnets is essential for their utilization in spintronic and memory technologies. Here, defects in mono- to multilayer CrSBr are correlated with structural, vibrational, and magnetic properties. Resonant Raman scattering is used to reveal distinct vibrational defect signatures. In pristine CrSBr, it is shown that bromine atoms mediate vibrational interlayer coupling, allowing for distinguishing between surface and bulk defect modes. Environmental exposure is shown to cause drastic degrdn. in monolayers, with the formation of intralayer defects. This is in contrast to multilayers that predominantly show bromine surface defects. Through deliberate ion irradn., the formation of defect modes is tuned: these are strongly polarized and resonantly enhanced, reflecting the quasi--1D electronic character of CrSBr. Strikingly, pronounced signatures of spin-phonon coupling of the intrinsic phonon modes and the ion beam-induced defect modes are obsd. throughout the magnetic transition temp. Overall, defect engineering of magnetic properties is possible, with resonant Raman spectroscopy serving as a direct fingerprint of magnetic phases and defects in CrSBr.
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    21. 21
      Pawbake, A. Raman scattering signatures of strong spin-phonon coupling in the bulk magnetic van der Waals material CrSBr. Phys. Rev. B 2023, 107, 075421  DOI: 10.1103/PhysRevB.107.075421
      21
      Raman scattering signatures of strong spin-phonon coupling in the bulk magnetic van der Waals material CrSBr
      Pawbake, Amit; Pelini, Thomas; Wilson, Nathan P.; Mosina, Kseniia; Sofer, Zdenek; Heid, Rolf; Faugeras, Clement
      Physical Review B (2023), 107 (7), 075421CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Magnetic excitations in layered magnetic materials that can be thinned down to the two-dimensional (2D) monolayer limit are of great interest from a fundamental point of view and for applications. Raman scattering has played a crucial role in exploring the properties of magnetic layered materials and, even though it is essentially a probe of lattice vibrations, it can reflect magnetic ordering in solids through the spin-phonon interaction or through the observation of magnon excitations. In bulk CrSBr, a layered A-type antiferromagnet (AF), we show that the magnetic ordering can be directly obsd. in the temp. dependence of the Raman scattering response (i) through the variations of the scattered intensities, (ii) through the activation of new phonon lines reflecting the change of symmetry with the appearance of the addnl. magnetic periodicity, and (iii) through the observation below the Neel temp. (TN) of second-order Raman scattering processes. We addnl. show that the three different magnetic phases encountered in CrSBr, including the recently identified low-temp. phase, have a particular Raman scattering signature. This work demonstrates that magnetic ordering can be obsd. directly in the Raman scattering response of bulk CrSBr with in-plane magnetization and that it can provide unique insight into the magnetic phases encountered in magnetic layered materials.
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    22. 22
      Klein, J. The Bulk van der Waals Layered Magnet CrSBr is a Quasi-1D Material. ACS Nano 2023, 17, 5316– 5328,  DOI: 10.1021/acsnano.2c07316
      22
      The Bulk van der Waals Layered Magnet CrSBr is a Quasi-1D Material
      Klein, Julian; Pingault, Benjamin; Florian, Matthias; Heissenbuttel, Marie-Christin; Steinhoff, Alexander; Song, Zhigang; Torres, Kierstin; Dirnberger, Florian; Curtis, Jonathan B.; Weile, Mads; Penn, Aubrey; Deilmann, Thorsten; Dana, Rami; Bushati, Rezlind; Quan, Jiamin; Luxa, Jan; Sofer, Zdenek; Alu, Andrea; Menon, Vinod M.; Wurstbauer, Ursula; Rohlfing, Michael; Narang, Prineha; Loncar, Marko; Ross, Frances M.
      ACS Nano (2023), 17 (6), 5316-5328CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for the study of fundamental interactions and excitations, such as Tomonaga-Luttinger liqs. and topol. orders and defects with properties completely different from the quasiparticles expected in their higher-dimensional counterparts. However, clean 1D electronic systems are difficult to realize exptl., particularly for magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr-S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielec. screening, with an effective electron mass ratio of mXe/mYe ~ 50. This extreme anisotropy exptl. manifests in strong electron-phonon and exciton-phonon interactions, a Peierls-like structural instability, and a Fano resonance from a van Hove singularity of similar strength to that of metallic carbon nanotubes. Moreover, because of the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an exptl. attractive candidate for the study of exotic exciton and 1D-correlated many-body physics in the presence of magnetic order.
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    23. 23
      Bianchi, M. Paramagnetic Electronic Structure of CrSBr: Comparison between Ab Initio GW Theory and Angle-Resolved Photoemission Spectroscopy. Phys. Rev. B 2023, 107, 235107  DOI: 10.1103/PhysRevB.107.235107
      There is no corresponding record for this reference.
    24. 24
      Zheng, Y. Paramagnon drag in high thermoelectric figure of merit Li-doped MnTe. Sci. Adv. 2019, 5, eaat9461  DOI: 10.1126/sciadv.aat9461
      There is no corresponding record for this reference.
    25. 25
      Wang, L. Paramagnons and high-temperature superconductivity in a model family of cuprates. Nat. Commun. 2022, 13, 3163,  DOI: 10.1038/s41467-022-30918-z
      25
      Paramagnons and high-temperature superconductivity in a model family of cuprates
      Wang, Lichen; He, Guanhong; Yang, Zichen; Garcia-Fernandez, Mirian; Nag, Abhishek; Zhou, Kejin; Minola, Matteo; Tacon, Matthieu Le; Keimer, Bernhard; Peng, Yingying; Li, Yuan
      Nature Communications (2022), 13 (1), 3163CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
      Cuprate superconductors have the highest crit. temps. (Tc) at ambient pressure, yet a consensus on the superconducting mechanism remains to be established. Finding an empirical parameter that limits the highest reachable Tc can provide crucial insight into this outstanding problem. Here, in the first two Ruddlesden-Popper members of the model Hg-family of cuprates, which are chem. nearly identical and have the highest Tc among all cuprate families, we use inelastic photon scattering to reveal that the energy of magnetic fluctuations may play such a role. In particular, we observe the single-paramagnon spectra to be nearly identical between the two compds., apart from an energy scale difference of ∼30% which matches their difference in Tc. The empirical correlation between paramagnon energy and maximal Tc is further found to extend to other cuprate families with relatively high Tc's, hinting at a fundamental connection between them.
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    26. 26
      Leitenstorfer, A. The 2023 terahertz science and technology roadmap. J. Phys. D. Appl. Phys. 2023, 56, 223001  DOI: 10.1088/1361-6463/acbe4c
      There is no corresponding record for this reference.
    27. 27
      Plankl, M. Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures. Nat. Photonics 2021, 15, 594– 600,  DOI: 10.1038/s41566-021-00813-y
      27
      Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures
      Plankl, M.; Faria Junior, P. E.; Mooshammer, F.; Siday, T.; Zizlsperger, M.; Sandner, F.; Schiegl, F.; Maier, S.; Huber, M. A.; Gmitra, M.; Fabian, J.; Boland, J. L.; Cocker, T. L.; Huber, R.
      Nature Photonics (2021), 15 (8), 594-600CODEN: NPAHBY; ISSN:1749-4885. (Nature Portfolio)
      Abstr Tunnelling is one of the most fundamental manifestations of quantum mechanics. The recent advent of lightwave-driven scanning tunnelling microscopy has revolutionized ultrafast nanoscience by directly resolving electron tunnelling in elec. conducting samples on the relevant ultrashort length- and timescales. Here, we introduce a complementary approach based on terahertz near-field microscopy to perform ultrafast nano-videog. of tunnelling processes even in insulators. The central idea is to probe the evolution of the local polarizability of electron-hole pairs with evanescent terahertz fields, which we detect with subcycle temporal resoln. In a proof of concept, we resolve femtosecond interlayer transport in van der Waals heterobilayers and reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometer scales. Such contact-free nanoscopy of tunnelling-induced dynamics should be universally applicable to conducting and non-conducting samples and reveal how ultrafast transport processes shape functionalities in a wide range of condensed matter systems.
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    28. 28
      Siday, T. Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2. Nano Lett. 2022, 22, 2561– 2568,  DOI: 10.1021/acs.nanolett.1c04741
      28
      Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2
      Siday, Thomas; Sandner, Fabian; Brem, Samuel; Zizlsperger, Martin; Perea-Causin, Raul; Schiegl, Felix; Nerreter, Svenja; Plankl, Markus; Merkl, Philipp; Mooshammer, Fabian; Huber, Markus A.; Malic, Ermin; Huber, Rupert
      Nano Letters (2022), 22 (6), 2561-2568CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The d.-driven transition of an exciton gas into an electron-hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron-hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quant. investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the d.-dependent recombination dynamics of electron-hole pairs within a WSe2 homobilayer. For increasing carrier d., an initial monomol. recombination of optically dark excitons transitions continuously into a bimol. recombination of an unbound electron-hole plasma above 7 x 1012 cm-2. We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted two-dimensional materials.
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    29. 29
      Eisele, M. Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution. Nat. Photonics 2014, 8, 841– 845,  DOI: 10.1038/nphoton.2014.225
      29
      Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution
      Eisele, M.; Cocker, T. L.; Huber, M. A.; Plankl, M.; Viti, L.; Ercolani, D.; Sorba, L.; Vitiello, M. S.; Huber, R.
      Nature Photonics (2014), 8 (11), 841-845CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      A review. Phase-locked ultrashort pulses in the rich terahertz spectral range have provided key insights into phenomena as diverse as quantum confinement, first-order phase transitions, high-temp. supercond. and carrier transport in nanomaterials. Ultrabroadband electro-optic sampling of few-cycle field transients can even reveal novel dynamics that occur faster than a single oscillation cycle of light. However, conventional terahertz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielec. functions averaged over the size, structure, orientation and d. of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved terahertz spectroscopy to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM). We trace the time-dependent dielec. function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (<50 fs) formation of a local carrier depletion layer.
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    30. 30
      Wagner, M. Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump–Probe Nanoscopy. Nano Lett. 2014, 14, 894– 900,  DOI: 10.1021/nl4042577
      30
      Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump-Probe Nanoscopy
      Wagner, Martin; Fei, Zhe; McLeod, Alexander S.; Rodin, Aleksandr S.; Bao, Wenzhong; Iwinski, Eric G.; Zhao, Zeng; Goldflam, Michael; Liu, Mengkun; Dominguez, Gerardo; Thiemens, Mark; Fogler, Michael M.; Castro Neto, Antonio H.; Lau, Chun Ning; Amarie, Sergiu; Keilmann, Fritz; Basov, D. N.
      Nano Letters (2014), 14 (2), 894-900CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe expts. inherently av. over local chem., compositional, and electronic inhomogeneities. This deficiency is circumvented and pump-probe IR spectroscopy with ∼20 nm spatial resoln., far below the diffraction limit, is introduced, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows study of exfoliated graphene single-layers on SiO2 at technol. significant mid-IR (MIR) frequencies where the local optical cond. becomes exptl. accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-IR (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 fs time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temp. up to several thousand Kelvin, as deduced directly from the Drude wt. assocd. with the plasmonic resonances.
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    31. 31
      Ni, G. X. Ultrafast optical switching of infrared plasmon polaritons in high-mobility graphene. Nat. Photonics 2016, 10, 244– 247,  DOI: 10.1038/nphoton.2016.45
      31
      Ultrafast optical switching of infrared plasmon polaritons in high-mobility graphene
      Ni, G. X.; Wang, L.; Goldflam, M. D.; Wagner, M.; Fei, Z.; McLeod, A. S.; Liu, M. K.; Keilmann, F.; Ozyilmaz, B.; Castro Neto, A. H.; Hone, J.; Fogler, M. M.; Basov, D. N.
      Nature Photonics (2016), 10 (4), 244-247CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      The success of metal-based plasmonics for manipulating light at the nanoscale has been empowered by imaginative designs and advanced nano-fabrication. However, the fundamental optical and electronic properties of elemental metals, the prevailing plasmonic media, are difficult to alter using external stimuli. This limitation is particularly restrictive in applications that require modification of the plasmonic response at sub-picosecond timescales. This handicap has prompted the search for alternative plasmonic media, with graphene emerging as one of the most capable candidates for IR wavelengths. Here we visualize and elucidate the properties of non-equil. photo-induced plasmons in a high-mobility graphene monolayer. We activate plasmons with femtosecond optical pulses in a specimen of graphene that otherwise lacks IR plasmonic response at equil. In combination with static nano-imaging results on plasmon propagation, our IR pump-probe nano-spectroscopy investigation reveals new aspects of carrier relaxation in heterostructures based on high-purity graphene.
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    32. 32
      Huber, M. A. Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures. Nat. Nanotechnol. 2017, 12, 207– 211,  DOI: 10.1038/nnano.2016.261
      32
      Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures
      Huber, Markus A.; Mooshammer, Fabian; Plankl, Markus; Viti, Leonardo; Sandner, Fabian; Kastner, Lukas Z.; Frank, Tobias; Fabian, Jaroslav; Vitiello, Miriam S.; Cocker, Tyler L.; Huber, Rupert
      Nature Nanotechnology (2017), 12 (3), 207-211CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      The possibility of hybridizing collective electronic motion with mid-IR light to form surface polaritons has made van der Waals layered materials a versatile platform for extreme light confinement and tailored nanophotonics. Graphene and its heterostructures have attracted particular attention because the absence of an energy gap allows plasmon polaritons to be tuned continuously. Here, we introduce black phosphorus as a promising new material in surface polaritonics that features key advantages for ultrafast switching. Unlike graphene, black phosphorus is a van der Waals bonded semiconductor, which enables high-contrast interband excitation of electron-hole pairs by ultrashort near-IR pulses. Here, we design a SiO2/black phosphorus/SiO2 heterostructure in which the surface phonon modes of the SiO2 layers hybridize with surface plasmon modes in black phosphorus that can be activated by photo-induced interband excitation. Within the Reststrahlen band of SiO2, the hybrid interface polariton assumes surface-phonon-like properties, with a well-defined frequency and momentum and excellent coherence. During the lifetime of the photogenerated electron-hole plasma, coherent hybrid polariton waves can be launched by a broadband mid-IR pulse coupled to the tip of a scattering-type scanning near-field optical microscopy set-up. The scattered radiation allows us to trace the new hybrid mode in time, energy and space. We find that the surface mode can be activated within ∼50 fs and disappears within 5 ps, as the electron-hole pairs in black phosphorus recombine. The excellent switching contrast and switching speed, the coherence properties and the const. wavelength of this transient mode make it a promising candidate for ultrafast nanophotonic devices.
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    33. 33
      Sternbach, A. J. Programmable hyperbolic polaritons in van der Waals semiconductors. Science 2021, 371, 617– 620,  DOI: 10.1126/science.abe9163
      33
      Programmable hyperbolic polaritons in van der Waals semiconductors
      Sternbach, A. J.; Chae, S. H.; Latini, S.; Rikhter, A. A.; Shao, Y.; Li, B.; Rhodes, D.; Kim, B.; Schuck, P. J.; Xu, X.; Zhu, X.-Y.; Averitt, R. D.; Hone, J.; Fogler, M. M.; Rubio, A.; Basov, D. N.
      Science (Washington, DC, United States) (2021), 371 (6529), 617-620CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      Collective electronic modes or lattice vibrations usually prohibit propagation of electromagnetic radiation through the bulk of common materials over a frequency range assocd. with these oscillations. However, this textbook tenet does not necessarily apply to layered crystals. Highly anisotropic materials often display nonintuitive optical properties and can permit propagation of sub-diffractional waveguide modes, with hyperbolic dispersion, throughout their bulk. Here, we report on the observation of optically induced electronic hyperbolicity in the layered transition metal dichalcogenide tungsten diselenide (WSe2). We used photoexcitation to inject electron-hole pairs in WSe2 and then visualized, by transient nanoimaging, the hyperbolic rays that traveled along conical trajectories inside of the crystal. We establish here the signatures of programmable hyperbolic electrodynamics and assess the role of quantum transitions of excitons within the Rydberg series in the obsd. polaritonic response.
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    34. 34
      Kim, R. H. J. Terahertz Nanoimaging of Perovskite Solar Cell Materials. ACS Photonics 2022, 9, 3550– 3556,  DOI: 10.1021/acsphotonics.2c00861
      34
      Terahertz Nanoimaging of Perovskite Solar Cell Materials
      Kim, Richard H. J.; Liu, Zhaoyu; Huang, Chuankun; Park, Joong-Mok; Haeuser, Samuel J.; Song, Zhaoning; Yan, Yanfa; Yao, Yongxin; Luo, Liang; Wang, Jigang
      ACS Photonics (2022), 9 (11), 3550-3556CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Direct visualization and quant. evaluation of charge filling in grain boundary (GB) traps of hybrid metal halide perovskites require dynamic cond. imaging simultaneously at the terahertz (THz) frequency and nanometer (nm) spatial scales not accessible by conventional transport and imaging methods used thus far. Here, we apply a THz near-field nanocond. mapping to the archetypal metal halide perovskite photovoltaic films and demonstrate that it is a powerful tool to reveal distinct dielec. heterogeneity due to charge trapping and degrdn. at the single GB level. Our approach visualizes the filled defect ion traps by local THz charge cond. and allows for extg. a quant. profile of trapping d. in the vicinity of GBs with sub-20 nm resoln. Furthermore, imaging material degrdn. by tracking local nanodefect distributions overtime identifies a distinct degrdn. pathway that starts from the GBs and propagates inside the grains over time. The single GB, nano-THz cond. imaging demonstrated here can be extended to benchmark various perovskite materials and devices for their global photoenergy conversion performance and local charge transfer proprieties of absorbers and interfaces.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38Xis1ehtbrK&md5=0019859dabb53b6e7fba8796b0edf22e
    35. 35
      Klarskov, P. Nanoscale Laser Terahertz Emission Microscopy. ACS Photonics 2017, 4, 2676– 2680,  DOI: 10.1021/acsphotonics.7b00870
      35
      Nanoscale Laser Terahertz Emission Microscopy
      Klarskov, Pernille; Kim, Hyewon; Colvin, Vicki L.; Mittleman, Daniel M.
      ACS Photonics (2017), 4 (11), 2676-2680CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Laser terahertz emission microscopy (LTEM) has become a powerful tool for studying ultrafast dynamics and local fields in many different types of materials. This technique, which relies on acceleration of charge carriers in a material upon femtosecond excitation, can provide insight into the physics of charge transport, built-in fields, grain boundaries or surface states. We describe a new implementation of LTEM with a spatial resoln. in the nanoscale regime based on a scattering-type near-field tip-based approach. We observe a spectral reshaping of the signal compared to conventional LTEM, which is analyzed using a resonant antenna model. Our exptl. and computational results clarify the importance of the mechanisms for both the plasmonic in-coupling of the near-IR pulses into the near field and the out-coupling of the generated terahertz pulses. We demonstrate a tip-size-limited spatial resoln. of ∼20 nm by imaging a gold nanorod using terahertz emission from the underlying substrate. This work enables for the first time the possibility of performing LTEM measurements on individual nanostructures.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhs1CisbrK&md5=d5d396b63e3eccad211a8fa1ff69d24e
    36. 36
      Zhang, J. Terahertz Nanoimaging of Graphene. ACS Photonics 2018, 5, 2645– 2651,  DOI: 10.1021/acsphotonics.8b00190
      36
      Terahertz nanoimaging of graphene
      Zhang, Jiawei; Chen, Xinzhong; Mills, Scott; Ciavatti, Thomas; Yao, Ziheng; Mescall, Ryan; Hu, Hai; Semenenko, Vyacheslav; Fei, Zhe; Li, Hua; Perebeinos, Vasili; Tao, Hu; Dai, Qing; Du, Xu; Liu, Mengkun
      ACS Photonics (2018), 5 (7), 2645-2651CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Accessing the nonradiative near-field electromagnetic interactions with high in-plane momentum (q) is the key to achieve super resoln. imaging far beyond the diffraction limit. At far-IR and terahertz (THz) wavelengths (e.g., 300 μm = 1 THz = 4 meV), the study of high q response and nanoscale near-field imaging is still a nascent research field. In this work, we report on THz nanoimaging of exfoliated single and multilayer graphene flakes by using a state-of-the-art scattering-type near-field optical microscope (s-SNOM). We exptl. demonstrated that the single layer graphene is close to a perfect near-field reflector at ambient environment, comparable to that of the noble metal films at the same frequency range. Further modeling and anal. considering the nonlocal graphene cond. indicate that the high near-field reflectivity of graphene is a rather universal behavior: graphene operates as a perfect high-q reflector at room temp. Our work uncovers the unique high-q THz response of graphene, which is essential for future applications of graphene in nano-optics or tip-enhanced technologies.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhtFOntb3N&md5=ba7971ec359aba1fd8c57958abb760bd
    37. 37
      Pizzuto, A. Nonlocal Time-Resolved Terahertz Spectroscopy in the Near Field. ACS Photonics 2021, 8, 2904– 2911,  DOI: 10.1021/acsphotonics.1c01367
      37
      Nonlocal Time-Resolved Terahertz Spectroscopy in the Near Field
      Pizzuto, Angela; Castro-Camus, Enrique; Wilson, William; Choi, Wonsik; Li, Xiuling; Mittleman, Daniel M.
      ACS Photonics (2021), 8 (10), 2904-2911CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Scattering-type near-field optical microscopy (s-SNOM) has enabled subwavelength spectroscopy measurements on a wide variety of materials and over a large spectral range. These tip-based measurements are of particular interest in the long wavelength regimes, where the study of individual nanoscale samples is very challenging. The combination of s-SNOM techniques with short pulse durations has opened a new realm of possibilities in which nanosystems can be characterized with both high spatial and temporal resoln., for example via optical-pump, terahertz-probe measurements. Here, we demonstrate the first "nonlocal" pump-probe measurement using a scattering-type scanning near-field microscopy technique, in which the pump spot is laterally displaced from the probe location. We observe nonlocal effects corresponding to this pump-probe offset, assocd. with carrier drift into the s-SNOM near-field probe region.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXitFCls7fJ&md5=10b58fb756b2009bff5ac79251c948e1
    38. 38
      Wang, T. Magnetically-dressed CrSBr exciton-polaritons in ultrastrong coupling regime. Nat. Commun. 2023, 14, 5966,  DOI: 10.1038/s41467-023-41688-7
      38
      Magnetically-dressed CrSBr exciton-polaritons in ultrastrong coupling regime
      Wang, Tingting; Zhang, Dingyang; Yang, Shiqi; Lin, Zhongchong; Chen, Quan; Yang, Jinbo; Gong, Qihuang; Chen, Zuxin; Ye, Yu; Liu, Wenjing
      Nature Communications (2023), 14 (1), 5966CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
      Over the past few decades, exciton-polaritons have attracted substantial research interest due to their half-light-half-matter bosonic nature. Coupling exciton-polaritons with magnetic orders grants access to rich many-body phenomena, but has been limited by the availability of material systems that exhibit simultaneous exciton resonances and magnetic ordering. Here we report magnetically-dressed microcavity exciton-polaritons in the van der Waals antiferromagnetic (AFM) semiconductor CrSBr coupled to a Tamm plasmon microcavity. Using angle-resolved spectroscopy, we reveal an exceptionally high exciton-photon coupling strength, up to 169 meV, demonstrating ultrastrong coupling that persists up to room temp. By performing temp.-dependent spectroscopy, we show the magnetic nature of the exciton-polaritons in CrSBr microcavity as the magnetic order changes from AFM to paramagnetic. By applying an out-of-plane magnetic field, we achieve effective tuning of the polariton energy while maintaining the ultrastrong exciton-photon coupling strength. We attribute this to the spin canting process that modulates the interlayer exciton interaction.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3sXitVWrs7jM&md5=2c90c1f7a5a8d5d071cfd26bdf773cf8
    39. 39
      Esteras, D. L. Straintronics in the 2D van der Waals Ferromagnet CrSBr from First-Principles. Nano Lett. 2022, 22, 8771– 8778,  DOI: 10.1021/acs.nanolett.2c02863
      There is no corresponding record for this reference.
    40. 40
      Cenker, J. Reversible strain-induced magnetic phase transition in a van der Waals magnet. Nat. Nanotechnol. 2022, 17, 256– 261,  DOI: 10.1038/s41565-021-01052-6
      40
      Reversible strain-induced magnetic phase transition in a van der Waals magnet
      Cenker, John; Sivakumar, Shivesh; Xie, Kaichen; Miller, Aaron; Thijssen, Pearl; Liu, Zhaoyu; Dismukes, Avalon; Fonseca, Jordan; Anderson, Eric; Zhu, Xiaoyang; Roy, Xavier; Xiao, Di; Chu, Jiun-Haw; Cao, Ting; Xu, Xiaodong
      Nature Nanotechnology (2022), 17 (3), 256-261CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
      Abstr.: Mech. deformation of a crystal can have a profound effect on its phys. properties. Notably, even small modifications of bond geometry can completely change the size and sign of magnetic exchange interactions and thus the magnetic ground state. Here we report the strain tuning of the magnetic properties of the A-type layered antiferromagnetic semiconductor CrSBr achieved by designing a strain device that can apply continuous, in situ uniaxial tensile strain to two-dimensional materials, reaching several percent at cryogenic temps. Using this app., we realize a reversible strain-induced antiferromagnetic-to-ferromagnetic phase transition at zero magnetic field and strain control of the out-of-plane spin-canting process. First-principles calcns. reveal that the tuning of the in-plane lattice const. strongly modifies the interlayer magnetic exchange interaction, which changes sign at the crit. strain. Our work creates new opportunities for harnessing the strain control of magnetism and other electronic states in low-dimensional materials and heterostructures.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38Xhs1CmtrY%253D&md5=5329c579eba4cbc67962e74e0c7cc58c
    41. 41
      Boix-Constant, C. Multistep magnetization switching in orthogonally twisted ferromagnetic monolayers. Nat. Mater. 2024, 23, 212– 218,  DOI: 10.1038/s41563-023-01735-6
      There is no corresponding record for this reference.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.3c05010.

    • Ultrafast THz near-field spectroscopy setup, tunable optical pump pulses from a noncollinear optical parametric amplifier (NOPA), estimation of the electron–hole pair density, rate-equation model for the ultrafast dynamics of electron–hole pairs, estimating the radiative exciton lifetime in monolayer CrSBr, modeling the spectral near-field response, steady-state nanospectroscopy of monolayer CrSBr, modeling the nonequilibrium dielectric function of monolayer CrSBr, and dependence of the pump–probe dynamics on the pump polarization (PDF)


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