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Delicate Ferromagnetism in MnBi6Te10

  • Chenhui Yan
    Chenhui Yan
    Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois60637, United States
    More by Chenhui Yan
    Orcidhttps://orcid.org/0000-0002-5440-9536
  • , 
  • Yanglin Zhu
    Yanglin Zhu
    Department of Physics, Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
    More by Yanglin Zhu
  • , 
  • Leixin Miao
    Leixin Miao
    Department of Materials Science and Engineering, The Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
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  • , 
  • Sebastian Fernandez-Mulligan
    Sebastian Fernandez-Mulligan
    Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois60637, United States
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  • Emanuel Green
    Emanuel Green
    Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois60637, United States
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  • , 
  • Ruobing Mei
    Ruobing Mei
    Department of Physics, Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
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  • , 
  • Hengxin Tan
    Hengxin Tan
    Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot7610001, Israel
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  • , 
  • Binghai Yan
    Binghai Yan
    Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot7610001, Israel
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    Orcidhttps://orcid.org/0000-0003-2164-5839
  • , 
  • Chao-Xing Liu
    Chao-Xing Liu
    Department of Physics, Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
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  • , 
  • Nasim Alem
    Nasim Alem
    Department of Materials Science and Engineering, The Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
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    Orcidhttps://orcid.org/0000-0003-0009-349X
  • , 
  • Zhiqiang Mao
    Zhiqiang Mao
    Department of Physics, Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
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  • , and 
  • Shuolong Yang*
    Shuolong Yang
    Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois60637, United States
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-8200-9898
Cite this: Nano Lett. 2022, 22, 24, 9815–9822
Publication Date (Web):October 31, 2022
https://doi.org/10.1021/acs.nanolett.2c02500

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Abstract

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Tailoring magnetic orders in topological insulators is critical to the realization of topological quantum phenomena. An outstanding challenge is to find a material where atomic defects lead to tunable magnetic orders while maintaining a nontrivial topology. Here, by combining magnetization measurements, angle-resolved photoemission spectroscopy, and transmission electron microscopy, we reveal disorder-enabled, tunable magnetic ground states in MnBi6Te10. In the ferromagnetic phase, an energy gap of 15 meV is resolved at the Dirac point on the MnBi2Te4 termination. In contrast, antiferromagnetic MnBi6Te10 exhibits gapless topological surface states on all terminations. Transmission electron microscopy and magnetization measurements reveal substantial Mn vacancies and Mn migration in ferromagnetic MnBi6Te10. We provide a conceptual framework where a cooperative interplay of these defects drives a delicate change of overall magnetic ground state energies and leads to tunable magnetic topological orders. Our work provides a clear pathway for nanoscale defect-engineering toward the realization of topological quantum phases.

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The first intrinsic magnetic topological insulator MnBi2Te4 (MBT) (1−16) integrates topology with magnetism, and provides a fertile ground for realizing fascinating topological phases. Importantly, MBT exhibits intralayer ferromagnetism and interlayer antiferromagnetism. The compensated magnetic moments in even-layer MBT and the uncompensated ones in odd-layer counterparts give rise to axion insulators (1) and quantum anomalous Hall (QAH) insulators, (14) respectively. The key to tuning and optimizing these topological phases is to precisely control the interlayer magnetic interactions.
A promising route is to construct MnBi2nTe3n+1 superlattices, where Bi2Te3 (BT) buffer layers are inserted between the MBT layers. Previous studies have revealed that superlattices with n ≤ 3 exhibit interlayer antiferromagnetism, (17−23) whereas those with n > 3 display ferromagnetism. (23−25) On the other hand, for higher order superlattices the interlayer magnetic interactions become progressively weaker, (23) which puts a strong limit on the tunability of magnetic interactions and subsequently on the onset temperatures of the resulting topological phases. Notably, previous studies on powder MnBi6Te10 (26) or hydrostatically pressurized MnBi6Te10 (27) revealed the possibility of tunable magnetic ground states, yet the nature of these samples disallowed angle-resolved photoemission spectroscopy (ARPES) to directly determine the band topology.
An alternative route is to explore the prevalent disorder effects in MBT and related compounds, (28−33) and potentially use disorder to control the interlayer magnetic interactions. Recent experiments on MnSb2Te4 (MST) and Sb-doped MnBi2nTe3n+1 have demonstrated that the Mn/Sb and Mn/Bi antisite defects play an important role in tuning the system between antiferromagnetic (AFM) and ferromagnetic (FM) phases. (29,34) However, no direct evidence has been found in the momentum space for the broken-symmetry gap in these materials. Moreover, the topological nature of MST is still under intense debates due to the reduced spin–orbit coupling (SOC) effect. (12,35,36) It is thus an intriguing question whether such disorder-mediated magnetic interactions can be realized in the MBT-derived compounds without compromising the SOC effect, which potentially enables tunable topological phases by a sensitive control of the disorder.
In this Letter, we report a delicate FM topological insulator state in MnBi6Te10, which is attributed to disorder-mediated magnetic interactions. We employ high-resolution laser-based ARPES to detail the electronic structures of the FM and AFM phases, respectively. In the FM phase, a broken-symmetry gap is unambiguously observed on the topological surface state (TSS) of the MBT termination, with a gap onset temperature coinciding with the Curie temperature. In stark contrast, all terminations of the AFM phase of MnBi6Te10 exhibit negligible energy gaps on the TSS. Furthermore, our structural and magnetic characterizations reveal that Mn vacancies in MBT layers and Mn migration from MBT to BT layers are prevalent in FM MnBi6Te10. We provide a conceptual framework where a delicate interplay of Mn vacancies and Mn migration leads to the tunable magnetic phases in MnBi6Te10. Our work not only establishes the first unequivocal FM topological insulator phase in MnBi6Te10, but also demonstrates one of the highest FM Tc’s (13 K) among all MBT-derived compounds. (24,25,37) The proposed new scheme of disorder-mediated ferromagnetism provides a pathway toward sensitive nanoscale tuning of topological phases and future topological quantum devices.
The MnBi6Te10 single crystals were synthesized through a self-flux method. (20) So far, only the AFM phase was reported in MnBi6Te10, which is the ground state for MnBi2nTe3n+1 (n ≤ 3). The FM phase has 0.05 meV higher total energy per Mn atom that may be compensated by introducing certain type of defects. (23,38) To meet this challenge, we increased the growth temperature window by 5 °C for FM samples comparing with the growth temperature for AFM samples in order to create Mn/Bi antisites and Mn vacancies, as both types of defects can weaken the interlayer AFM exchange coupling.
As shown in Figure 1b, the X-ray diffraction (XRD) patterns for the two magnetic phases exhibit the same (00l) diffraction peaks in good agreement with previous studies. (20,39,40)Figure 1c displays the zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibilities measured with a c-axis applied field H = 100 Oe. A sharp Λ-like peak at 10.2 K is observed in the magnetic susceptibility of AFM MnBi6Te10, which indicates an AFM transition. (20,23,25) The ZFC-FC bifurcation below 8 K seen in the AFM sample can be attributed to the evolution from a long-range AFM order to a cluster spin glass state, (23) which is likely driven by the magnetic frustration caused by competing FM and AFM interactions. In contrast, the magnetic susceptibility for the FM material shows signatures of a typical FM transition: a rapid increase and plateauing of the susceptibility below the Curie temperature of 13 K. For FM MBT-derived materials, magneto-optical imaging has demonstrated that the ZFC-FC bifurcation in magnetic susceptibilities is likely due to the movement of FM domains. (41) The FM phase is further confirmed by the isothermal magnetization curves in Figure 1e: a typical hysteresis loop expected for FM materials is revealed. In contrast, a spin-flop-like transition is observed in the AFM phase (Figure 1d), in agreement with previous reports on AFM MnBi4Te7. (19)

Figure 1

Figure 1. Structural and magnetic characterizations of MnBi6Te10. (a) Schematic crystal structure of MnBi6Te10. (b) X-ray diffraction of ferromagnetic (FM, red) and antiferromagnetic (AFM, blue) MnBi6Te10. (c) Temperature dependent zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibilities of FM (red) and AFM (blue) MnBi6Te10 using an external field H = 100 Oe along the c-axis. The results corresponding to the AFM samples are offset vertically for clarity. (d,e) Isothermal magnetization curves with the magnetic field applied along the c-axis and in the ab plane at various temperatures in (d) AFM and (e) FM MnBi6Te10.

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High-resolution ARPES results on FM MnBi6Te10 are presented in Figure 2. Three possible terminations are expected after cleaving, denoted by the top layer as MBT, single BT (1-BT), or double BT (2-BT) terminations. It is crucial to employ micron-scale laser beams to distinguish different terminations. Figure 2 presents three types of electronic structures found on FM MnBi6Te10. First, we point out that the termination assignment cannot be based on direct comparison between ARPES data and first-principles calculations, as the latter is often unable to properly model the impacts of defects and quantum confinement in MnBi2nTe3n+1 materials. (16,40,42) A common spectroscopic feature characteristic of the MBT termination for all MnBi2nTe3n+1 superlattices is a Dirac surface state hybridized with parabolic Rashba bands. (16,40,43) Hence, we associate the electronic structure in Figure 2a with the MBT termination. We assign the spectrum in Figure 2d to the 1-BT termination due to its strong resemblance with the counterpart on the 1-BT termination of FM Mn(Bi0.85Sb0.15)4Te7. (37) Finally, since our material is phase-pure as demonstrated by the transmission electron microscopy measurement (Figure 3), we can only assign the last type of ARPES spectrum (Figure 2g) to the 2-BT termination. The two different types of BT-derived terminations exhibit electron (Figure 2d) and hole (Figure 2g) dopings, respectively. We notice that the qualitative trend is consistent with the previous ARPES data on the BT-derived terminations of AFM MnBi6Te10 (40) and FM MnBi8Te13, (25) where electron doping systematically decreases as the number of BT layers increases. Nevertheless, the doping change from the 1-BT to the 2-BT terminations in FM MnBi6Te10 is much more dramatic, which may be due to its specific defect configuration [Supporting Information (SI) Note 1].

Figure 2

Figure 2. Electronic structure of ferromagnetic MnBi6Te10. Energy-momentum spectra along Γ̅ – Μ̅ at (a) 7.5 K, and (b) 20 K. The insert in (a) illustrates the MnBi2Te4 (MBT) termination. (c) Comparison of energy distribution curves at Γ̅ . An energy gap is observed at the Dirac point (black dashed line) at 7.5 K. The counterpart results for the 1-Bi2Te3 (1-BT) termination are plotted in (d–f): (d,e) energy-momentum spectra, and (f) energy distribution curves at Γ̅ . The counterpart results for the 2-Bi2Te3 (2-BT) termination are plotted in (g–i): (g,h) energy-momentum spectra, and (i) energy distribution curves at Γ̅ .

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Figure 3

Figure 3. Structural characterizations of MnBi6Te10. Annular dark-field scanning transmission electron microscopy (ADF-STEM) images of (a) AFM and (b) FM MnBi6Te10. Scale bar indicates 5 nm. Mn vacancies are highlighted by red shading, where the intensity for the atomic column of Mn is lower than the value of two standard deviations below the Mn mean intensity. (c) Magnified image (yellow box in (b)). (d) A cartoon illustration showing HIL > HD in the AFM phase. (e) With increased Mn vacancies, HIL < HD in the FM phase.

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We focus on the MBT and 1-BT terminations as their Dirac points are clearly resolvable below the Fermi level. Circular dichroism in ARPES measurements exhibits antisymmetric patterns near the Dirac point for both terminations (SI Figure 1). A helical circular dichroism (CD) pattern in ARPES has been successfully used to identify the topological surface state in MnBi2nTe3n+1 compounds. (11,43) For the MBT termination, an energy gap of ∼15 meV at the Dirac point is visible in the ARPES data at 7.5 K and this gap disappears at 20 K (Figure 2a,b). The comparison of energy distribution curves (EDCs) taken at Γ̅ further highlights this temperature-dependent energy gap: a spectral peak at −0.18 eV at 20 K evolves into a dip upon cooling to 7.5 K. A temperature-dependent band dispersion analysis on the MBT termination yields a gap-closing temperature at 12.7 ± 1.4 K (SI Note 2 and SI Figure 2), yet we cannot completely rule out the existence of a finite gap above Tc. For the 1-BT termination, the spectra taken at both temperatures exhibit gapless Dirac cones (Figure 2d–f).
AFM MnBi6Te10 exhibits three types of ARPES spectra on three terminations (SI Figure 3 and SI Figure 4). (21,40,44,45) The energy gaps at the Dirac points are negligible on all terminations of AFM MnBi6Te10, consistent with previous reports. (22,40,43) While it is not the focus of our work, we point out that a likely scenario for the gapless TSS on the MBT termination of AFM MnBi6Te10 is that the Mn–Bi antisite defects may cause the TSS wave function to be relocated into the space between two adjacent MBT septuple layers. (46) Both the nonmagnetic BT layers and the opposite magnetic moments from the two adjacent MBT layers may reduce the magnetic gap.
We thus establish the spectroscopic evidence of a magnetically induced broken-symmetry gap in FM MnBi6Te10 for the first time. This gap originates from strong interactions between the TSS electrons and the magnetic moments in the MBT layer. In contrast, the TSS electrons on the 1-BT termination are localized to the top BT layer and spatially separate from the magnetic MBT layer, leading to a gapless Dirac point. Gap opening in the MnBi2nTe3n+1 compound family has been highly controversial. (2,3,6,7,11,16,22,23,25,40,44,47−50) Specifically on MnBi6Te10, a 60 meV gap was reported in the AFM phase (44) in contrast to the results from all other ARPES studies on MnBi6Te10 (22,23,25,40) including ours. However, this gap persists well above the magnetic ordering temperature and is likely due to extrinsic reasons such as local impurities. On the contrary, our clear spectroscopic gap in FM MnBi6Te10 which onsets at the magnetic ordering temperature clarifies the physics picture of a broken time-reversal symmetry. In addition, our resolved gap in FM MnBi6Te10 is fully consistent with previous observations on FM MnBi8Te13. (24,25) The crucial new discovery is that the magnetic phase of MnBi6Te10 can be controllably tuned between AFM and FM.
We proceed to investigate the microscopic mechanism leading to tunable magnetic phases. Notably, Mn-doped BT can host ferromagnetism, (51−54) yet the distinct XRD and ARPES results from our FM MnBi6Te10 compared to those from Mn-doped BT (51−53) suggest that the proportion of the latter phase in our materials is negligible. Furthermore, structural characterizations by annular dark field scanning transmission electron microscopy (ADF-STEM) do not find any observable impurity phases for AFM or FM samples, as shown in Figure 3. The atomic resolution images exhibit an interleaved structure composed of MBT septuple layers and 2 BT quintuple layers, which is consistent with the crystal structures determined by XRD in Figure 1b. The selected-area electron diffraction (SAED) patterns, determined by the atomic stacking sequences and periodicity along the c-axis (SI Figure 5), further confirm the single phase of the FM and AFM MnBi6Te10 samples.
Mn vacancies are observed in both AFM and FM MnBi6Te10, as highlighted in red in Figure 3a–c. Strikingly, the concentration of Mn vacancies in the FM samples (Figure 3b) is much higher than that in the AFM samples (Figure 3a). The quantitative percentages of Mn vacancies are hard to determine from ADF-STEM since the intensity is formed by atoms in the projection of the entire atomic columns perpendicular to the imaging plane. The energy dispersive X-ray (EDX) analysis confirms the higher concentration of Mn vacancies in the FM samples (SI Table 1).
Another important type of disorder is Mn migration. This effect is manifested as the ferrimagnetic order induced by the “antisite defects” in MnSb2Te4 (28,29) and Sb-doped MnBi4Te7. (34,55) Measurements of magnetic moments at low and high magnetic fields can be used to estimate the density of Mn migration. (32) Using our measured magnetic moments at 0.2 T (Figure 1) and 7 T (SI Figure 6), we obtain that 8.1% and 11.8% of Mn atoms in FM and AFM MnBi6Te10, respectively, have migrated from the original Mn sheets to the neighboring layers. Here we use the chemical formula of Mn1–y-6x(Bi1–xMnx)6Te10, where 6x and y indicate the densities of Mn migration and Mn vacancies, respectively. The comparable densities of Mn migration in FM and AFM MnBi6Te10 suggest that the true physical picture for the FM order is more complex than a simple migration-induced ferrimagnetism. (29)
We construct a conceptual model to illustrate that it is the delicate interplay of Mn vacancies and Mn migration that leads to the tunable magnetic phases in MnBi6Te10. We consider two Mn sheets where the intralayer and interlayer magnetic interactions are FM and AFM, respectively. An intermediate layer of migrated Mn ions interacts with the original Mn sheets antiferromagnetically. We obtain the energy difference between the FM and AFM alignments of the original Mn sheets (SI Note 3)
EFM−EAFM=2∑i,j(1),(2)JIL,ij−2∑i,j(1),(3)JD,ij=2HIL−2HD
(1)
Here JIL,ij is the interlayer magnetic coupling between site i and j in the two original Mn sheets; JD,ij is the defect-induced magnetic coupling across the original Mn sheets and the defect layer. Importantly, the scenarios of HIL > HD and HIL < HD lead to the AFM and FM alignment of the two original Mn sheets, respectively.
To evaluate the energy balance in the presence of Mn vacancies and Mn migration, we consider the scaling laws of HIL and HD in terms of the Mn density in the original sheets (no) and in the migrated space (nm): HIL ∝ no2 and HD ∝ nonm. These scaling laws are rooted in the microscopic nature of magnetic interactions (SI Note 4), and independent of the numerical details of JD,ij and JIL,ij. Subsequently
HD/HIL∝nm/no
(2)
Equation 2 reveals the microscopic mechanism for defect-induced ferromagnetism in MnBi6Te10. First, with substantial Mn vacancies in the MBT layer, both HIL and HD decrease (Figure 3e). The energy balance is determined by nm/no. Notably, the saturated magnetic moments at low and high magnetic fields (M1 and M2, respectively) allow us to estimate this ratio: (32)nm/no=(M2−M1)/(M2+M1). Our measurements lead to nm/no = 0.2 and 0.13 for the FM and AFM samples, respectively (SI Note 4). The values of nm/no for all the AFM MnBi6Te10 materials in the literature are smaller than 0.1. (20,25,38) The > 50% increase of nm/no tips the energy balance between HD and HIL, stabilizing a ferromagnetic phase (Figure 3e). We remark that the more accurate nm/no can be >0.2 for FM MnBi6Te10 due to the potentially unsaturated M2 at 7 T. (32)
We emphasize that the simple Ising model neglects other types of magnetic interactions and should only serve as a conceptual framework. Nevertheless, it offers powerful scaling laws under which the delicate adjustment of Mn vacancies and Mn migration leads to tunable magnetic phases in MnBi6Te10. We emphasize that this disorder tuning leads to a change of the “global” magnetic ground state, which is to be distinguished from the more trivial disorder effect resulting in local changes of magnetism. (36) Our new insight of quantitatively considering the ratio of nm/no is a substantial advance compared to previous studies which qualitatively pointed out the importance of disorder (36,56) and the theoretical modeling whose predictions sensitively depend on numerical details. (55) This insight, importantly, is obtained by the multimodal measurements combining magnetization, laser-based μARPES, and atomic-resolution TEM on the same batch of samples. Notably, FM MnBi6Te10 exhibits a Tc of 13 K, which is higher than the Tc of 10.5 K for FM MnBi8Te13 (24,25) and the Tc < 9 K for hydrostatically pressured MnBi6Te10, (27) suggesting that the disorder-induced ferromagnetism may be more robust than that induced by superlattice stacking or by hydrostatic pressure. The delicate ferromagnetism revealed in this work will serve as a general framework to understand the phenomenology in all MnBi2nTe3n+1 superlattices.
We hope that our work will serve as a milestone to motivate further theoretical and experimental studies, in particular spectroscopy and microscopy studies with atomic resolutions to resolve what defects form under what growth conditions in disordered FM MnBi2nTe3n+1. The defect engineering via controlling the growth temperature can potentially be adapted in thin film deposition of MnBi2nTe3n+1 as well. Understanding how to tune the magnetism and topological states without introducing new chemical elements in the same MBT system through delicately controlling the defects, allows us to selectively realize exotic phases such as the axion insulator state and the quantum anomalous Hall insulator state, paving the road toward functional topological quantum devices at realistic cryogenic temperatures.

Methods

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Sample Growth and Characterization

MnBi6Te10 single crystals were synthesized using the self-flux method, (20) in which Mn, Bi, and Te powders were mixed with a stoichiometric molar ratio and sealed in a carbon-coated quartz tube under high vacuum. For the synthesis of ferromagnetic (FM) MnBi6Te10, the mixture was heated to 900 °C in a furnace and held for 10 h for homogeneous melting. The mixture then underwent a series of cooling and annealing stages: from 900 to 595 °C in 5 h, from 595 to 580 °C at a rate of 0.1 °C/h, annealed at 580 °C for 48 h, and finally quenched in water at 0 °C. For the synthesis of antiferromagnetic (AFM) MnBi6Te10, the same melting was adopted. It was followed by cooling from 900 to 590 °C in 5 h and then from 590 to 575 °C at a rate of 0.1 °C/h, annealed at 575 °C for 48 h and then quenched in water at 0 °C. The as-grown single crystals were found to be plate-like with luster and lateral dimensions of 2 × 2 mm2.
The crystallization of the grown single crystals was verified by the sharp (00L) XRD peaks using a Malvern Panalytical Empyrean diffractometer (Cu Kα radiation), as shown in Figure 1b. Magnetization of crystals was measured by the MPMS3 SQUID magnetometer (Quantum Design).
The TEM specimens were prepared using ThermoFisher Helios 660 dual beam system. The specimens were thinned down to electron transparency using 30 kV and 5 kV Ga ion beam and cleaned with 2 kV and 1 kV ion beam. The aberration-corrected STEM imaging was performed using ThermoFisher Titan G2 S/TEM equipped with image and probe correctors. The operating voltage for the STEM imaging was 200 kV. The STEM images have been drift corrected using the nonlinear drift correction algorithm. The elemental mapping was acquired with energy dispersive X-ray spectroscopy (EDS) with STEM mode.

Ultrahigh Resolution Laser-Based Angle-Resolved Photoemission Spectroscopy (ARPES)

Our laser-based ARPES setup was based on a Coherent MIRA Ti:sapphire oscillator. With a 5 W, 532 nm continuous wave seed laser, the oscillator output >9 nJ pulses with a central wavelength at 820 nm, a bandwidth of 7.6 nm, a pulse duration of 130 fs, and a repetition rate of 80 MHz. One mm beta barium borate (BBO) crystals were used to generate the second harmonic (410 nm) and fourth harmonic (205 nm), the latter of which was used for ARPES measurements. The optical bandwidth of the 6 eV beam was expected to be 2.7 meV due to the finite BBO thicknesses. The overall energy resolution incorporating the ARPES analyzer resolution was characterized as 4 meV. (57) The beam waist at the optical focal point was less than 10 μm. (57)

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.2c02500.

  • Defects and doping, circular dichroism in the ARPES spectra of ferromagnetic MnBi6Te10, side-by-side comparison of FM and AFM MnBi6Te10 using the temperature evolutions of the TSS’s on the MBT termination, electronic structure of antiferromagnetic MnBi6Te10, circular dichroism in the ARPES spectra of antiferromagnetic MnBi6Te10, selected area electron diffraction (SAED) patterns of MnBi6Te10, high-field magnetization measurements of AFM and FM MnBi6Te10, estimates of the chemical concentrations of MnBi6Te10 based on the scanning transmission electron microscopy–energy dispersive X-ray (STEM-EDX) analysis, magnetic interactions, and the determination of the Mn density ratio between different layers (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Chenhui Yan - Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois60637, United States;  Orcidhttps://orcid.org/0000-0002-5440-9536
    • Yanglin Zhu - Department of Physics, Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
    • Leixin Miao - Department of Materials Science and Engineering, The Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
    • Sebastian Fernandez-Mulligan - Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois60637, United States
    • Emanuel Green - Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois60637, United States
    • Ruobing Mei - Department of Physics, Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
    • Hengxin Tan - Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot7610001, Israel
    • Binghai Yan - Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot7610001, Israel;  Orcidhttps://orcid.org/0000-0003-2164-5839
    • Chao-Xing Liu - Department of Physics, Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
    • Nasim Alem - Department of Materials Science and Engineering, The Pennsylvania State University, University Park, State College, Pennsylvania16802, United States;  Orcidhttps://orcid.org/0000-0003-0009-349X
    • Zhiqiang Mao - Department of Physics, Pennsylvania State University, University Park, State College, Pennsylvania16802, United States
  • Author Contributions

    C.Y., S.M., E.G., and S.Y. performed the ARPES measurements. Y.Z. and Z.M. grew the samples and performed the magnetization measurements. L.M. and N.A. performed the STEM measurements. All authors discussed the results and commented on the paper. C.Y. and S.Y. wrote the paper with input from all authors.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The ARPES work was in part supported by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division, under contract No. DE-AC02-06CH11357, and in part supported by NSF DMR-2145373. The financial support for sample preparation was provided by the National Science Foundation through the Penn State 2D Crystal Consortium-Materials Innovation Platform (2DCC-MIP) under NSF cooperative agreement DMR-1539916 and DMR-2039351. Z.Q.M. acknowledges the support from the US National Science Foundation under grant DM-1917579. C.X.L. and R.B.M. acknowledge the support of the U.S. Department of Energy (Grant DESC0019064). B.Y. acknowledges the financial support by the Willner Family Leadership Institute for the Weizmann Institute of Science, the Benoziyo Endowment Fund for the Advancement of Science, Ruth and Herman Albert Scholars Program for New Scientists, the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant 815869). N.A. and L.M acknowledge the support by NSF through the Pennsylvania State University Materials Research Science and Engineering Center DMR-2011839 (2020–2026). L.M and N.A. acknowledge the Air Force Office of Scientific Research (AFOSR) program FA9550-18-1-0277 as well as GAME MURI, 10059059-PENN for support.

References

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This article references 57 other publications.

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    The intricate interplay between non-trivial topol. and magnetism in two-dimensional materials can lead to the emergence of interesting phenomena such as the quantum anomalous Hall effect. Here we investigate the quantum transport of both bulk crystal and exfoliated MnBi2Te4 flakes in a field-effect transistor geometry. For the six septuple-layer device tuned into the insulating regime, we observe a large longitudinal resistance and zero Hall plateau, which are characteristics of an axion insulator state. The robust axion insulator state occurs in zero magnetic field, over a wide magnetic-field range and at relatively high temps. Moreover, a moderate magnetic field drives a quantum phase transition from the axion insulator phase to a Chern insulator phase with zero longitudinal resistance and quantized Hall resistance h/e2, where h is Planck's const. and e is electron charge. Our results pave the way for using even-no. septuple-layer MnBi2Te4 to realize the quantized topol. magnetoelec. effect and axion electrodynamics in condensed matter systems.
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  2. 2
    Otrokov, M. M.; Klimovskikh, I. I.; Bentmann, H.; Estyunin, D.; Zeugner, A.; Aliev, Z. S.; Gaß, S.; Wolter, A. U. B.; Koroleva, A. V.; Shikin, A. M.; Blanco-Rey, M.; Hoffmann, M.; Rusinov, I. P.; Vyazovskaya, A. Y.; Eremeev, S. V.; Koroteev, Y. M.; Kuznetsov, V. M.; Freyse, F.; Sánchez-Barriga, J.; Amiraslanov, I. R.; Babanly, M. B.; Mamedov, N. T.; Abdullayev, N. A.; Zverev, V. N.; Alfonsov, A.; Kataev, V.; Büchner, B.; Schwier, E. F.; Kumar, S.; Kimura, A.; Petaccia, L.; Di Santo, G.; Vidal, R. C.; Schatz, S.; Kißner, K.; Ünzelmann, M.; Min, C. H.; Moser, S.; Peixoto, T. R. F.; Reinert, F.; Ernst, A.; Echenique, P. M.; Isaeva, A.; Chulkov, E. V. Prediction and Observation of an Antiferromagnetic Topological Insulator. Nature 2019, 576 (7787), 416– 422,  DOI: 10.1038/s41586-019-1840-9
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    2
    Prediction and observation of an antiferromagnetic topological insulator
    Otrokov, M. M.; Klimovskikh, I. I.; Bentmann, H.; Estyunin, D.; Zeugner, A.; Aliev, Z. S.; Gass, S.; Wolter, A. U. B.; Koroleva, A. V.; Shikin, A. M.; Blanco-Rey, M.; Hoffmann, M.; Rusinov, I. P.; Vyazovskaya, A. Yu.; Eremeev, S. V.; Koroteev, Yu. M.; Kuznetsov, V. M.; Freyse, F.; Sanchez-Barriga, J.; Amiraslanov, I. R.; Babanly, M. B.; Mamedov, N. T.; Abdullayev, N. A.; Zverev, V. N.; Alfonsov, A.; Kataev, V.; Buchner, B.; Schwier, E. F.; Kumar, S.; Kimura, A.; Petaccia, L.; Di Santo, G.; Vidal, R. C.; Schatz, S.; Kissner, K.; Unzelmann, M.; Min, C. H.; Moser, Simon; Peixoto, T. R. F.; Reinert, F.; Ernst, A.; Echenique, P. M.; Isaeva, A.; Chulkov, E. V.
    Nature (London, United Kingdom) (2019), 576 (7787), 416-422CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Magnetic topol. insulators are narrow-gap semiconductor materials that combine non-trivial band topol. and magnetic order. Unlike their nonmagnetic counterparts, magnetic topol. insulators may have some of the surfaces gapped, which enables a no. of exotic phenomena that have potential applications in spintronics, such as the quantum anomalous Hall effect and chiral Majorana fermions3. So far, magnetic topol. insulators have only been created by means of doping nonmagnetic topol. insulators with 3d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temps. An intrinsic magnetic topol. insulator-a stoichiometric well ordered magnetic compd.-could be an ideal soln. to these problems, but no such material has been obsd. so far. Here, the authors predict by ab initio calcns. and further confirm using various exptl. techniques the realization of an antiferromagnetic topol. insulator in the layered van der Waals compd. MnBi2Te4. The antiferromagnetic ordering that MnBi2Te4 shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a Z2 topol. classification; Z2 = 1 for MnBi2Te4, confirming its topol. nontrivial nature. The expts. indicate that the symmetry-breaking (0001) surface of MnBi2Te4 exhibits a large band gap in the topol. surface state. The authors expect this property to eventually enable the observation of a no. of fundamental phenomena, among them quantized magnetoelec. coupling and axion electrodynamics. Other exotic phenomena could become accessible at much higher temps. than those reached so far, such as the quantum anomalous Hall effect and chiral Majorana fermions.
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  3. 3
    Rienks, E. D. L.; Wimmer, S.; Sánchez-Barriga, J.; Caha, O.; Mandal, P. S.; Růžička, J.; Ney, A.; Steiner, H.; Volobuev, V. V.; Groiss, H.; Albu, M.; Kothleitner, G.; Michalička, J.; Khan, S. A.; Minár, J.; Ebert, H.; Bauer, G.; Freyse, F.; Varykhalov, A.; Rader, O.; Springholz, G. Large Magnetic Gap at the Dirac Point in Bi2Te3/MnBi2Te4 Heterostructures. Nature 2019, 576 (7787), 423– 428,  DOI: 10.1038/s41586-019-1826-7
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    3
    Large magnetic gap at the Dirac point in Bi2Te3/MnBi2Te4 heterostructures
    Rienks, E. D. L.; Wimmer, S.; Sanchez-Barriga, J.; Caha, O.; Mandal, P. S.; Ruzicka, J.; Ney, A.; Steiner, H.; Volobuev, V. V.; Groiss, H.; Albu, M.; Kothleitner, G.; Michalicka, J.; Khan, S. A.; Minar, J.; Ebert, H.; Bauer, G.; Freyse, F.; Varykhalov, A.; Rader, O.; Springholz, G.
    Nature (London, United Kingdom) (2019), 576 (7787), 423-428CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Magnetically doped topol. insulators enable the quantum anomalous Hall effect (QAHE), which provides quantized edge states for lossless charge-transport applications1-8. The edge states are hosted by a magnetic energy gap at the Dirac point2, but hitherto all attempts to observe this gap directly have been unsuccessful. Observing the gap is considered to be essential to overcoming the limitations of the QAHE, which so far occurs only at temps. that are one to two orders of magnitude below the ferromagnetic Curie temp., TC (ref. 8). Here we use low-temp. photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi2Te3, which displays ferromagnetic out-of-plane spin texture and opens up only below TC. Surprisingly, our anal. reveals large gap sizes at 1 K of up to 90 millielectronvolts, which is five times larger than theor. predicted9. Using multiscale anal. we show that this enhancement is due to a remarkable structure modification induced by Mn doping: instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3 quintuple layers is formed. This enhances the wavefunction overlap and size of the magnetic gap10. Mn-doped Bi2Se3 (ref. 11) and Mn-doped Sb2Te3 form similar heterostructures, but for Bi2Se3 only a nonmagnetic gap is formed and the magnetization is in the surface plane. This is explained by the smaller spin-orbit interaction by comparison with Mn-doped Bi2Te3. Our findings provide insights that will be crucial in pushing lossless transport in topol. insulators towards room-temp. applications.
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  4. 4
    Gong, Y.; Guo, J.; Li, J.; Zhu, K.; Liao, M.; Liu, X.; Zhang, Q.; Gu, L.; Tang, L.; Feng, X.; Zhang, D.; Li, W.; Song, C.; Wang, L.; Yu, P.; Chen, X.; Wang, Y.; Yao, H.; Duan, W.; Xu, Y.; Zhang, S.-C.; Ma, X.; Xue, Q.-K.; He, K. Experimental Realization of an Intrinsic Magnetic Topological Insulator. Chin. Phys. Lett. 2019, 36 (7), 076801,  DOI: 10.1088/0256-307X/36/7/076801
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    4
    Experimental realization of an intrinsic magnetic topological insulator
    Gong, Yan; Guo, Jingwen; Li, Jiaheng; Zhu, Kejing; Liao, Menghan; Liu, Xiaozhi; Zhang, Qinghua; Gu, Lin; Tang, Lin; Feng, Xiao; Zhang, Ding; Li, Wei; Song, Canli; Wang, Lili; Yu, Pu; Chen, Xi; Wang, Yayu; Yao, Hong; Duan, Wenhui; Xu, Yong; Zhang, Shou-Cheng; Ma, Xucun; Xue, Qi-Kun; He, Ke
    Chinese Physics Letters (2019), 36 (7), 076801CODEN: CPLEEU; ISSN:1741-3540. (IOP Publishing Ltd.)
    An intrinsic magnetic topol. insulator (TI) is a stoichiometric magnetic compd. possessing both inherent magnetic order and topol. electronic states. Such a material can provide a shortcut to various novel topol. quantum effects but remained elusive exptl. for a long time. Here we report the exptl. realization of thin films of an intrinsic magnetic TI, MnBi2Te4, by alternate growth of a Bi2Te3quintuple layer and a MnTe bilayer with mol. beam epitaxy. The material shows the archetypical Dirac surface states in angle-resolved photoemission spectroscopy and is demonstrated to be an antiferromagnetic topol. insulator with ferromagnetic surfaces by magnetic and transport measurements as well as first-principles calcns. The unique magnetic and topol. electronic structures and their interplays enable the material to embody rich quantum phases such as quantum anomalous Hall insulators and axion insulators at higher temp. and in a well-controlled way.
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  5. 5
    Zeugner, A.; Nietschke, F.; Wolter, A. U. B.; Gaß, S.; Vidal, R. C.; Peixoto, T. R. F.; Pohl, D.; Damm, C.; Lubk, A.; Hentrich, R.; Moser, S. K.; Fornari, C.; Min, C. H.; Schatz, S.; Kißner, K.; Ünzelmann, M.; Kaiser, M.; Scaravaggi, F.; Rellinghaus, B.; Nielsch, K.; Hess, C.; Büchner, B.; Reinert, F.; Bentmann, H.; Oeckler, O.; Doert, T.; Ruck, M.; Isaeva, A. Chemical Aspects of the Candidate Antiferromagnetic Topological Insulator MnBi2Te4. Chem. Mater. 2019, 31 (8), 2795– 2806,  DOI: 10.1021/acs.chemmater.8b05017
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    5
    Chemical Aspects of the Candidate Antiferromagnetic Topological Insulator MnBi2Te4
    Zeugner, Alexander; Nietschke, Frederik; Wolter, Anja U. B.; Gass, Sebastian; Vidal, Raphael C.; Peixoto, Thiago R. F.; Pohl, Darius; Damm, Christine; Lubk, Axel; Hentrich, Richard; Moser, Simon K.; Fornari, Celso; Min, Chul Hee; Schatz, Sonja; Kissner, Katharina; Uenzelmann, Maximilian; Kaiser, Martin; Scaravaggi, Francesco; Rellinghaus, Bernd; Nielsch, Kornelius; Hess, Christian; Buechner, Bernd; Reinert, Friedrich; Bentmann, Hendrik; Oeckler, Oliver; Doert, Thomas; Ruck, Michael; Isaeva, Anna
    Chemistry of Materials (2019), 31 (8), 2795-2806CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    High-quality single crystals of MnBi2Te4 are grown for the first time by slow cooling within a narrow range between the m.ps. of Bi2Te3 (586 °C) and MnBi2Te4 (600 °C). Single-crystal X-ray diffraction and electron microscopy reveal ubiquitous antisite defects in both cation sites and, possibly, Mn vacancies (Mn0.85(3)Bi2.10(3)Te4). Thermochem. studies complemented with high-temp. X-ray diffraction establish a limited high-temp. range of phase stability and metastability at room temp. Nevertheless, the synthesis of MnBi2Te4 can be scaled-up as powders can be obtained at subsolidus temps. and quenched at room temp. Bulk samples exhibit long-range antiferromagnetic ordering below 24 K. The Mn(II) out-of-plane magnetic state is confirmed by the magnetization, X-ray photoemission, X-ray absorption, and linear dichroism measurements. The compd. shows a metallic type of resistivity in the range 4.5-300 K and is an n-type conductor that reaches a thermoelec. figure of merit up to ZT = 0.17. Angle-resolved photoemission expts. show a surface state forming a gapped Dirac cone, thus strengthening MnBi2Te4 as a promising candidate for the intrinsic magnetic topol. insulator, in accordance with theor. predictions. The developed synthetic protocols enable further exptl. studies of a crossover between magnetic ordering and nontrivial topol. in bulk MnBi2Te4.
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  6. 6
    Swatek, P.; Wu, Y.; Wang, L.-L.; Lee, K.; Schrunk, B.; Yan, J.; Kaminski, A. Gapless Dirac Surface States in the Antiferromagnetic Topological Insulator MnBi2Te4. Phys. Rev. B 2020, 101 (16), 161109,  DOI: 10.1103/PhysRevB.101.161109
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    6
    Gapless Dirac surface states in the antiferromagnetic topological insulator MnBi2Te4
    Swatek, Przemyslaw; Wu, Yun; Wang, Lin-Lin; Lee, Kyungchan; Schrunk, Benjamin; Yan, Jiaqiang; Kaminski, Adam
    Physical Review B (2020), 101 (16), 161109CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    A review. We used angle-resolved photoemission spectroscopy (ARPES) and d. functional theory calcns. to study the electronic properties of MnBi2Te4, a material that was predicted to be an intrinsic antiferromagnetic (AFM) topol. insulator. In striking contrast to earlier literature showing a full gap opening between two surface band manifolds on the (0001) surface, we obsd. a gapless Dirac surface state with a Dirac point sitting at EB=-280meV. Furthermore, our ARPES data revealed the existence of a second Dirac cone sitting closer to the Fermi level. Surprisingly, these surface states remain intact across the AFM transition. The presence of gapless Dirac states in this material may be caused by different ordering at the surface from the bulk or weaker magnetic coupling between the bulk and surface. Whereas the surface Dirac cones seem to be remarkably insensitive to the AFM ordering most likely due to weak coupling to magnetism, we did observe a splitting of the bulk band accompanying the AFM transition. With a moderately high ordering temp. and interesting gapless Dirac surface states, MnBi2Te4 provides a unique platform for studying the interplay between magnetic ordering and topol.
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  7. 7
    Hao, Y.-J.; Liu, P.; Feng, Y.; Ma, X.-M.; Schwier, E. F.; Arita, M.; Kumar, S.; Hu, C.; Lu, R.; Zeng, M.; Wang, Y.; Hao, Z.; Sun, H.-Y.; Zhang, K.; Mei, J.; Ni, N.; Wu, L.; Shimada, K.; Chen, C.; Liu, Q.; Liu, C. Gapless Surface Dirac Cone in Antiferromagnetic Topological Insulator MnBi2Te4. Phys. Rev. X 2019, 9 (4), 041038,  DOI: 10.1103/PhysRevX.9.041038
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    7
    Gapless Surface Dirac Cone in Antiferromagnetic Topological Insulator MnBi2Te4
    Hao, Yu-Jie; Liu, Pengfei; Feng, Yue; Ma, Xiao-Ming; Schwier, Eike F.; Arita, Masashi; Kumar, Shiv; Hu, Chaowei; Lu, Rui-e; Zeng, Meng; Wang, Yuan; Hao, Zhanyang; Sun, Hong-Yi; Zhang, Ke; Mei, Jiawei; Ni, Ni; Wu, Liusuo; Shimada, Kenya; Chen, Chaoyu; Liu, Qihang; Liu, Chang
    Physical Review X (2019), 9 (4), 041038CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
    The recently discovered antiferromagnetic topol. insulators in the Mn-Bi-Te family with intrinsic magnetic ordering have rapidly drawn broad interest since its cleaved surface state is believed to be gapped, hosting the unprecedented axion states with a half-integer quantum Hall effect. Here, however, we show unambiguously by using high-resoln. angle resolved photoemission spectroscopy that a gapless Dirac cone at the (0001) surface of MnBi2Te4 exists inside the bulk band gap. Such an unexpected surface state remains unchanged across the bulk N´eel temp., and is even robust against severe surface degrdn., indicating addnl. topol. protection. Through symmetry anal. and ab initio calcns. we consider different types of surface reconstruction of the magnetic moments as possible origins giving rise to such linear dispersion. Our results unveil the exptl. topol. properties of MnBi2Te4, revealing that the intrinsic magnetic topol. insulator hosts a rich platform to realize various topol. phases by tuning the magnetic or structural configurations, and thus push forward the comprehensive understanding of magnetic topol. materials.
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  8. 8
    Chen, Y. J.; Xu, L. X.; Li, J. H.; Li, Y. W.; Wang, H. Y.; Zhang, C. F.; Li, H.; Wu, Y.; Liang, A. J.; Chen, C.; Jung, S. W.; Cacho, C.; Mao, Y. H.; Liu, S.; Wang, M. X.; Guo, Y. F.; Xu, Y.; Liu, Z. K.; Yang, L. X.; Chen, Y. L. Topological Electronic Structure and Its Temperature Evolution in Antiferromagnetic Topological Insulator MnBi2Te4. Phys. Rev. X 2019, 9 (4), 041040,  DOI: 10.1103/PhysRevX.9.041040
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    8
    Topological Electronic Structure and Its Temperature Evolution in Antiferromagnetic Topological Insulator MnBi2Te4
    Chen, Y. J.; Xu, L. X.; Li, J. H.; Li, Y. W.; Wang, H. Y.; Zhang, C. F.; Li, H.; Wu, Y.; Liang, A. J.; Chen, C.; Jung, S. W.; Cacho, C.; Mao, Y. H.; Liu, S.; Wang, M. X.; Guo, Y. F.; Xu, Y.; Liu, Z. K.; Yang, L. X.; Chen, Y. L.
    Physical Review X (2019), 9 (4), 041040CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
    The intrinsic magnetic topol. insulator MnBi2Te4 exhibits rich topol. effects such as quantum anomalous Hall effect and axion electrodynamics. Here, by combining the use of synchrotron and laser light sources, we carry out comprehensive and high-resoln. angle-resolved photoemission spectroscopy studies on MnBi2Te4 and clearly identify its topol. electronic structure. In contrast to theor. predictions and previous studies, we observe topol. surface states with diminished gap forming a characteristic Dirac cone. We argue that the topol. surface states are mediated by multidomains of different magnetization orientations. In addn., the temp. evolution of the energy bands clearly reveals their interplay with the magnetic phase transition by showing interesting differences between the bulk and surface states, resp. The investigation of the detailed electronic structure of MnBi2Te4 and its temp. evolution provides important insight into not only the exotic properties of MnBi2Te4, but also the generic understanding of the interplay between magnetism and topol. electronic structure in magnetic topol. quantum materials.
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  9. 9
    Li, H.; Gao, S.-Y.; Duan, S.-F.; Xu, Y.-F.; Zhu, K.-J.; Tian, S.-J.; Gao, J.-C.; Fan, W.-H.; Rao, Z.-C.; Huang, J.-R.; Li, J.-J.; Yan, D.-Y.; Liu, Z.-T.; Liu, W.-L.; Huang, Y.-B.; Li, Y.-L.; Liu, Y.; Zhang, G.-B.; Zhang, P.; Kondo, T.; Shin, S.; Lei, H.-C.; Shi, Y.-G.; Zhang, W.-T.; Weng, H.-M.; Qian, T.; Ding, H. Dirac Surface States in Intrinsic Magnetic Topological Insulators EuSn2As2 and MnBi2nTe3n+1. Phys. Rev. X 2019, 9 (4), 041039,  DOI: 10.1103/PhysRevX.9.041039
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    9
    Dirac Surface States in Intrinsic Magnetic Topological Insulators EuSn2As2 and MnBi2nTe3n+1
    Li, Hang; Gao, Shun-Ye; Duan, Shao-Feng; Xu, Yuan-Feng; Zhu, Ke-Jia; Tian, Shang-Jie; Gao, Jia-Cheng; Fan, Wen-Hui; Rao, Zhi-Cheng; Huang, Jie-Rui; Li, Jia-Jun; Yan, Da-Yu; Liu, Zheng-Tai; Liu, Wan-Ling; Huang, Yao-Bo; Li, Yu-Liang; Liu, Yi; Zhang, Guo-Bin; Zhang, Peng; Kondo, Takeshi; Shin, Shik; Lei, He-Chang; Shi, You-Guo; Zhang, Wen-Tao; Weng, Hong-Ming; Qian, Tian; Ding, Hong
    Physical Review X (2019), 9 (4), 041039CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
    In magnetic topol. insulators (TIs), the interplay between magnetic order and nontrivial topol. can induce fascinating topol. quantum phenomena, such as the quantum anomalous Hall effect, chiral Majorana fermions, and axion electrodynamics. Recently, a great deal of attention has been focused on the intrinsic magnetic TIs, where disorder effects can be eliminated to a large extent, which is expected to facilitate the emergence of topol. quantum phenomena. Despite intensive efforts, exptl. evidence of the topol. surface states (SSs) remains elusive. Here, by combining first-principles calcns. and angle-resolved photoemission spectroscopy (ARPES) expts., we reveal that EuSn2As2 is an antiferromagnetic TI with the observation of Dirac SSs consistent with our prediction. We also observe nearly gapless Dirac SSs in antiferromagnetic TIs MnBi2nTe3n+1 (n=1 and 2), which are absent in previous ARPES results. These results provide clear evidence for nontrivial topol. of these intrinsic magnetic TIs. Furthermore, we find that the topol. SSs show no observable changes across the magnetic transition within the exptl. resoln., indicating that the magnetic order has a quite small effect on the topol. SSs, which can be attributed to weak hybridization between the localized magnetic moments, from either 4f or 3d orbitals, and the topol. electronic states. This finding provides insights for further research that the correlations between magnetism and topol. states need to be strengthened to induce larger gaps in the topol. SSs, which will facilitate the realization of topol. quantum phenomena at higher temps.
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  10. 10
    Vidal, R. C.; Bentmann, H.; Peixoto, T. R. F.; Zeugner, A.; Moser, S.; Min, C.-H.; Schatz, S.; Kißner, K.; Ünzelmann, M.; Fornari, C. I.; Vasili, H. B.; Valvidares, M.; Sakamoto, K.; Mondal, D.; Fujii, J.; Vobornik, I.; Jung, S.; Cacho, C.; Kim, T. K.; Koch, R. J.; Jozwiak, C.; Bostwick, A.; Denlinger, J. D.; Rotenberg, E.; Buck, J.; Hoesch, M.; Diekmann, F.; Rohlf, S.; Kalläne, M.; Rossnagel, K.; Otrokov, M. M.; Chulkov, E. V.; Ruck, M.; Isaeva, A.; Reinert, F. Surface States and Rashba-Type Spin Polarization in Antiferromagnetic MnBi2Te4 (0001). Phys. Rev. B 2019, 100 (12), 121104,  DOI: 10.1103/PhysRevB.100.121104
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    10
    Surface states and Rashba-type spin polarization in antiferromagnetic MnBi2Te4(0001)
    Vidal, R. C.; Bentmann, H.; Peixoto, T. R. F.; Zeugner, A.; Moser, S.; Min, C.-H.; Schatz, S.; Kissner, K.; Uenzelmann, M.; Fornari, C. I.; Vasili, H. B.; Valvidares, M.; Sakamoto, K.; Mondal, D.; Fujii, J.; Vobornik, I.; Jung, S.; Cacho, C.; Kim, T. K.; Koch, R. J.; Jozwiak, C.; Bostwick, A.; Denlinger, J. D.; Rotenberg, E.; Buck, J.; Hoesch, M.; Diekmann, F.; Rohlf, S.; Kallaene, M.; Rossnagel, K.; Otrokov, M. M.; Chulkov, E. V.; Ruck, M.; Isaeva, A.; Reinert, F.
    Physical Review B (2019), 100 (12), 121104CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    The layered van der Waals antiferromagnet MnBi2Te4 has been predicted to combine the band ordering of archetypical topol. insulators such as Bi2Te3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topol. states. We have systematically investigated the surface electronic structure of MnBi2Te4(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy expts. In line with theor. predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.
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  11. 11
    Nevola, D.; Li, H. X.; Yan, J.-Q.; Moore, R. G.; Lee, H.-N.; Miao, H.; Johnson, P. D. Coexistence of Surface Ferromagnetism and a Gapless Topological State in MnBi2Te4. Phys. Rev. Lett. 2020, 125 (11), 117205,  DOI: 10.1103/PhysRevLett.125.117205
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    11
    Coexistence of Surface Ferromagnetism and a Gapless Topological State in MnBi2Te4
    Nevola, D.; Li, H. X.; Yan, J.-Q.; Moore, R. G.; Lee, H.-N.; Miao, H.; Johnson, P. D.
    Physical Review Letters (2020), 125 (11), 117205CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
    Surface magnetism and its correlation with the electronic structure are crit. to understanding the topol. surface state in the intrinsic magnetic topol. insulator MnBi2Te4. Here, using static and time resolved angle-resolved photoemission spectroscopy (ARPES), we find a significant ARPES intensity change together with a gap opening on a Rashba-like conduction band. Comparison with a model simulation strongly indicates that the surface magnetism on cleaved MnBi2Te4 is the same as its bulk state. The inability of surface ferromagnetism to open a gap in the topol. surface state uncovers the novel complexity of MnBi2Te4 that may be responsible for the low quantum anomalous Hall temp. of exfoliated MnBi2Te4.
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  12. 12
    Chen, B.; Fei, F.; Zhang, D.; Zhang, B.; Liu, W.; Zhang, S.; Wang, P.; Wei, B.; Zhang, Y.; Zuo, Z.; Guo, J.; Liu, Q.; Wang, Z.; Wu, X.; Zong, J.; Xie, X.; Chen, W.; Sun, Z.; Wang, S.; Zhang, Y.; Zhang, M.; Wang, X.; Song, F.; Zhang, H.; Shen, D.; Wang, B. Intrinsic Magnetic Topological Insulator Phases in the Sb Doped MnBi2Te4 Bulks and Thin Flakes. Nat. Commun. 2019, 10, 4469,  DOI: 10.1038/s41467-019-12485-y
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    12
    Intrinsic magnetic topological insulator phases in the Sb doped MnBi2Te4 bulks and thin flakes
    Chen Bo; Fei Fucong; Zhang Dongqin; Zhang Shuai; Wei Boyuan; Zhang Yong; Zuo Zewen; Guo Jingwen; Liu Qianqian; Zong Junyu; Xie Xuedong; Chen Wang; Zhang Yi; Zhang Minhao; Song Fengqi; Zhang Haijun; Wang Baigeng; Chen Bo; Fei Fucong; Zhang Shuai; Wei Boyuan; Zhang Yong; Zuo Zewen; Guo Jingwen; Liu Qianqian; Zhang Minhao; Wang Xuefeng; Song Fengqi; Zhang Bo; Wang Pengdong; Sun Zhe; Liu Wanling; Liu Wanling; Shen Dawei; Liu Wanling; Shen Dawei; Wang Zilu; Wu Xuchuan; Wang Shancai; Wang Xuefeng
    Nature communications (2019), 10 (1), 4469 ISSN:.
    Magnetic topological insulators (MTIs) offer a combination of topologically nontrivial characteristics and magnetic order and show promise in terms of potentially interesting physical phenomena such as the quantum anomalous Hall (QAH) effect and topological axion insulating states. However, the understanding of their properties and potential applications have been limited due to a lack of suitable candidates for MTIs. Here, we grow two-dimensional single crystals of Mn(SbxBi(1-x))2Te4 bulk and exfoliate them into thin flakes in order to search for intrinsic MTIs. We perform angle-resolved photoemission spectroscopy, low-temperature transport measurements, and first-principles calculations to investigate the band structure, transport properties, and magnetism of this family of materials, as well as the evolution of their topological properties. We find that there exists an optimized MTI zone in the Mn(SbxBi(1-x))2Te4 phase diagram, which could possibly host a high-temperature QAH phase, offering a promising avenue for new device applications.
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  13. 13
    Otrokov, M. M.; Rusinov, I. P.; Blanco-Rey, M.; Hoffmann, M.; Vyazovskaya, A. Yu.; Eremeev, S. V.; Ernst, A.; Echenique, P. M.; Arnau, A.; Chulkov, E. V. Unique Thickness-Dependent Properties of the van Der Waals Interlayer Antiferromagnet MnBi2Te4 Films. Phys. Rev. Lett. 2019, 122 (10), 107202,  DOI: 10.1103/PhysRevLett.122.107202
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    13
    Unique Thickness-Dependent Properties of the van der Waals Interlayer Antiferromagnet MnBi2Te4 Films
    Otrokov, M. M.; Rusinov, I. P.; Blanco-Rey, M.; Hoffmann, M.; Vyazovskaya, A. Yu.; Eremeev, S. V.; Ernst, A.; Echenique, P. M.; Arnau, A.; Chulkov, E. V.
    Physical Review Letters (2019), 122 (10), 107202CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
    Using d. functional theory and Monte Carlo calcns., we study the thickness dependence of the magnetic and electronic properties of a van der Waals interlayer antiferromagnet in the two-dimensional limit. Considering MnBi2Te4 as a model material, we find it to demonstrate a remarkable set of thickness-dependent magnetic and topol. transitions. While a single septuple layer block of MnBi2Te4 is a topol. trivial ferromagnet, the thicker films made of an odd (even) no. of blocks are uncompensated (compensated) interlayer antiferromagnets, which show wide band gap quantum anomalous Hall (zero plateau quantum anomalous Hall) states. Thus, MnBi2Te4 is the first stoichiometric material predicted to realize the zero plateau quantum anomalous Hall state intrinsically. This state has been theor. shown to host the exotic axion insulator phase.
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  14. 14
    Deng, Y.; Yu, Y.; Shi, M. Z.; Guo, Z.; Xu, Z.; Wang, J.; Chen, X. H.; Zhang, Y. Quantum Anomalous Hall Effect in Intrinsic Magnetic Topological Insulator MnBi2Te4. Science 2020, 367 (6480), 895– 900,  DOI: 10.1126/science.aax8156
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    14
    Quantum anomalous Hall effect in intrinsic magnetic topological insulator MnBi2Te4
    Deng, Yujun; Yu, Yijun; Shi, Meng Zhu; Guo, Zhongxun; Xu, Zihan; Wang, Jing; Chen, Xian Hui; Zhang, Yuanbo
    Science (Washington, DC, United States) (2020), 367 (6480), 895-900CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    In a magnetic topol. insulator, nontrivial band topol. combines with magnetic order to produce exotic states of matter, such as quantum anomalous Hall (QAH) insulators and axion insulators. In this work, the authors probe quantum transport in MnBi2Te4 thin flakes-a topol. insulator with intrinsic magnetic order. In this layered van der Waals crystal, the ferromagnetic layers couple antiparallel to each other; atomically thin MnBi2Te4, however, becomes ferromagnetic when the sample has an odd no. of septuple layers. The authors observe a zero-field QAH effect in a five-septuple-layer specimen at 1.4 K, and an external magnetic field further raises the quantization temp. to 6.5 K by aligning all layers ferromagnetically. The results establish MnBi2Te4 as an ideal arena for further exploring various topol. phenomena with a spontaneously broken time-reversal symmetry.
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  15. 15
    Estyunin, D. A.; Klimovskikh, I. I.; Shikin, A. M.; Schwier, E. F.; Otrokov, M. M.; Kimura, A.; Kumar, S.; Filnov, S. O.; Aliev, Z. S.; Babanly, M. B.; Chulkov, E. V. Signatures of Temperature Driven Antiferromagnetic Transition in the Electronic Structure of Topological Insulator MnBi2Te4. APL Mater. 2020, 8 (2), 021105,  DOI: 10.1063/1.5142846
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    15
    Signatures of temperature driven antiferromagnetic transition in the electronic structure of topological insulator MnBi2Te4
    Estyunin, D. A.; Klimovskikh, I. I.; Shikin, A. M.; Schwier, E. F.; Otrokov, M. M.; Kimura, A.; Kumar, S.; Filnov, S. O.; Aliev, Z. S.; Babanly, M. B.; Chulkov, E. V.
    APL Materials (2020), 8 (2), 021105CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    In this work, we employed angle resolved photoemission spectroscopy (ARPES) to analyze the temp. dependent changes in the electronic structure of the first antiferromagnetic topol. insulator MnBi2Te4 upon crossing the Ne´el temp. TN ≈25 K. We obsd. an exchange splitting of the bulk conduction band, which has a power law dependence on temp. (1 - T/T0)2β with an onset temp. T0 well matching the measured bulk TN. We found a matching temp. evolution of the topol. surface states integrated spectral wt. in the vicinity of the Dirac point. Furthermore, we obsd. an addnl. quasi-2D state with Rashba-type splitting, which is also affected by the emerged magnetism and exhibits an opening of a gap, reminiscent of the effect of an out-of-plane magnetic field, below TN. All these findings point toward strong evidence of the interplay between emerged magnetism with bulk and topol. surface states. The obsd. temp.-dependent effects in MnBi2Te4 may be used as an exptl. fingerprint for the presence of magnetism and may guide the future anal. of ARPES spectra in magnetic topol. insulators. (c) 2020 American Institute of Physics.
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  16. 16
    Yan, C.; Fernandez-Mulligan, S.; Mei, R.; Lee, S. H.; Protic, N.; Fukumori, R.; Yan, B.; Liu, C.; Mao, Z.; Yang, S. Origins of Electronic Bands in the Antiferromagnetic Topological Insulator MnBi2Te4. Phys. Rev. B 2021, 104 (4), L041102,  DOI: 10.1103/PhysRevB.104.L041102
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    16
    Origins of electronic bands in the antiferromagnetic topological insulator MnBi2Te4
    Yan, Chenhui; Fernandez-Mulligan, Sebastian; Mei, Ruobing; Lee, Seng Huat; Protic, Nikola; Fukumori, Rikuto; Yan, Binghai; Liu, Chaoxing; Mao, Zhiqiang; Yang, Shuolong
    Physical Review B (2021), 104 (4), L041102CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Despite the rapid progress in understanding the first intrinsic magnetic topol. insulator MnBi2Te4, its electronic structure remains a topic under debates. Here we perform a thorough spectroscopic investigation into the electronic structure of MnBi2Te4 via laser-based angle-resolved photoemission spectroscopy. Through quant. anal., we est. an upper bound of 3 meV for the gap size of the topol. surface state. Furthermore, our CD measurements reveal band chiralities for both the topol. surface state and quasi-2D bands, which can be well reproduced in a band hybridization model. A numerical simulation of energy-momentum dispersions based on a four-band model with an addnl. step potential near the surface provides a promising explanation for the origin of the quasi-2D bands. Our study represents a solid step forward in reconciling the existing controversies in the electronic structure of MnBi2Te4, and provides an important framework to understand the electronic structures of other relevant topol. materials MnBi2nTe3n+1.
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  17. 17
    Hu, C.; Gordon, K. N.; Liu, P.; Liu, J.; Zhou, X.; Hao, P.; Narayan, D.; Emmanouilidou, E.; Sun, H.; Liu, Y.; Brawer, H.; Ramirez, A. P.; Ding, L.; Cao, H.; Liu, Q.; Dessau, D.; Ni, N. A van Der Waals Antiferromagnetic Topological Insulator with Weak Interlayer Magnetic Coupling. Nat. Commun. 2020, 11, 97,  DOI: 10.1038/s41467-019-13814-x
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    17
    A van der Waals antiferromagnetic topological insulator with weak interlayer magnetic coupling
    Hu, Chaowei; Gordon, Kyle N.; Liu, Pengfei; Liu, Jinyu; Zhou, Xiaoqing; Hao, Peipei; Narayan, Dushyant; Emmanouilidou, Eve; Sun, Hongyi; Liu, Yuntian; Brawer, Harlan; Ramirez, Arthur P.; Ding, Lei; Cao, Huibo; Liu, Qihang; Dessau, Dan; Ni, Ni
    Nature Communications (2020), 11 (1), 97CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Magnetic topol. insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential tech. revolutions. By realizing a bulk van der Waals material MnBi4Te7 with alternating septuple [MnBi2Te4] and quintuple [Bi2Te3] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane satn. field of ∼0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calcns. further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states assocd. with the [MnBi2Te4] or [Bi2Te3] termination, resp. Addnl., its superlattice nature may make various heterostructures of [MnBi2Te4] and [Bi2Te3] layers possible by exfoliation. Therefore, the low satn. field and the superlattice nature of MnBi4Te7 make it an ideal system to investigate rich emergent phenomena.
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  18. 18
    Wu, J.; Liu, F.; Sasase, M.; Ienaga, K.; Obata, Y.; Yukawa, R.; Horiba, K.; Kumigashira, H.; Okuma, S.; Inoshita, T.; Hosono, H. Natural van Der Waals Heterostructural Single Crystals with Both Magnetic and Topological Properties. Sci. Adv. 2019, 5 (11), eaax9989  DOI: 10.1126/sciadv.aax9989
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    There is no corresponding record for this reference.
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    Vidal, R. C.; Zeugner, A.; Facio, J. I.; Ray, R.; Haghighi, M. H.; Wolter, A. U. B.; Corredor Bohorquez, L. T.; Caglieris, F.; Moser, S.; Figgemeier, T.; Peixoto, T. R. F.; Vasili, H. B.; Valvidares, M.; Jung, S.; Cacho, C.; Alfonsov, A.; Mehlawat, K.; Kataev, V.; Hess, C.; Richter, M.; Büchner, B.; van den Brink, J.; Ruck, M.; Reinert, F.; Bentmann, H.; Isaeva, A. Topological Electronic Structure and Intrinsic Magnetization in MnBi4Te7: A Bi2Te3 Derivative with a Periodic Mn Sublattice. Phys. Rev. X 2019, 9 (4), 041065,  DOI: 10.1103/PhysRevX.9.041065
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    19
    Topological Electronic Structure and Intrinsic Magnetization in MnBi4Te7: A Bi2Te3 Derivative with a Periodic Mn Sublattice
    Vidal, Raphael C.; Zeugner, Alexander; Facio, Jorge I.; Ray, Rajyavardhan; Haghighi, M. Hossein; Wolter, Anja U. B.; Corredor Bohorquez, Laura T.; Caglieris, Federico; Moser, Simon; Figgemeier, Tim; Peixoto, Thiago R. F.; Vasili, Hari Babu; Valvidares, Manuel; Jung, Sungwon; Cacho, Cephise; Alfonsov, Alexey; Mehlawat, Kavita; Kataev, Vladislav; Hess, Christian; Richter, Manuel; Buechner, Bernd; van den Brink, Jeroen; Ruck, Michael; Reinert, Friedrich; Bentmann, Hendrik; Isaeva, Anna
    Physical Review X (2019), 9 (4), 041065CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
    Combinations of nontrivial band topol. and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topol. magnetoelec. effect. Following theor. advances, material candidates are emerging. Yet, so far a compd. that combines a band-inverted electronic structure with an intrinsic net magnetization remains unrealized. MnBi2Te4 has been established as the first antiferromagnetic topol. insulator and constitutes the progenitor of a modular (Bi2Te3)n(MnBi2Te4) series. Here, for n=1, we confirm a nonstoichiometric compn. proximate to MnBi4Te7. We establish an antiferromagnetic state below 13 K followed by a state with a net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission expts. and d.-functional calcns. reveal a topol. nontrivial surface state on the MnBi4Te7(0001) surface, analogous to the nonmagnetic parent compd. Bi2Te3. Our results establish MnBi4Te7 as the first band-inverted compd. with intrinsic net magnetization providing a versatile platform for the realization of magnetic topol. states of matter.
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  20. 20
    Shi, M. Z.; Lei, B.; Zhu, C. S.; Ma, D. H.; Cui, J. H.; Sun, Z. L.; Ying, J. J.; Chen, X. H. Magnetic and Transport Properties in the Magnetic Topological Insulators MnB2Te4(Bi2Te3)n (N = 1,2). Phys. Rev. B 2019, 100 (15), 155144,  DOI: 10.1103/PhysRevB.100.155144
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    20
    Magnetic and transport properties in the magnetic topological insulators MnBi2Te4(Bi2Te3)n (n=1,2)
    Shi, M. Z.; Lei, B.; Zhu, C. S.; Ma, D. H.; Cui, J. H.; Sun, Z. L.; Ying, J. J.; Chen, X. H.
    Physical Review B (2019), 100 (15), 155144CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    The observation of quantized anomalous Hall conductance in the forced ferromagnetic state of MnBi2Te4 thin flakes has attracted much attention. However, a strong magnetic field is needed to fully polarize the magnetic moments due to the large antiferromagnetic interlayer exchange coupling. Here, we reported the magnetic and elec. transport properties of the magnetic van der Waals MnBi2Te4(Bi2Te3)n (n=1,2) single crystals, in which the interlayer antiferromagnetic exchange coupling is greatly suppressed with the increase of the sepn. layers Bi2Te3. MnBi4Te7 and MnBi6Te10 show weak antiferromagnetic transition at 12.3 and 10.5 K, resp. The ferromagnetic hysteresis was obsd. at low temp. for both of the crystals, which is quite crucial for realizing the quantum anomalous Hall effect without external magnetic field. Our work indicates that MnBi2Te4(Bi2Te3)n (n=1,2) provides an ideal platform to investigate the rich topol. phases with their two-dimensional limits.
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  21. 21
    Tian, S.; Gao, S.; Nie, S.; Qian, Y.; Gong, C.; Fu, Y.; Li, H.; Fan, W.; Zhang, P.; Kondo, T.; Shin, S.; Adell, J.; Fedderwitz, H.; Ding, H.; Wang, Z.; Qian, T.; Lei, H. Magnetic Topological Insulator MnBi6Te10with a Zero-Field Ferromagnetic State and Gapped Dirac Surface States. Phys. Rev. B 2020, 102 (3), 035144,  DOI: 10.1103/PhysRevB.102.035144
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    21
    Magnetic topological insulator MnBi6Te10 with a zero-field ferromagnetic state and gapped Dirac surface states
    Tian, Shangjie; Gao, Shunye; Nie, Simin; Qian, Yuting; Gong, Chunsheng; Fu, Yang; Li, Hang; Fan, Wenhui; Zhang, Peng; Kondo, Takesh; Shin, Shik; Adell, Johan; Fedderwitz, Hanna; Ding, Hong; Wang, Zhijun; Qian, Tian; Lei, Hechang
    Physical Review B (2020), 102 (3), 035144CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Magnetic topol. insulators (TIs) with nontrivial topol. electronic structure and broken time-reversal symmetry exhibit various exotic topol. quantum phenomena. The realization of such exotic phenomena at high temp. is one of the central topics in this area. We reveal that MnBi6Te10 is a magnetic TI with an antiferromagnetic ground state below 10.8 K whose nontrivial topol. is manifested by Dirac-like surface states. The ferromagnetic axion insulator state with Z4=2 emerges once spins are polarized at a field as low as 0.1 T, accompanied with satd. anomalous Hall resistivity up to 10 K. Such a ferromagnetic state is preserved even with an external field down to zero at 2 K. Theor. calcns. indicate that the few-layer ferromagnetic MnBi6Te10 is also topol. nontrivial with a nonzero Chern no. Angle-resolved photoemission spectroscopy expts. further reveal three types of Dirac surface states arising from different terminations on the cleavage surfaces, one of which has insulating behavior with an energy gap of ~ 28 meV at the Dirac point. These outstanding features suggest that MnBi6Te10 is a promising system to realize various topol. quantum effects at zero field and high temp.
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  22. 22
    Hu, Y.; Xu, L.; Shi, M.; Luo, A.; Peng, S.; Wang, Z. Y.; Ying, J. J.; Wu, T.; Liu, Z. K.; Zhang, C. F.; Chen, Y. L.; Xu, G.; Chen, X.-H.; He, J.-F. Universal Gapless Dirac Cone and Tunable Topological States in (MnBi2Te4)m(Bi2Te3)n Heterostructures. Phys. Rev. B 2020, 101 (16), 161113,  DOI: 10.1103/PhysRevB.101.161113
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    22
    Universal gapless Dirac cone and tunable topological states in (MnBi2Te4)m(Bi2Te3)n heterostructures
    Hu, Yong; Xu, Lixuan; Shi, Mengzhu; Luo, Aiyun; Peng, Shuting; Wang, Z. Y.; Ying, J. J.; Wu, T.; Liu, Z. K.; Zhang, C. F.; Chen, Y. L.; Xu, G.; Chen, X.-H.; He, J.-F.
    Physical Review B (2020), 101 (16), 161113CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    A review The newly discovered magnetic topol. insulators (MnBi2Te4)m(Bi2Te3)n are predicted to be a versatile platform for exploring novel topol. states. Here, we report angle-resolved photoemission spectroscopy studies on a series of (MnBi2Te4)m(Bi2Te3)n heterostructures. An unexpected but universal gapless Dirac cone is obsd. on the (MnBi2Te4) terminated (0001) surfaces in all systems, indicating an altered magnetic structure near the surface. The specific band dispersion of the surface states, presumably dominated by the top surface, is found to be sensitive to different stackings of the underlying MnBi2Te4 and Bi2Te3 layers. Our results suggest the high tunability of both magnetic and electronic structures of the topol. surface states in (MnBi2Te4)m(Bi2Te3)n heterostructures, which is essential in realizing and manipulating various topol. states.
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  23. 23
    Klimovskikh, I. I.; Otrokov, M. M.; Estyunin, D.; Eremeev, S. V.; Filnov, S. O.; Koroleva, A.; Shevchenko, E.; Voroshnin, V.; Rybkin, A. G.; Rusinov, I. P.; Blanco-Rey, M.; Hoffmann, M.; Aliev, Z. S.; Babanly, M. B.; Amiraslanov, I. R.; Abdullayev, N. A.; Zverev, V. N.; Kimura, A.; Tereshchenko, O. E.; Kokh, K. A.; Petaccia, L.; Di Santo, G.; Ernst, A.; Echenique, P. M.; Mamedov, N. T.; Shikin, A. M.; Chulkov, E. V. Tunable 3D/2D Magnetism in the (MnBi2Te4)(Bi2Te3)m Topological Insulators Family. Npj Quantum Mater. 2020, 5 (1), 54,  DOI: 10.1038/s41535-020-00255-9
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    23
    Tunable 3D/2D magnetism in the (MnBi2Te4)(Bi2Te3)m topological insulators family
    Klimovskikh, Ilya I.; Otrokov, Mikhail M.; Estyunin, Dmitry; Eremeev, Sergey V.; Filnov, Sergey O.; Koroleva, Alexandra; Shevchenko, Eugene; Voroshnin, Vladimir; Rybkin, Artem G.; Rusinov, Igor P.; Blanco-Rey, Maria; Hoffmann, Martin; Aliev, Ziya S.; Babanly, Mahammad B.; Amiraslanov, Imamaddin R.; Abdullayev, Nadir A.; Zverev, Vladimir N.; Kimura, Akio; Tereshchenko, Oleg E.; Kokh, Konstantin A.; Petaccia, Luca; Di Santo, Giovanni; Ernst, Arthur; Echenique, Pedro M.; Mamedov, Nazim T.; Shikin, Alexander M.; Chulkov, Eugene V.
    npj Quantum Materials (2020), 5 (1), 54CODEN: QMUADP; ISSN:2397-4648. (Nature Research)
    Feasibility of many emergent phenomena that intrinsic magnetic topol. insulators (TIs) may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic structures. Here we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compds. (MnBi2Te4)(Bi2Te3)m with m = 0, ···, 6. Magnetic, electronic and, consequently, topol. properties of these materials depend strongly on the m value and are thus highly tunable. The antiferromagnetic (AFM) coupling between the neighboring Mn layers strongly weakens on moving from MnBi2Te4 (m = 0) to MnBi4Te7 (m = 1) and MnBi6Te10 (m = 2). Further increase in m leads to change of the overall magnetic behavior to ferromagnetic (FM) one for (m = 3), while the interlayer coupling almost disappears. In this way, the AFM and FM TI states are, resp., realized in the m = 0, 1, 2 and m = 3 cases. For large m nos. a hitherto-unknown topol. nontrivial phase can be created, in which below the corresponding crit. temp. the magnetizations of the non-interacting 2D ferromagnets, formed by the MnBi2Te4 building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi2Te4)(Bi2Te3)m allows efficient engineering of functional van der Waals heterostructures for topol. quantum computation, as well as antiferromagnetic and 2D spintronics.
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  24. 24
    Lu, R.; Sun, H.; Kumar, S.; Wang, Y.; Gu, M.; Zeng, M.; Hao, Y.-J.; Li, J.; Shao, J.; Ma, X.-M.; Hao, Z.; Zhang, K.; Mansuer, W.; Mei, J.; Zhao, Y.; Liu, C.; Deng, K.; Huang, W.; Shen, B.; Shimada, K.; Schwier, E. F.; Liu, C.; Liu, Q.; Chen, C. Half-Magnetic Topological Insulator with Magnetization-Induced Dirac Gap at a Selected Surface. Phys. Rev. X 2021, 11 (1), 011039,  DOI: 10.1103/PhysRevX.11.011039
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    24
    Half-Magnetic Topological Insulator with Magnetization-Induced Dirac Gap at a Selected Surface
    Lu, Ruie; Sun, Hongyi; Kumar, Shiv; Wang, Yuan; Gu, Mingqiang; Zeng, Meng; Hao, Yu-Jie; Li, Jiayu; Shao, Jifeng; Ma, Xiao-Ming; Hao, Zhanyang; Zhang, Ke; Mansuer, Wumiti; Mei, Jiawei; Zhao, Yue; Liu, Cai; Deng, Ke; Huang, Wen; Shen, Bing; Shimada, Kenya; Schwier, Eike F.; Liu, Chang; Liu, Qihang; Chen, Chaoyu
    Physical Review X (2021), 11 (1), 011039CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
    Topol. magnets are a new family of quantum materials providing great potential to realize emergent phenomena, such as the quantum anomalous Hall effect and the axion-insulator state. Here, we present our discovery that the stoichiometric ferromagnet MnBi8Te13 with natural heterostructure MnBi2Te4/(Bi2Te3)3 is an unprecedented "half-magnetic topol. insulator," with the magnetization existing at the MnBi2Te4 surface but not at the opposite surface terminated by triple Bi2Te3 layers. Our angle-resolved photoemission spectroscopy measurements unveil a massive Dirac gap at the MnBi2Te4 surface and a gapless Dirac cone on the other side. Remarkably, the Dirac gap (about 28 meV) at the MnBi2Te4 surface decreases monotonically with increasing temp. and closes right at the Curie temp., thereby representing the first smoking-gun spectroscopic evidence of a magnetization-induced topol. surface gap among all known magnetic topol. materials. We further demonstrate theor. that the half-magnetic topol. insulator is desirable to realize the surface anomalous Hall effect, which serves as direct proof of the general concept of axion electrodynamics in condensed matter systems.
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  25. 25
    Hu, C.; Ding, L.; Gordon, K. N.; Ghosh, B.; Tien, H.-J.; Li, H.; Linn, A. G.; Lien, S.-W.; Huang, C.-Y.; Mackey, S.; Liu, J.; Reddy, P. V. S.; Singh, B.; Agarwal, A.; Bansil, A.; Song, M.; Li, D.; Xu, S.-Y.; Lin, H.; Cao, H.; Chang, T.-R.; Dessau, D.; Ni, N. Realization of an Intrinsic Ferromagnetic Topological State in MnBi8Te13. Sci. Adv. 2020, 6 (30), eaba4275  DOI: 10.1126/sciadv.aba4275
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    Souchay, D.; Nentwig, M.; Günther, D.; Keilholz, S.; de Boor, J.; Zeugner, A.; Isaeva, A.; Ruck, M.; Wolter, A. U.; Büchner, B.; Oeckler, O. Layered Manganese Bismuth Tellurides with GeBi4Te7 and GeBi6Te10 Type Structures: Towards Multifunctional Materials. J. Mater. Chem. C 2019, 7 (32), 9939– 9953,  DOI: 10.1039/C9TC00979E
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    26
    Layered manganese bismuth tellurides with GeBi4Te7- and GeBi6Te10-type structures: towards multifunctional materials
    Souchay, Daniel; Nentwig, Markus; Guenther, Daniel; Keilholz, Simon; de Boor, Johannes; Zeugner, Alexander; Isaeva, Anna; Ruck, Michael; Wolter, Anja U. B.; Buechner, Bernd; Oeckler, Oliver
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2019), 7 (32), 9939-9953CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    The crystal structures of new layered manganese bismuth tellurides with the compns. Mn0.85(3)Bi4.10(2)Te7 and Mn0.73(4)Bi6.18(2)Te10 were detd. by single-crystal X-ray diffraction, including the use of microfocused synchrotron radiation. These analyses reveal that the layered structures deviate from the idealized stoichiometry of the 12P-GeBi4Te7 (space group P‾3m1) and 51R-GeBi6Te10 (space group R‾3m) structure types they adopt. Modified compns. Mn1-xBi4+2x/3Te7 (x = 0.15-0.2) and Mn1-xBi6+2x/3Te10 (x = 0.19-0.26) assume cation vacancies and lead to homogenous bulk samples as confirmed by Rietveld refinements. Electron diffraction patterns exhibit no diffuse streaks that would indicate stacking disorder. The alternating quintuple-layer [M2Te3] and septuple-layer [M3Te4] slabs (M = mixed occupied by Bi and Mn) with 1:1 sequence (12P stacking) in Mn0.85Bi4.10Te7 and 2:1 sequence (51R stacking) in Mn0.81Bi6.13Te10 were also obsd. in HRTEM images. Temp.-dependent powder diffraction and differential scanning calorimetry show that the compds. are high-temp. phases, which are metastable at ambient temp. Magnetization measurements are in accordance with a MnII oxidn. state and point at predominantly ferromagnetic coupling in both compds. The thermoelec. figures of merit of n-type conducting Mn0.85Bi4.10Te7 and Mn0.81Bi6.13Te10 reach zT = 0.25 at 375° and zT = 0.28 at 325°, resp. Although the compds. are metastable, compact ingots exhibit still ≤80% of the main phases after thermoelec. measurements ≤400°.
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  27. 27
    Shao, J.; Liu, Y.; Zeng, M.; Li, J.; Wu, X.; Ma, X.-M.; Jin, F.; Lu, R.; Sun, Y.; Gu, M.; Wang, K.; Wu, W.; Wu, L.; Liu, C.; Liu, Q.; Zhao, Y. Pressure-Tuned Intralayer Exchange in Superlattice-Like MnBi2Te4/(Bi2Te3)n Topological Insulators. Nano Lett. 2021, 21 (13), 5874– 5880,  DOI: 10.1021/acs.nanolett.1c01874
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    27
    Pressure-Tuned Intralayer Exchange in Superlattice-Like MnBi2Te4/(Bi2Te3)n Topological Insulators
    Shao, Jifeng; Liu, Yuntian; Zeng, Meng; Li, Jingyuan; Wu, Xuefeng; Ma, Xiao-Ming; Jin, Feng; Lu, Ruie; Sun, Yichen; Gu, Mingqiang; Wang, Kedong; Wu, Wenbin; Wu, Liusuo; Liu, Chang; Liu, Qihang; Zhao, Yue
    Nano Letters (2021), 21 (13), 5874-5880CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The magnetic structures of MnBi2Te4(Bi2Te3)n can be manipulated by tuning the interlayer coupling via the no. of Bi2Te3 spacer layers n, while the intralayer ferromagnetic (FM) exchange coupling is considered too robust to control. By applying hydrostatic pressure up to 3.5 GPa, we discover opposite responses of magnetic properties for n = 1 and 2. MnBi4Te7 stays at A-type antiferromagnetic (AFM) phase with a decreasing Neel temp. and an increasing satn. field. In sharp contrast, MnBi6Te10 experiences a phase transition from A-type AFM to a quasi-two-dimensional FM state with a suppressed satn. field under pressure. First-principles calcns. reveal the essential role of intralayer exchange coupling from lattice compression in detg. these magnetic properties. Such magnetic phase transition is also obsd. in 20% Sb-doped MnBi6Te10 because of the in-plane lattice compression.
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  28. 28
    Liu, Y.; Wang, L.-L.; Zheng, Q.; Huang, Z.; Wang, X.; Chi, M.; Wu, Y.; Chakoumakos, B. C.; McGuire, M. A.; Sales, B. C.; Wu, W.; Yan, J. Site Mixing for Engineering Magnetic Topological Insulators. Phys. Rev. X 2021, 11 (2), 021033,  DOI: 10.1103/PhysRevX.11.021033
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    28
    Site Mixing for Engineering Magnetic Topological Insulators
    Liu, Yaohua; Wang, Lin-Lin; Zheng, Qiang; Huang, Zengle; Wang, Xiaoping; Chi, Miaofang; Wu, Yan; Chakoumakos, Bryan C.; McGuire, Michael A.; Sales, Brian C.; Wu, Weida; Yan, Jiaqiang
    Physical Review X (2021), 11 (2), 021033CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
    The van der Waals compd., MnBi2Te4, is the first intrinsic magnetic topol. insulator, providing a materials platform for exploring exotic quantum phenomena such as the axion insulator state and the quantum anomalous Hall effect. However, intrinsic structural imperfections lead to bulk cond., and the roles of magnetic defects are still unknown. With higher concns. of the same types of magnetic defects, the isostructural compd. MnSb2Te4 is a better model system for a systematic investigation of the connections among magnetism, topol., and lattice defects. In this work, the impact of antisite defects on the magnetism and electronic structure is studied in MnSb2Te4. Mn-Sb site mixing leads to complex magnetic structures and tunes the interlayer magnetic coupling between antiferromagnetic and ferromagnetic. The detailed nonstoichiometry and site mixing of MnSb2Te4 crystals depend on the growth parameters, which can lead to ≈40% of Mn sites occupied by Sb and ≈15% of Sb sites by Mn in as-grown crystals. Single-crystal neutron diffraction and electron microscopy studies show nearly random distribution of the antisite defects. Band structure calcns. suggest that the Mn-Sb site mixing favors a ferromagnetic interlayer coupling, consistent with exptl. observation, but is detrimental to the band inversion required for a nontrivial topol. Our results suggest a long-range magnetic order of Mn ions sitting on Bi sites in MnBi2Te4. The effects of site mixing should be considered in all layered heterostructures that consist of alternating magnetic and topol. layers, including the entire family of MnTe(Bi2Te3)n, its Sb analogs, and their solid soln.
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  29. 29
    Murakami, T.; Nambu, Y.; Koretsune, T.; Xiangyu, G.; Yamamoto, T.; Brown, C. M.; Kageyama, H. Realization of Interlayer Ferromagnetic Interaction in MnSb2Te4 toward the Magnetic Weyl Semimetal State. Phys. Rev. B 2019, 100 (19), 195103,  DOI: 10.1103/PhysRevB.100.195103
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    29
    Realization of interlayer ferromagnetic interaction in MnSb2Te4 toward the magnetic Weyl semimetal state
    Murakami, Taito; Nambu, Yusuke; Koretsune, Takashi; Gu, Xiangyu; Yamamoto, Takafumi; Brown, Craig M.; Kageyama, Hiroshi
    Physical Review B (2019), 100 (19), 195103CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Magnetic properties of MnSb2Te4 were examd. through magnetic susceptibility, specific-heat, and neutron-diffraction measurements. As opposed to isostructural MnBi2Te4 with the antiferromagnetic ground state, MnSb2Te4 develops a spontaneous magnetization below 25 K. From our first-principles calcns. on the material in a ferromagnetic state, the state could be interpreted as a type-II Weyl semimetal state with broken time-reversal symmetry. Detailed structural refinements using x-ray-diffraction and neutron-diffraction data reveal the presence of site mixing between Mn and Sb sites, leading to the ferrimagnetic ground state. With theor. calcns., we found that the presence of site mixing plays an important role for the interlayer Mn-Mn ferromagnetic interactions.
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  30. 30
    Hou, F.; Yao, Q.; Zhou, C.-S.; Ma, X.-M.; Han, M.; Hao, Y.-J.; Wu, X.; Zhang, Y.; Sun, H.; Liu, C.; Zhao, Y.; Liu, Q.; Lin, J. Te-Vacancy-Induced Surface Collapse and Reconstruction in Antiferromagnetic Topological Insulator MnBi2Te4. ACS Nano 2020, 14 (9), 11262– 11272,  DOI: 10.1021/acsnano.0c03149
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    30
    Te-Vacancy-Induced Surface Collapse and Reconstruction in Antiferromagnetic Topological Insulator MnBi2Te4
    Hou, Fuchen; Yao, Qiushi; Zhou, Chun-Sheng; Ma, Xiao-Ming; Han, Mengjiao; Hao, Yu-Jie; Wu, Xuefeng; Zhang, Yu; Sun, Hongyi; Liu, Chang; Zhao, Yue; Liu, Qihang; Lin, Junhao
    ACS Nano (2020), 14 (9), 11262-11272CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    MnBi2Te4 is an antiferromagnetic topol. insulator that has stimulated intense interest due to its exotic quantum phenomena and promising device applications. The surface structure is a determinant factor to understand the magnetic and topol. behavior of MnBi2Te4, yet its precise at. structure remains elusive. Here the authors discovered a surface collapse and reconstruction of few-layer MnBi2Te4 exfoliated under delicate protection. Instead of the ideal septuple-layer structure in the bulk, the collapsed surface is shown to reconstruct as a Mn-doped Bi2Te3 quintuple layer and a MnxBiyTe double layer with a clear van der Waals gap in between. Combined with 1st-principles calcns., such surface collapse is attributed to the abundant intrinsic Mn-Bi antisite defects and the Te vacancy in the exfoliated surface, which is further supported by in situ annealing and electron irradn. expts. Results shed light on the understanding of the intricate surface-bulk correspondence of MnBi2Te4 and provide an insightful perspective on the surface-related quantum measurements in MnBi2Te4 few-layer devices.
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    Huang, Z.; Du, M.-H.; Yan, J.; Wu, W. Native Defects in Antiferromagnetic Topological Insulator MnBi2Te4. Phys. Rev. Mater. 2020, 4 (12), 121202,  DOI: 10.1103/PhysRevMaterials.4.121202
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    Native defects in antiferromagnetic topological insulator MnBi2Te4
    Huang, Zengle; Du, Mao-Hua; Yan, Jiaqiang; Wu, Weida
    Physical Review Materials (2020), 4 (12), 121202CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
    Using scanning tunneling microscopy and spectroscopy, we visualized the native defects in antiferromagnetic topol. insulator MnBi2Te4. Two native defects, MnBi and BiTe antisites, can be well resolved in the topog. images. MnBi tend to suppress the d. of states at the conduction band edge. Spectroscopy imaging reveals a localized peaklike local d. of state at ∼80 meV below the Fermi energy. A careful inspection of topog. and spectroscopic images, combined with d. functional theory calcn., suggests this results from BiMn antisites at Mn sites. The random distribution of MnBi and BiMn antisites results in spatial fluctuation of local d. of states near the Fermi level in MnBi2Te4.
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    Lai, Y.; Ke, L.; Yan, J.; McDonald, R. D.; McQueeney, R. J. Defect-Driven Ferrimagnetism and Hidden Magnetization in MnBi2Te4. Phys. Rev. B 2021, 103 (18), 184429,  DOI: 10.1103/PhysRevB.103.184429
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    32
    Defect-driven ferrimagnetism and hidden magnetization in MnBi2Te4
    Lai, You; Ke, Liqin; Yan, Jiaqiang; McDonald, Ross D.; McQueeney, Robert J.
    Physical Review B (2021), 103 (18), 184429CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    MnBi2Te4 (MBT) materials are promising antiferromagnetic topol. insulators in which field-driven ferromagnetism is predicted to cause a transition between axion insulator and Weyl semimetallic states. However, the presence of antiferromagnetic coupling between Mn/Bi antisite defects and the main Mn layer can reduce the low-field magnetization, and it has been shown that such defects are more prevalent in the structurally identical magnetic insulator MnSb2Te4 (MST). We use high-field magnetization measurements to show that the magnetization of MBT and MST occur in stages and full satn. requires fields of ∼60 T. As a consequence, the low-field magnetization plateau state in MBT, where many detns. of the quantum anomalous Hall state are studied, actually consists of ferrimagnetic septuple blocks contg. both uniform and staggered magnetization components.
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    Du, M.-H.; Yan, J.; Cooper, V. R.; Eisenbach, M. Tuning Fermi Levels in Intrinsic Antiferromagnetic Topological Insulators MnBi2Te4 and MnBi4Te7 by Defect Engineering and Chemical Doping. Adv. Funct. Mater. 2021, 31 (3), 2006516,  DOI: 10.1002/adfm.202006516
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    33
    Tuning Fermi Levels in Intrinsic Antiferromagnetic Topological Insulators MnBi2Te4 and MnBi4Te7 by Defect Engineering and Chemical Doping
    Du, Mao-Hua; Yan, Jiaqiang; Cooper, Valentino R.; Eisenbach, Markus
    Advanced Functional Materials (2021), 31 (3), 2006516CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    MnBi2Te4 and MnBi4Te7 are intrinsic antiferromagnetic topol. insulators, offering a promising materials platform for realizing exotic topol. quantum states. However, high densities of intrinsic defects in these materials not only cause bulk metallic cond., preventing the measurement of quantum transport in surface states, but may also affect magnetism and topol. properties. In this paper, systematic d. functional theory calcns. reveal specific material chem. and growth conditions that det. the defect formation and dopant incorporation in MnBi2Te4 and MnBi4Te7. The large strain induced by the internal heterostructure promotes the formation of large-size-mismatched antisite defects and substitutional dopants. The results here show that the abundance of antisite defects is responsible for the obsd. n-type metallic cond. A Te-rich growth condition is predicted to reduce the bulk free electron d., which is confirmed by exptl. synthesis and transport measurements in MnBi2Te4. Furthermore, Na doping is proposed to be an effective acceptor dopant to pin the Fermi level within the bulk band gap to enable the observation of surface quantum transport. The defect engineering and doping strategies proposed here should stimulate further studies for improving synthesis and for manipulating magnetic and topol. properties in MnBi2Te4, MnBi4Te7, and related magnetic topol. insulators.
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    Hu, C.; Lien, S.-W.; Feng, E.; Mackey, S.; Tien, H.-J.; Mazin, I. I.; Cao, H.; Chang, T.-R.; Ni, N. Tuning Magnetism and Band Topology through Antisite Defects in Sb-Doped MnBi4Te7. Phys. Rev. B 2021, 104 (5), 054422,  DOI: 10.1103/PhysRevB.104.054422
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    34
    Tuning magnetism and band topology through antisite defects in Sb-doped MnBi4Te7
    Hu, Chaowei; Lien, Shang-Wei; Feng, Erxi; Mackey, Scott; Tien, Hung-Ju; Mazin, Igor I.; Cao, Huibo; Chang, Tay-Rong; Ni, Ni
    Physical Review B (2021), 104 (5), 054422CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    The fine control of magnetism and electronic structure in a magnetic topol. insulator is crucial in order to realize various novel magnetic topol. states including axion insulators, magnetic Weyl semimetals, Chern insulators, etc. Through crystal growth, transport, thermodn., neutron diffraction measurements, we show that under Sb doping the newly discovered intrinsic antiferromagnetic (AFM) topol. insulator MnBi4Te7 evolves from AFM to ferromagnetic (FM) and then ferrimagnetic. We attribute this to the formation of Mn(Bi,Sb) antisites upon doping, which results in addnl. Mn sublattices that modify the delicate interlayer magnetic interactions and cause the dominant Mn sublattice to go from AFM to FM. We further investigate the effect of antisites on the band topol. using the first-principles calcns. Without considering antisites, the series evolves from AFM topol. insulator (x = 0) to FM axion insulators. In the exaggerated case of 16.7% of periodic antisites, the band topol. is modified and a type-I magnetic Weyl semimetal phase can be realized at intermediate dopings. Therefore, this doping series provides a fruitful platform with continuously tunable magnetism and topol. for investigating emergent phenomena, including quantum anomalous Hall effect, Fermi arc states, etc.
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    Chen, L.; Wang, D.; Shi, C.; Jiang, C.; Liu, H.; Cui, G.; Zhang, X.; Li, X. Electronic Structure and Magnetism of MnSb2Te4. J. Mater. Sci. 2020, 55 (29), 14292– 14300,  DOI: 10.1007/s10853-020-05005-7
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    35
    Electronic structure and magnetism of MnSb2Te4
    Chen, Li; Wang, Dongchao; Shi, Changmin; Jiang, Chuan; Liu, Hongmei; Cui, Guangliang; Zhang, Xiaoming; Li, Xiaolong
    Journal of Materials Science (2020), 55 (29), 14292-14300CODEN: JMTSAS; ISSN:0022-2461. (Springer)
    MnSb2Te4 has the same crystal structure as MnBi2Te4. Whether it is an intrinsic antiferromagnetic TI, quantum anomalous Hall insulator or axion insulator like MnBi2Te4 [CHIN. PHYS. LETT. 36, (2019) 076801] has not been reported yet. The electronic structure and magnetism of MnSb2Te4 have been studied using first-principles calcns. The results show that the MnSb2Te4 is an antiferromagnetic semiconductor with a trivial energy gap (∼ 0.132 eV). The band gap decreases to 0.057 eV under the tensile strain (1.03a0, 1.03c0). The feature of Weyl semimetal could be presented in MnSb2Te4 with ferromagnetic phase under strain 3%. Thin films (011) are metals with antiferromagnetic order and also metals with ferromagnetic order. Thin film (111) with alternate of thick (1 septuple layer-7 septuple layers) is an intrinsic magnetic semiconductor with a trivial energy gap (0.002-0.344 eV) rather than an intrinsic quantum anomalous Hall insulator or axion insulator.
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  36. 36
    Wimmer, S.; Sánchez-Barriga, J.; Küppers, P.; Ney, A.; Schierle, E.; Freyse, F.; Caha, O.; Michalička, J.; Liebmann, M.; Primetzhofer, D.; Hoffman, M.; Ernst, A.; Otrokov, M. M.; Bihlmayer, G.; Weschke, E.; Lake, B.; Chulkov, E. V.; Morgenstern, M.; Bauer, G.; Springholz, G.; Rader, O. Mn-Rich MnSb2Te4: A Topological Insulator with Magnetic Gap Closing at High Curie Temperatures of 45–50 K. Adv. Mater. 2021, 33 (42), 2102935,  DOI: 10.1002/adma.202102935
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    36
    Mn-Rich MnSb2Te4: A Topological Insulator with Magnetic Gap Closing at High Curie Temperatures of 45-50 K
    Wimmer, Stefan; Sanchez-Barriga, Jaime; Kueppers, Philipp; Ney, Andreas; Schierle, Enrico; Freyse, Friedrich; Caha, Ondrej; Michalicka, Jan; Liebmann, Marcus; Primetzhofer, Daniel; Hoffman, Martin; Ernst, Arthur; Otrokov, Mikhail M.; Bihlmayer, Gustav; Weschke, Eugen; Lake, Bella; Chulkov, Evgueni V.; Morgenstern, Markus; Bauer, Guenther; Springholz, Gunther; Rader, Oliver
    Advanced Materials (Weinheim, Germany) (2021), 33 (42), 2102935CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Ferromagnetic topol. insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high-precision metrol., edge channel spintronics, and topol. qubits. The stable 2+ state of Mn enables intrinsic magnetic topol. insulators. MnBi2Te4 is, however, antiferromagnetic with 25 K Neel temp. and is strongly n-doped. In this work, p-type MnSb2Te4, previously considered topol. trivial, is shown to be a ferromagnetic topol. insulator for a few percent Mn excess. (i) Ferromagnetic hysteresis with record Curie temp. of 45-50 K, (ii) out-of-plane magnetic anisotropy, (iii) a 2D Dirac cone with the Dirac point close to the Fermi level, (iv) out-of-plane spin polarization as revealed by photoelectron spectroscopy, and (v) a magnetically induced bandgap closing at the Curie temp., demonstrated by scanning tunneling spectroscopy (STS), are shown. Moreover, a crit. exponent of the magnetization β ≈ 1 is found, indicating the vicinity of a quantum crit. point. Ab initio calcns. reveal that Mn-Sb site exchange provides the ferromagnetic interlayer coupling and the slight excess of Mn nearly doubles the Curie temp. Remaining deviations from the ferromagnetic order open the inverted bulk bandgap and render MnSb2Te4 a robust topol. insulator and new benchmark for magnetic topol. insulators.
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    Guan, Y. D.; Yan, C. H.; Lee, S. H.; Gui, X.; Ning, W.; Ning, J. L.; Zhu, Y. L.; Kothakonda, M.; Xu, C. Q.; Ke, X. L.; Sun, J. W.; Xie, W. W.; Yang, S. L.; Mao, Z. Q. Ferromagnetic MnBi4Te7 Obtained with Low-Concentration Sb Doping: A Promising Platform for Exploring Topological Quantum States. Phys. Rev. Mater. 2022, 6 (5), 054203,  DOI: 10.1103/PhysRevMaterials.6.054203
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    37
    Ferromagnetic MnBi4Te7 obtained with low-concentration Sb doping: A promising platform for exploring topological quantum states
    Guan, Y. D.; Yan, C. H.; Lee, S. H.; Gui, X.; Ning, W.; Ning, J. L.; Zhu, Y. L.; Kothakonda, M.; Xu, C. Q.; Ke, X. L.; Sun, J. W.; Xie, W. W.; Yang, S. L.; Mao, Z. Q.
    Physical Review Materials (2022), 6 (5), 054203CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
    The tuning of the magnetic phase, chem. potential, and structure is crucial to observe diverse exotic topol. quantum states in MnBi2Te4(Bi2Te3)m(m=0-3). Here we show a ferromagnetic (FM) phase with a chiral crystal structure in Mn(Bi1-xSbx)4Te7, obtained via tuning the growth conditions and Sb concn. Unlike previously reported Mn(Bi1-xSbx)4Te7, which exhibits FM transitions only at high Sb doping levels, our samples show FM transitions (TC=13.5K) at 15%-27% doping levels. Furthermore, our single-crystal x-ray-diffraction structure refinements find Sb doping leads to a chiral structure with the space group of P3-, contrasted with the centrosym. P3‾ m1 crystal structure of the parent compd. MnBi4Te7. Through angle-resolved photoemission spectroscopy measurements, we also demonstrated that the nontrivial band topol. is preserved in the Sb-doped FM samples. Given that the nontrivial band topol. of this system remains robust for low Sb doping levels, our success in making FM Mn(Bi1-xSbx)4Te7 with x=0.15, 0.175, 0.2, and 0.27 paves the way for realizing the predicted topol. quantum states, such as the axion insulator and Weyl semimetals. Addnl., we also obsd. magnetic glassy behavior in both antiferromagnetic MnBi4Te7 and FM Mn(Bi1-xSbx)4Te7 samples, which we believe originates from cluster spin-glass phases coexisting with long-range antiferromagnetic/FM orders. We have also discussed how the antisite Mn ions impact the interlayer magnetic coupling and how FM interlayer coupling is stabilized in this system.
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  38. 38
    Xie, H.; Wang, D.; Cai, Z.; Chen, B.; Guo, J.; Naveed, M.; Zhang, S.; Zhang, M.; Wang, X.; Fei, F.; Zhang, H.; Song, F. The Mechanism Exploration for Zero-Field Ferromagnetism in Intrinsic Topological Insulator MnBi2Te4 by Bi2Te3 Intercalations. Appl. Phys. Lett. 2020, 116 (22), 221902,  DOI: 10.1063/5.0009085
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    38
    The mechanism exploration for zero-field ferromagnetism in intrinsic topological insulator MnBi2Te4 by Bi2Te3 intercalations
    Xie, Hangkai; Wang, Dinghui; Cai, Zixiu; Chen, Bo; Guo, Jingwen; Naveed, Muhammad; Zhang, Shuai; Zhang, Minhao; Wang, Xuefeng; Fei, Fucong; Zhang, Haijun; Song, Fengqi
    Applied Physics Letters (2020), 116 (22), 221902CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Recent research on intrinsic magnetic topol. insulators (MTIs), MnBi2Te4, sheds new light on the observation of a long-expected high-temp. quantum anomalous Hall effect (QAHE). However, the strong interlayered anti-ferromagnetic (AFM) coupling hinders the practical applications without applying a magnetic field. Thus, how to adjust the magnetism of this compd. under zero field is essential. Here, we theor. and exptl. study the magnetic properties of two new promising intrinsic MTI candidates MnBi4Te7 and MnBi6Te10, formed by intercalating the Bi2Te3 layer into MnBi2Te4. The first-principles calcns. reveal that the relative energy between ferromagnetic (FM) and AFM states is greatly reduced by Bi2Te3 intercalations. The calcd. energy barriers for the spin flipping process also point out that the metastable FM state is more easily retained by intercalation. Meanwhile, we also exptl. carry out magnetic and transport measurements on these materials. By increasing Bi2Te3 intercalations, the AFM coupling becomes weaker, and an almost fully polarized FM state can be preserved in MnBi6Te10 at low temps., which are consistent with our calcns. (c) 2020 American Institute of Physics.
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  39. 39
    Aliev, Z. S.; Amiraslanov, I. R.; Nasonova, D. I.; Shevelkov, A. V.; Abdullayev, N. A.; Jahangirli, Z. A.; Orujlu, E. N.; Otrokov, M. M.; Mamedov, N. T.; Babanly, M. B.; Chulkov, E. V. Novel Ternary Layered Manganese Bismuth Tellurides of the MnTe-Bi2Te3 System: Synthesis and Crystal Structure. J. Alloys Compd. 2019, 789, 443– 450,  DOI: 10.1016/j.jallcom.2019.03.030
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    39
    Novel ternary layered manganese bismuth tellurides of the MnTe-Bi2Te3 system: Synthesis and crystal structure
    Aliev, Ziya S.; Amiraslanov, Imamaddin R.; Nasonova, Daria I.; Shevelkov, Andrei V.; Abdullayev, Nadir A.; Jahangirli, Zakir A.; Orujlu, Elnur N.; Otrokov, Mikhail M.; Mamedov, Nazim T.; Babanly, Mahammad B.; Chulkov, Evgueni V.
    Journal of Alloys and Compounds (2019), 789 (), 443-450CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    It is shown that MnTe-Bi2Te3 system is quasi-binary and in fact hosts three intermediate phases. Along with already known MnBi2Te4 phase, another two, MnBi4Te7 and MnBi6Te10 have been found to exist. All the phases melt incongruently in a very narrow temp. range of 577-590°C via peritectic reactions. Directional crystal growth results in hetero-phase ingots due to the narrow compositional range and narrow primary crystn. fields. The crystal structure of each phase is a derivation of the prototype tetradymit-type layered structure and the phases constitute a new homologous series with the chem. formula (MnTe)·n(Bi2Te3). X-ray diffraction patterns and Raman spectroscopy of the sorted-out single phase samples show that different phases have different no. of the seven (7)- and five (5)-layer blocks and their different stacking manner in the unit cell. In particular, MnBi2Te4 exhibits the -7-7-7-, MnBi4Te7 -5-7-5-7-, and MnBi6Te10 -5-5-7-5-5-7- sequence of the blocks. Thus, these structures are the first derivs. of Bi2Te3 structure to contain a transition metal cation Mn2+.
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  40. 40
    Ma, X.-M.; Chen, Z.; Schwier, E. F.; Zhang, Y.; Hao, Y.-J.; Kumar, S.; Lu, R.; Shao, J.; Jin, Y.; Zeng, M.; Liu, X.-R.; Hao, Z.; Zhang, K.; Mansuer, W.; Song, C.; Wang, Y.; Zhao, B.; Liu, C.; Deng, K.; Mei, J.; Shimada, K.; Zhao, Y.; Zhou, X.; Shen, B.; Huang, W.; Liu, C.; Xu, H.; Chen, C. Hybridization-Induced Gapped and Gapless States on the Surface of Magnetic Topological Insulators. Phys. Rev. B 2020, 102 (24), 245136,  DOI: 10.1103/PhysRevB.102.245136
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    40
    Hybridization-induced gapped and gapless states on the surface of magnetic topological insulators
    Ma, Xiao-Ming; Chen, Zhongjia; Schwier, Eike F.; Zhang, Yang; Hao, Yu-Jie; Kumar, Shiv; Lu, Ruie; Shao, Jifeng; Jin, Yuanjun; Zeng, Meng; Liu, Xiang-Rui; Hao, Zhanyang; Zhang, Ke; Mansuer, Wumiti; Song, Chunyao; Wang, Yuan; Zhao, Boyan; Liu, Cai; Deng, Ke; Mei, Jiawei; Shimada, Kenya; Zhao, Yue; Zhou, Xingjiang; Shen, Bing; Huang, Wen; Liu, Chang; Xu, Hu; Chen, Chaoyu
    Physical Review B (2020), 102 (24), 245136CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    The layered MnBi2nTe3n+1 family represents the first intrinsic antiferromagnetic (AFM) topol. insulator (protected by a combination symmetry S) ever discovered, providing an ideal platform to explore novel areas of physics such as the quantum anomalous Hall effect at elevated temp. and axion electrodynamics. Some of the recent angle-resolved photoemission spectroscopy (ARPES) expts. on this family have revealed that all terminations exhibit (nearly) gapless topol. surface states (TSSs) in the AFM state. The gapless behavior is inconsistent with the theor. expectation, as the surfaces being studied are S-breaking and shall therefore open a gap. Here we explain this curious paradox using a surface-bulk band hybridization picture. Combining CD ARPES and first-principles calcns. on MnBi6Te10, we prove that gaplike features are induced through hybridization between TSSs and certain bulk bands with Rashba character. The obsd. (nearly) gapless features are consistently reproduced by tight-binding simulations where TSSs are coupled to a pair of Rashba-split bands (RSBs). The Dirac-cone-like spectral features actually originate from the RSBs. Our findings highlight the role of band hybridization, superior to magnetism in this case, in shaping the general surface band structure in this family of magnetic topol. materials.
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  41. 41
    Hu, C.; Tanatar, M. A.; Prozorov, R.; Ni, N. Unusual Dynamic Susceptibility Arising from Soft Ferromagnetic Domains in MnBi 8 Te 13 and Sb-Doped MnBi2nTe3n+1(n = 2, 3). J. Phys. Appl. Phys. 2022, 55 (5), 054003,  DOI: 10.1088/1361-6463/ac3032
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    41
    Unusual dynamic susceptibility arising from soft ferromagnetic domains in MnBi8Te13 and Sb-doped MnBi2nTe3n + 1 (n = 2, 3)
    Hu, Chaowei; Tanatar, Makariy A.; Prozorov, Ruslan; Ni, Ni
    Journal of Physics D: Applied Physics (2022), 55 (5), 054003CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
    MnBi2nTe3n + 1 (MBT) is the first intrinsic magnetic topol. insulator and is promising to host emergent phenomena such as quantum anomalous Hall effect. They can be made ferromagnetic by having n ≥ 4 or with Sb doping. We studied the magnetic dynamics in a few selected ferromagnetic (FM) MBT compds., including MnBi8Te13 and Sb doped MnBi2nTe3n + 1 (n = 2, 3) using AC susceptibility and magneto-optical imaging. Slow relaxation behavior is obsd. in all three compds., suggesting its universality among FM MBT. We attribute the origin of the relaxation behavior to the irreversible domain movements since they only appear below the satn. fields when ferromagnetic domains form. The very soft ferromagnetic domain nature is revealed by the low-field fine-structured domains and high-field sea-urchin-shaped remanent-state domains imaged via our magneto-optical measurements. Finally, we ascribe the rare 'double-peak' behavior obsd. in the AC susceptibility under small DC bias fields to the very soft ferromagnetic domain formations.
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  42. 42
    Wu, X.; Li, J.; Ma, X.-M.; Zhang, Y.; Liu, Y.; Zhou, C.-S.; Shao, J.; Wang, Q.; Hao, Y.-J.; Feng, Y.; Schwier, E. F.; Kumar, S.; Sun, H.; Liu, P.; Shimada, K.; Miyamoto, K.; Okuda, T.; Wang, K.; Xie, M.; Chen, C.; Liu, Q.; Liu, C.; Zhao, Y. Distinct Topological Surface States on the Two Terminations of MnBi4Te7. Phys. Rev. X 2020, 10 (3), 031013,  DOI: 10.1103/PhysRevX.10.031013
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    42
    Distinct Topological Surface States on the Two Terminations of MnBi4Te7
    Wu, Xuefeng; Li, Jiayu; Ma, Xiao-Ming; Zhang, Yu; Liu, Yuntian; Zhou, Chun-Sheng; Shao, Jifeng; Wang, Qiaoming; Hao, Yu-Jie; Feng, Yue; Schwier, Eike F.; Kumar, Shiv; Sun, Hongyi; Liu, Pengfei; Shimada, Kenya; Miyamoto, Koji; Okuda, Taichi; Wang, Kedong; Xie, Maohai; Chen, Chaoyu; Liu, Qihang; Liu, Chang; Zhao, Yue
    Physical Review X (2020), 10 (3), 031013CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
    Intrinsic magnetic topol. insulator MnBi2Te4 has got unusual success in hosting emergent phenomena such as quantum anomalous Hall effect and axion insulator states. By using scanning tunneling microscopy and angle-resolved photoemission spectroscopy techniques, we unambiguously assign the two distinct surface states of MnBi4Te7 (n=1) to the quintuple-layer (QL) Bi2Te3 termination and the septuple-layer (SL) MnBi2Te4 termination, resp. A comparison of the exptl. observations with theor. calcns. reveals diverging topol. behaviors, esp. the hybridization effect between the QL and SL, on the two terminations. We identify a gap on the QL termination, originating from the hybridization between the topol. surface states of the QL and the bands of the SL beneath, and a gapless Dirac-cone band structure on the SL termination with time-reversal symmetry. The quasiparticle interference patterns further confirm the topol. nature of the surface states for both terminations, continuing far above the Fermi energy. The QL termination carries a spin-helical Dirac state with hexagonal warping, while at the SL termination, a strongly canted helical state from the surface lies between a pair of Rashba-like splitting bands from its neighboring layer. Our work elucidates an unprecedented hybridization effect between the building blocks of the topol. surface states and also reveals the termination-dependent time-reversal symmetry breaking in a magnetic topol. insulator.
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  43. 43
    Vidal, R. C.; Bentmann, H.; Facio, J. I.; Heider, T.; Kagerer, P.; Fornari, C. I.; Peixoto, T. R. F.; Figgemeier, T.; Jung, S.; Cacho, C.; Büchner, B.; van den Brink, J.; Schneider, C. M.; Plucinski, L.; Schwier, E. F.; Shimada, K.; Richter, M.; Isaeva, A.; Reinert, F. Orbital Complexity in Intrinsic Magnetic Topological Insulator MnBi4Te7 and MnBi6Te10. Phys. Rev. Lett. 2021, 126 (17), 176403,  DOI: 10.1103/PhysRevLett.126.176403
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    43
    Orbital Complexity in Intrinsic Magnetic Topological Insulators MnBi4Te7 and MnBi6Te10
    Vidal, R. C.; Bentmann, H.; Facio, J. I.; Heider, T.; Kagerer, P.; Fornari, C. I.; Peixoto, T. R. F.; Figgemeier, T.; Jung, S.; Cacho, C.; Buchner, B.; van den Brink, J.; Schneider, C. M.; Plucinski, L.; Schwier, E. F.; Shimada, K.; Richter, M.; Isaeva, A.; Reinert, F.
    Physical Review Letters (2021), 126 (17), 176403CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
    Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compds. MnBi4Te7 and MnBi6Te10, the n=1 and 2 members of a modular (Bi2Te3)n(MnBi2Te4) series, which have attracted recent interest as intrinsic magnetic topol. insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calcns. based on d. functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topol. from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for Bi2Te3-terminated surfaces but remains preserved for MnBi2Te4-terminated surfaces. Our results firmly establish the topol. nontrivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall cond. depends on surface termination.
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    Jo, N. H.; Wang, L.-L.; Slager, R.-J.; Yan, J.; Wu, Y.; Lee, K.; Schrunk, B.; Vishwanath, A.; Kaminski, A. Intrinsic Axion Insulating Behavior in Antiferromagnetic MnBi6Te10. Phys. Rev. B 2020, 102 (4), 045130,  DOI: 10.1103/PhysRevB.102.045130
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    Intrinsic axion insulating behavior in antiferromagnetic MnBi6Te10
    Jo, Na Hyun; Wang, Lin-Lin; Slager, Robert-Jan; Yan, Jiaqiang; Wu, Yun; Lee, Kyungchan; Schrunk, Benjamin; Vishwanath, Ashvin; Kaminski, Adam
    Physical Review B (2020), 102 (4), 045130CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    A striking feature of time-reversal symmetry (TRS) protected topol. insulators (TIs) is that they are characterized by a half integer quantum Hall effect on the boundary when the surface states are gapped by time-reversal breaking perturbations. While TRS-protected TIs have become increasingly under control, magnetic analogs are still a largely unexplored territory with novel rich structures. In particular, magnetic topol. insulators can also host a quantized axion term in the presence of lattice symmetries. Since these symmetries are naturally broken on the boundary, the surface states can develop a gap without external manipulation. In this paper, we combine theor. anal., d.-functional calcns. and exptl. evidence to reveal intrinsic axion insulating behavior in MnBi6Te10. The quantized axion term arises from the simplest possible mechanism in the antiferromagnetic regime where it is protected by inversion symmetry and the product of a fractional translation and TRS. The anticipated gapping of the Dirac surface state at the edge is subsequently exptl. established using angle resolved photoemission spectroscopy (ARPES). As a result, this system provides the magnetic analog of the simplest TRS-protected TI with a single, gapped Dirac cone at the surface.
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    Gordon, K. N.; Sun, H.; Hu, C.; Linn, A. G.; Li, H.; Liu, Y.; Liu, P.; Mackey, S.; Liu, Q.; Ni, N.; Dessau, D. Strongly Gapped Topological Surface States on Protected Surfaces of Antiferromagnetic MnBi4Te7 and MnBi6Te10. 2019, 1910.13943. arXiv.  DOI: 10.48550/arXiv.1910.13943 (accessed June 24, 2022).
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    Tan, H.; Yan, B. Distinct Magnetic Gaps between Antiferromagnetic and Ferromagnetic Orders Driven by Surface Defects in the Topological Magnet MnBi2Te4. 2022, 2207.13511. arXiv.  DOI: 10.48550/arXiv.2207.13511 (accessed September 08, 2022).
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    Garnica, M.; Otrokov, M. M.; Aguilar, P. C.; Klimovskikh, I. I.; Estyunin, D.; Aliev, Z. S.; Amiraslanov, I. R.; Abdullayev, N. A.; Zverev, V. N.; Babanly, M. B.; Mamedov, N. T.; Shikin, A. M.; Arnau, A.; de Parga, A. L. V.; Chulkov, E. V.; Miranda, R. Native Point Defects and Their Implications for the Dirac Point Gap at MnBi2Te4(0001). Npj Quantum Mater. 2022, 7 (1), 1– 9,  DOI: 10.1038/s41535-021-00414-6
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    Shikin, A. M.; Estyunin, D. A.; Klimovskikh, I. I.; Filnov, S. O.; Schwier, E. F.; Kumar, S.; Miyamoto, K.; Okuda, T.; Kimura, A.; Kuroda, K.; Yaji, K.; Shin, S.; Takeda, Y.; Saitoh, Y.; Aliev, Z. S.; Mamedov, N. T.; Amiraslanov, I. R.; Babanly, M. B.; Otrokov, M. M.; Eremeev, S. V.; Chulkov, E. V. Nature of the Dirac Gap Modulation and Surface Magnetic Interaction in Axion Antiferromagnetic Topological Insulator MnBi2Te4. Sci. Rep. 2020, 10 (1), 13226,  DOI: 10.1038/s41598-020-70089-9
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    48
    Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator MnBi2Te4
    Shikin, A. M.; Estyunin, D. A.; Klimovskikh, I. I.; Filnov, S. O.; Schwier, E. F.; Kumar, S.; Miyamoto, K.; Okuda, T.; Kimura, A.; Kuroda, K.; Yaji, K.; Shin, S.; Takeda, Y.; Saitoh, Y.; Aliev, Z. S.; Mamedov, N. T.; Amiraslanov, I. R.; Babanly, M. B.; Otrokov, M. M.; Eremeev, S. V.; Chulkov, E. V.
    Scientific Reports (2020), 10 (1), 13226CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
    Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topol. insulator MnBi2Te4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temps. (9-35 K), light polarizations and photon energies. We have distinguished both large (60-70 meV) and reduced (< 20meV) gaps at the DP in the ARPES dispersions, which remain open above the Neel temp. (TN = 24.5K). We propose that the gap above TN remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and CD ARPES measurements show a surface ferromagnetic ordering for the "large gap" sample and apparently significantly reduced effective magnetic moment for the "reduced gap" sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calcns. surface structural modification can result in a significant modulation of the DP gap.
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  49. 49
    Ma, X.-M.; Zhao, Y.; Zhang, K.; Kumar, S.; Lu, R.; Li, J.; Yao, Q.; Shao, J.; Hou, F.; Wu, X.; Zeng, M.; Hao, Y.-J.; Hao, Z.; Wang, Y.; Liu, X.-R.; Shen, H.; Sun, H.; Mei, J.; Miyamoto, K.; Okuda, T.; Arita, M.; Schwier, E. F.; Shimada, K.; Deng, K.; Liu, C.; Lin, J.; Zhao, Y.; Chen, C.; Liu, Q.; Liu, C. Realization of a Tunable Surface Dirac Gap in Sb-Doped MnBi2Te4. Phys. Rev. B 2021, 103 (12), L121112,  DOI: 10.1103/PhysRevB.103.L121112
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    49
    Realization of a tunable surface Dirac gap in Sb-doped MnBi2Te4
    Ma, Xiao-Ming; Zhao, Yufei; Zhang, Ke; Kumar, Shiv; Lu, Ruie; Li, Jiayu; Yao, Qiushi; Shao, Jifeng; Hou, Fuchen; Wu, Xuefeng; Zeng, Meng; Hao, Yu-Jie; Hao, Zhanyang; Wang, Yuan; Liu, Xiang-Rui; Shen, Huiwen; Sun, Hongyi; Mei, Jiawei; Miyamoto, Koji; Okuda, Taichi; Arita, Masashi; Schwier, Eike F.; Shimada, Kenya; Deng, Ke; Liu, Cai; Lin, Junhao; Zhao, Yue; Chen, Chaoyu; Liu, Qihang; Liu, Chang
    Physical Review B (2021), 103 (12), L121112CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Signatures of both the quantum anomalous Hall effect and axion electrodynamics have been recently obsd. to exist in thin films of MnBi2Te4, a stoichiometric antiferromagnetic topol. insulator. Direct evidence of the bulk topol. magnetoelec. response in an axion insulator requires an energy gap at its topol. surface state (TSS). However, independent spectroscopic expts. revealed that such a surface gap is much smaller than previously thought. Here we utilize angle resolved photoemission spectroscopy and d. functional theory calcns. to demonstrate that a sizable TSS gap unexpectedly exists in Sb-doped MnBi2Te4 where the bulk system remains topol. nontrivial. This gap is found to be insensitive to the bulk antiferromagnetic-paramagnetic transition, while it enlarges along with increasing Sb concn., enabling simultaneous tunability of the Fermi level and the TSS gap size (up to >100 meV). Our work shows that Sb dopants in MnBi2Te4 can not only control the Fermi level but also induce a tunable surface gap, providing a potential platform to observe the key features of the high-temp. axion-insulator phase.
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  50. 50
    Shikin, A. M.; Estyunin, D. A.; Zaitsev, N. L.; Glazkova, D.; Klimovskikh, I. I.; Filnov, S. O.; Rybkin, A. G.; Schwier, E. F.; Kumar, S.; Kimura, A.; Mamedov, N.; Aliev, Z.; Babanly, M. B.; Kokh, K.; Tereshchenko, O. E.; Otrokov, M. M.; Chulkov, E. V.; Zvezdin, K. A.; Zvezdin, A. K. Sample-Dependent Dirac-Point Gap in MnBi2Te4 and Its Response to Applied Surface Charge: A Combined Photoemission and Ab Initio Study. Phys. Rev. B 2021, 104 (11), 115168,  DOI: 10.1103/PhysRevB.104.115168
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    50
    Sample-dependent Dirac-point gap in MnBi2Te4 and its response to applied surface charge: A combined photoemission and ab initio study
    Shikin, A. M.; Estyunin, D. A.; Zaitsev, N. L.; Glazkova, D.; Klimovskikh, I. I.; Filnov, S. O.; Rybkin, A. G.; Schwier, E. F.; Kumar, S.; Kimura, A.; Mamedov, N.; Aliev, Z.; Babanly, M. B.; Kokh, K.; Tereshchenko, O. E.; Otrokov, M. M.; Chulkov, E. V.; Zvezdin, K. A.; Zvezdin, A. K.
    Physical Review B (2021), 104 (11), 115168CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Recently discovered intrinsic antiferromagnetic topol. insulator MnBi2Te4 presents an exciting platform for realization of the quantum anomalous Hall effect and a no. of related phenomena at elevated temps. An important characteristic making this material attractive for applications is its predicted large magnetic gap at the Dirac point (DP). However, while the early exptl. measurements reported on large DP gaps, a no. of recent studies claimed to observe a gapless dispersion of the MnBi2Te4 Dirac cone. Here, using micro(μ)-laser angle-resolved photoemission spectroscopy, we study the electronic structure of 15 different MnBi2Te4 samples, grown by two different chemists groups. Based on the careful energy distribution curves anal., the DP gaps between 15 and 65 meV are obsd., as measured below the Neel temp. at about 10-16 K. At that, roughly half of the studied samples show the DP gap of about 30 meV, while for a quarter of the samples the gaps are in the 50 to 60 meV range. Summarizing the results of both our and other groups, in the currently available MnBi2Te4 samples the DP gap can acquire an arbitrary value between a few and several tens of meV. Furthermore, based on the d. functional theory, we discuss a possible factor that might contribute to the redn. of the DP gap size, which is the excess surface charge that can appear due to various defects in surface region. We demonstrate that the DP gap is influenced by the applied surface charge and even can be closed, which can be taken advantage of to tune the MnBi2Te4 DP gap size.
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    Hor, Y. S.; Roushan, P.; Beidenkopf, H.; Seo, J.; Qu, D.; Checkelsky, J. G.; Wray, L. A.; Hsieh, D.; Xia, Y.; Xu, S.-Y.; Qian, D.; Hasan, M. Z.; Ong, N. P.; Yazdani, A.; Cava, R. J. Development of Ferromagnetism in the Doped Topological Insulator Bi2Te3. Phys. Rev. B 2010, 81 (19), 195203,  DOI: 10.1103/PhysRevB.81.195203
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    Development of ferromagnetism in the doped topological insulator Bi2-xMnxTe3
    Hor, Y. S.; Roushan, P.; Beidenkopf, H.; Seo, J.; Qu, D.; Checkelsky, J. G.; Wray, L. A.; Hsieh, D.; Xia, Y.; Xu, S.-Y.; Qian, D.; Hasan, M. Z.; Ong, N. P.; Yazdani, A.; Cava, R. J.
    Physical Review B: Condensed Matter and Materials Physics (2010), 81 (19), 195203/1-195203/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The development of ferromagnetism in Mn-doped Bi2Te3 is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy anal. shows that the Mn substitutes on the Bi sites, forming compds. of the type Bi2-xMnxTe3, and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compns. Bi1.96Mn0.04Te3 and Bi1.91Mn0.09Te3, a second-order ferromagnetic transition is obsd., with TC ∼9-12 K. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the Bi2Te3 basal plane. Thermoelec. power and Hall effect measurements show that the Mn-doped Bi2Te3 crystals are p-type. Angle-resolved photoemission spectroscopy measurements show that the topol. surface states that are present in pristine Bi2Te3 are also present at 15 K in ferromagnetic Mn-doped Bi2-xMnxTe3 and that the dispersion relations of the surface states are changed in a subtle fashion.
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    Lee, J. S.; Richardella, A.; Rench, D. W.; Fraleigh, R. D.; Flanagan, T. C.; Borchers, J. A.; Tao, J.; Samarth, N. Ferromagnetism and Spin-Dependent Transport in n-Type Mn-Doped Bismuth Telluride Thin Films. Phys. Rev. B 2014, 89 (17), 174425,  DOI: 10.1103/PhysRevB.89.174425
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    Ferromagnetism and spin-dependent transport in n-type Mn-doped bismuth telluride thin films
    Lee, Joon Sue; Richardella, Anthony; Rench, David W.; Fraleigh, Robert D.; Flanagan, Thomas C.; Borchers, Julie A.; Tao, Jing; Samarth, Nitin
    Physical Review B: Condensed Matter and Materials Physics (2014), 89 (17), 174425/1-174425/8, 8 pp.CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We describe a detailed study of the structural, magnetic and magnetotransport properties of single-crystal, n-type, Mn-doped bismuth telluride thin films grown by mol. beam epitaxy. With increasing Mn concn., the crystal structure changes from the tetradymite structure of the Bi2Te3 parent crystal at low Mn concns. towards a BiTe phase in the (Bi2Te3)m(Bi2)n homologous series. Magnetization measurements reveal the onset of ferromagnetism with a Curie temp. in the range 13.8-17 K in films with ∼2%-∼10% Mn concn. Magnetization hysteresis loops reveal that the magnetic easy axis is along the c axis of the crystal (perpendicular to the plane). Polarized neutron reflectivity measurements of a 68-nm-thick sample show that the magnetization is uniform through the film. The presence of ferromagnetism is also manifest in a strong anomalous Hall effect and a hysteretic magnetoresistance arising from domain-wall scattering. Ordinary Hall effect measurements show that the carrier d. is n type, increases with Mn doping and is high enough (≥2.8 × 1013 cm-2) to place the chem. potential in the conduction band. Thus the obsd. ferromagnetism is likely assocd. with both bulk and surface states. Surprisingly, the Curie temp. does not show any clear dependence on the carrier d. but does increase with Mn concn. Our results suggest that the ferromagnetism probed in these Mn-doped bismuth telluride films is not mediated by carriers in the conduction band or in an impurity band.
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    Vobornik, I.; Panaccione, G.; Fujii, J.; Zhu, Z.-H.; Offi, F.; Salles, B. R.; Borgatti, F.; Torelli, P.; Rueff, J. P.; Ceolin, D.; Artioli, A.; Unnikrishnan, M.; Levy, G.; Marangolo, M.; Eddrief, M.; Krizmancic, D.; Ji, H.; Damascelli, A.; van der Laan, G.; Egdell, R. G.; Cava, R. J. Observation of Distinct Bulk and Surface Chemical Environments in a Topological Insulator under Magnetic Doping. J. Phys. Chem. C 2014, 118 (23), 12333– 12339,  DOI: 10.1021/jp502729u
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    Observation of Distinct Bulk and Surface Chemical Environments in a Topological Insulator under Magnetic Doping
    Vobornik, Ivana; Panaccione, Giancarlo; Fujii, Jun; Zhu, Zhi-Huai; Offi, Francesco; Salles, Benjamin R.; Borgatti, Francesco; Torelli, Piero; Rueff, Jean Pascal; Ceolin, Denis; Artioli, Alberto; Unnikrishnan, Manju; Levy, Giorgio; Marangolo, Massimiliano; Eddrief, Mamhoud; Krizmancic, Damjan; Ji, Huiwen; Damascelli, Andrea; van der Laan, Gerrit; Egdell, Russell G.; Cava, Robert J.
    Journal of Physical Chemistry C (2014), 118 (23), 12333-12339CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The influence of magnetic dopants on the electronic and chem. environments in topol. insulators (TIs) is a key factor when considering possible spintronic applications based on topol. surface state properties. Here the authors provide spectroscopic evidence for the presence of distinct chem. and electronic behavior for surface and bulk magnetic doping of Bi2Te3. The inclusion of Mn in the bulk of Bi2Te3 induces a genuine dil. ferromagnetic state, with redn. of the bulk band gap as the Mn content is increased. Deposition of Fe on the Bi2Te3 surface, however, favors the formation of iron telluride already at coverage ≥0.07 monolayer, as a consequence of the reactivity of the Te-rich surface. The authors' results identify the factors that need to be controlled in the realization of magnetic nanosystems and interfaces based on TIs.
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    Teng, J.; Liu, N.; Li, Y. Mn-Doped Topological Insulators: A Review. J. Semicond. 2019, 40 (8), 081507,  DOI: 10.1088/1674-4926/40/8/081507
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    Mn-doped topological insulators: a review
    Teng, Jing; Liu, Nan; Li, Yongqing
    Journal of Semiconductors (2019), 40 (8), 081507CODEN: JSOEB4; ISSN:1674-4926. (IOP Publishing Ltd.)
    A review. Topol. insulators (TIs) host robust edge or surface states protected by time-reversal symmetry (TRS), which makes them prime candidates for applications in spintronic devices. A promising avenue of research for the development of functional TI devices has involved doping of three-dimensional (3D) TI thin film and bulk materials with magnetic elements. This approach aims to break the TRS and open a surface band gap near the Dirac point. Utilizing this gapped surface state allows for a wide range of novel phys. effects to be obsd., paving a way for applications in spintronics and quantum computation. This review focuses on the research of 3D TIs doped with manganese (Mn). We summarize major progress in the study of Mn doped chalcogenide TIs, including Bi2Se3, Bi2Te3, and Bi2(Te,Se)3. The transport properties, in particular the anomalous Hall effect, of the Mn-doped Bi2Se3 are discussed in detail. Finally, we conclude with future prospects and challenges in further studies of Mn doped TIs.
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    Chen, B.; Wang, D.; Jiang, Z.; Zhang, B.; Cui, S.; Guo, J.; Xie, H.; Zhang, Y.; Naveed, M.; Du, Y.; Wang, X.; Zhang, H.; Fei, F.; Shen, D.; Sun, Z.; Song, F. Coexistence of Ferromagnetism and Topology by Charge Carrier Engineering in the Intrinsic Magnetic Topological Insulator Mn Bi4Te7. Phys. Rev. B 2021, 104 (7), 075134,  DOI: 10.1103/PhysRevB.104.075134
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    Coexistence of ferromagnetism and topology by charge carrier engineering in the intrinsic magnetic topological insulator MnBi4Te7
    Chen, Bo; Wang, Dinghui; Jiang, Zhicheng; Zhang, Bo; Cui, Shengtao; Guo, Jingwen; Xie, Hangkai; Zhang, Yong; Naveed, Muhammad; Du, Yu; Wang, Xuefeng; Zhang, Haijun; Fei, Fucong; Shen, Dawei; Sun, Zhe; Song, Fengqi
    Physical Review B (2021), 104 (7), 075134CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Intrinsic magnetic topol. insulators (MTIs) MnBi2Te4 and MnBi2Te4/(Bi2Te3)n are expected to realize the high-temp. quantum anomalous Hall effect and dissipationless elec. transport. However, there is still a lack of ideal MTI candidates with magnetic ordering of the ferromagnetic (FM) ground state. Here, we show a MTI sample of Mn(Bi0.7Sb0.3)4Te7 which holds the coexistence of a FM behavior state and topol. nontriviality. The dramatic modulation of the magnetism is induced by a charge carrier engineering process via the Sb substitution in the MnBi4Te7 matrix with antiferromagnetic ordering. The evolution of magnetism in Mn(Bi1-xSbx)4Te7 is systematically investigated by our magnetic measurements and theor. calcns. The clear topol. surface states of the FM sample of Mn(Bi0.7Sb0.3)4Te7 are further verified by angle-resolved photoemission spectroscopy. The demonstration of the intrinsic FM-MTI of Mn(Bi0.7Sb0.3)4Te7 in this paper sheds light on further material optimization of intrinsic MTIs and paves the way for further studies to clarify the relationships between topol., magnetism, and charge carriers in topol. materials.
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  56. 56
    Sitnicka, J.; Park, K.; Skupinski, P.ł; Grasza, K.; Reszka, A.; Sobczak, K.; Borysiuk, J.; Adamus, Z.; Tokarczyk, M.; Avdonin, A.; Fedorchenko, I.; Abaloszewa, I.; Turczyniak-Surdacka, S.; Olszowska, N.; Kołodziej, J.; Kowalski, B. J; Deng, H.; Konczykowski, M.; Krusin-Elbaum, L.; Wołos, A. Systemic Consequences of Disorder in Magnetically Self-Organized Topological MnBi2Te4/(Bi2Te3) n Superlattices. 2D Mater. 2022, 9 (1), 015026,  DOI: 10.1088/2053-1583/ac3cc6
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    Systemic consequences of disorder in magnetically self-organized topological MnBi2Te4/(Bi2Te3)n superlattices
    Sitnicka, Joanna; Park, Kyungwha; Skupinski, Pawel; Grasza, Krzysztof; Reszka, Anna; Sobczak, Kamil; Borysiuk, Jolanta; Adamus, Zbigniew; Tokarczyk, Mateusz; Avdonin, Andrei; Fedorchenko, Irina; Abaloszewa, Irina; Turczyniak-Surdacka, Sylwia; Olszowska, Natalia; Kolodziej, Jacek; Kowalski, Bogdan J.; Deng, Haiming; Konczykowski, Marcin; Krusin-Elbaum, Lia; Wolos, Agnieszka
    2D Materials (2022), 9 (1), 015026CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    MnBi2Te4/(Bi2Te3)n materials system has recently generated strong interest as a natural platform for the realization of the quantum anomalous Hall (QAH) state. The system is magnetically much better ordered than substitutionally doped materials, however, the detrimental effects of certain disorders are becoming increasingly acknowledged. Here, from compiling structural, compositional, and magnetic metrics of disorder in ferromagnetic (FM) MnBi2Te4/(Bi2Te3)n it is found that migration of Mn between MnBi2Te4 septuple layers (SLs) and otherwise non-magnetic Bi2Te3 quintuple layers (QLs) has systemic consequences-it induces FM coupling of Mn-depleted SLs with Mn-doped QLs, seen in ferromagnetic resonance as an acoustic and optical resonance mode of the two coupled spin subsystems. Even for a large SL sepn. (n ⪆ 4 QLs) the structure cannot be considered as a stack of uncoupled two-dimensional layers. Angle-resolved photoemission spectroscopy and d. functional theory studies show that Mn disorder within an SL causes delocalization of electron wave functions and a change of the surface band structure as compared to the ideal MnBi2Te4/(Bi2Te3)n. These findings highlight the crit. importance of inter- and intra-SL disorder towards achieving new QAH platforms as well as exploring novel axion physics in intrinsic topol. magnets.
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    Yan, C.; Green, E.; Fukumori, R.; Protic, N.; Lee, S. H.; Fernandez-Mulligan, S.; Raja, R.; Erdakos, R.; Mao, Z.; Yang, S. An Integrated Quantum Material Testbed with Multi-Resolution Photoemission Spectroscopy. Rev. Sci. Instrum. 2021, 92 (11), 113907,  DOI: 10.1063/5.0072979
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    An integrated quantum material testbed with multi-resolution photoemission spectroscopy
    Yan, Chenhui; Green, Emanuel; Fukumori, Riku; Protic, Nikola; Lee, Seng Huat; Fernandez-Mulligan, Sebastian; Raja, Rahim; Erdakos, Robin; Mao, Zhiqiang; Yang, Shuolong
    Review of Scientific Instruments (2021), 92 (11), 113907CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
    The development of a multi-resoln. photoemission spectroscopy (MRPES) setup, which probes quantum materials in energy, momentum, space, and time is presented. This versatile setup integrates 3 light sources in 1 photoelectron setup and can conveniently switch between traditional angle-resolved photoelectron spectroscopy (ARPES), time-resolved ARPES (trARPES), and micrometer-scale spatially resolved ARPES. It provides a 1st-time all-in-one soln. to achieve an energy resoln. of <4 meV, a time resoln. of <35 fs, and a spatial resoln. of ∼10μm in photoelectron spectroscopy. The shortest time resoln. among the trARPES setups was obtained using solid-state nonlinear crystals for frequency upconversion. This MRPES setup is integrated with a shadow-mask assisted MBE system, which transforms the traditional photoemission spectroscopy into a quantum device characterization instrument. The functionalities of this novel quantum material testbed were demonstrated using FeSe/SrTiO3 films and MnBi4Te7 magnetic topol. insulators. (c) 2021 American Institute of Physics.
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  • Abstract

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    Figure 1

    Figure 1. Structural and magnetic characterizations of MnBi6Te10. (a) Schematic crystal structure of MnBi6Te10. (b) X-ray diffraction of ferromagnetic (FM, red) and antiferromagnetic (AFM, blue) MnBi6Te10. (c) Temperature dependent zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibilities of FM (red) and AFM (blue) MnBi6Te10 using an external field H = 100 Oe along the c-axis. The results corresponding to the AFM samples are offset vertically for clarity. (d,e) Isothermal magnetization curves with the magnetic field applied along the c-axis and in the ab plane at various temperatures in (d) AFM and (e) FM MnBi6Te10.

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    Figure 2

    Figure 2. Electronic structure of ferromagnetic MnBi6Te10. Energy-momentum spectra along Γ̅ – Μ̅ at (a) 7.5 K, and (b) 20 K. The insert in (a) illustrates the MnBi2Te4 (MBT) termination. (c) Comparison of energy distribution curves at Γ̅ . An energy gap is observed at the Dirac point (black dashed line) at 7.5 K. The counterpart results for the 1-Bi2Te3 (1-BT) termination are plotted in (d–f): (d,e) energy-momentum spectra, and (f) energy distribution curves at Γ̅ . The counterpart results for the 2-Bi2Te3 (2-BT) termination are plotted in (g–i): (g,h) energy-momentum spectra, and (i) energy distribution curves at Γ̅ .

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    Figure 3

    Figure 3. Structural characterizations of MnBi6Te10. Annular dark-field scanning transmission electron microscopy (ADF-STEM) images of (a) AFM and (b) FM MnBi6Te10. Scale bar indicates 5 nm. Mn vacancies are highlighted by red shading, where the intensity for the atomic column of Mn is lower than the value of two standard deviations below the Mn mean intensity. (c) Magnified image (yellow box in (b)). (d) A cartoon illustration showing HIL > HD in the AFM phase. (e) With increased Mn vacancies, HIL < HD in the FM phase.

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    1. 1
      Liu, C.; Wang, Y.; Li, H.; Wu, Y.; Li, Y.; Li, J.; He, K.; Xu, Y.; Zhang, J.; Wang, Y. Robust Axion Insulator and Chern Insulator Phases in a Two-Dimensional Antiferromagnetic Topological Insulator. Nat. Mater. 2020, 19, 522,  DOI: 10.1038/s41563-019-0573-3
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      Nature Materials (2020), 19 (5), 522-527CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
      The intricate interplay between non-trivial topol. and magnetism in two-dimensional materials can lead to the emergence of interesting phenomena such as the quantum anomalous Hall effect. Here we investigate the quantum transport of both bulk crystal and exfoliated MnBi2Te4 flakes in a field-effect transistor geometry. For the six septuple-layer device tuned into the insulating regime, we observe a large longitudinal resistance and zero Hall plateau, which are characteristics of an axion insulator state. The robust axion insulator state occurs in zero magnetic field, over a wide magnetic-field range and at relatively high temps. Moreover, a moderate magnetic field drives a quantum phase transition from the axion insulator phase to a Chern insulator phase with zero longitudinal resistance and quantized Hall resistance h/e2, where h is Planck's const. and e is electron charge. Our results pave the way for using even-no. septuple-layer MnBi2Te4 to realize the quantized topol. magnetoelec. effect and axion electrodynamics in condensed matter systems.
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      Nature (London, United Kingdom) (2019), 576 (7787), 416-422CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Magnetic topol. insulators are narrow-gap semiconductor materials that combine non-trivial band topol. and magnetic order. Unlike their nonmagnetic counterparts, magnetic topol. insulators may have some of the surfaces gapped, which enables a no. of exotic phenomena that have potential applications in spintronics, such as the quantum anomalous Hall effect and chiral Majorana fermions3. So far, magnetic topol. insulators have only been created by means of doping nonmagnetic topol. insulators with 3d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temps. An intrinsic magnetic topol. insulator-a stoichiometric well ordered magnetic compd.-could be an ideal soln. to these problems, but no such material has been obsd. so far. Here, the authors predict by ab initio calcns. and further confirm using various exptl. techniques the realization of an antiferromagnetic topol. insulator in the layered van der Waals compd. MnBi2Te4. The antiferromagnetic ordering that MnBi2Te4 shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a Z2 topol. classification; Z2 = 1 for MnBi2Te4, confirming its topol. nontrivial nature. The expts. indicate that the symmetry-breaking (0001) surface of MnBi2Te4 exhibits a large band gap in the topol. surface state. The authors expect this property to eventually enable the observation of a no. of fundamental phenomena, among them quantized magnetoelec. coupling and axion electrodynamics. Other exotic phenomena could become accessible at much higher temps. than those reached so far, such as the quantum anomalous Hall effect and chiral Majorana fermions.
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      Rienks, E. D. L.; Wimmer, S.; Sánchez-Barriga, J.; Caha, O.; Mandal, P. S.; Růžička, J.; Ney, A.; Steiner, H.; Volobuev, V. V.; Groiss, H.; Albu, M.; Kothleitner, G.; Michalička, J.; Khan, S. A.; Minár, J.; Ebert, H.; Bauer, G.; Freyse, F.; Varykhalov, A.; Rader, O.; Springholz, G. Large Magnetic Gap at the Dirac Point in Bi2Te3/MnBi2Te4 Heterostructures. Nature 2019, 576 (7787), 423– 428,  DOI: 10.1038/s41586-019-1826-7
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      Large magnetic gap at the Dirac point in Bi2Te3/MnBi2Te4 heterostructures
      Rienks, E. D. L.; Wimmer, S.; Sanchez-Barriga, J.; Caha, O.; Mandal, P. S.; Ruzicka, J.; Ney, A.; Steiner, H.; Volobuev, V. V.; Groiss, H.; Albu, M.; Kothleitner, G.; Michalicka, J.; Khan, S. A.; Minar, J.; Ebert, H.; Bauer, G.; Freyse, F.; Varykhalov, A.; Rader, O.; Springholz, G.
      Nature (London, United Kingdom) (2019), 576 (7787), 423-428CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Magnetically doped topol. insulators enable the quantum anomalous Hall effect (QAHE), which provides quantized edge states for lossless charge-transport applications1-8. The edge states are hosted by a magnetic energy gap at the Dirac point2, but hitherto all attempts to observe this gap directly have been unsuccessful. Observing the gap is considered to be essential to overcoming the limitations of the QAHE, which so far occurs only at temps. that are one to two orders of magnitude below the ferromagnetic Curie temp., TC (ref. 8). Here we use low-temp. photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi2Te3, which displays ferromagnetic out-of-plane spin texture and opens up only below TC. Surprisingly, our anal. reveals large gap sizes at 1 K of up to 90 millielectronvolts, which is five times larger than theor. predicted9. Using multiscale anal. we show that this enhancement is due to a remarkable structure modification induced by Mn doping: instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3 quintuple layers is formed. This enhances the wavefunction overlap and size of the magnetic gap10. Mn-doped Bi2Se3 (ref. 11) and Mn-doped Sb2Te3 form similar heterostructures, but for Bi2Se3 only a nonmagnetic gap is formed and the magnetization is in the surface plane. This is explained by the smaller spin-orbit interaction by comparison with Mn-doped Bi2Te3. Our findings provide insights that will be crucial in pushing lossless transport in topol. insulators towards room-temp. applications.
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    4. 4
      Gong, Y.; Guo, J.; Li, J.; Zhu, K.; Liao, M.; Liu, X.; Zhang, Q.; Gu, L.; Tang, L.; Feng, X.; Zhang, D.; Li, W.; Song, C.; Wang, L.; Yu, P.; Chen, X.; Wang, Y.; Yao, H.; Duan, W.; Xu, Y.; Zhang, S.-C.; Ma, X.; Xue, Q.-K.; He, K. Experimental Realization of an Intrinsic Magnetic Topological Insulator. Chin. Phys. Lett. 2019, 36 (7), 076801,  DOI: 10.1088/0256-307X/36/7/076801
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      4
      Experimental realization of an intrinsic magnetic topological insulator
      Gong, Yan; Guo, Jingwen; Li, Jiaheng; Zhu, Kejing; Liao, Menghan; Liu, Xiaozhi; Zhang, Qinghua; Gu, Lin; Tang, Lin; Feng, Xiao; Zhang, Ding; Li, Wei; Song, Canli; Wang, Lili; Yu, Pu; Chen, Xi; Wang, Yayu; Yao, Hong; Duan, Wenhui; Xu, Yong; Zhang, Shou-Cheng; Ma, Xucun; Xue, Qi-Kun; He, Ke
      Chinese Physics Letters (2019), 36 (7), 076801CODEN: CPLEEU; ISSN:1741-3540. (IOP Publishing Ltd.)
      An intrinsic magnetic topol. insulator (TI) is a stoichiometric magnetic compd. possessing both inherent magnetic order and topol. electronic states. Such a material can provide a shortcut to various novel topol. quantum effects but remained elusive exptl. for a long time. Here we report the exptl. realization of thin films of an intrinsic magnetic TI, MnBi2Te4, by alternate growth of a Bi2Te3quintuple layer and a MnTe bilayer with mol. beam epitaxy. The material shows the archetypical Dirac surface states in angle-resolved photoemission spectroscopy and is demonstrated to be an antiferromagnetic topol. insulator with ferromagnetic surfaces by magnetic and transport measurements as well as first-principles calcns. The unique magnetic and topol. electronic structures and their interplays enable the material to embody rich quantum phases such as quantum anomalous Hall insulators and axion insulators at higher temp. and in a well-controlled way.
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    5. 5
      Zeugner, A.; Nietschke, F.; Wolter, A. U. B.; Gaß, S.; Vidal, R. C.; Peixoto, T. R. F.; Pohl, D.; Damm, C.; Lubk, A.; Hentrich, R.; Moser, S. K.; Fornari, C.; Min, C. H.; Schatz, S.; Kißner, K.; Ünzelmann, M.; Kaiser, M.; Scaravaggi, F.; Rellinghaus, B.; Nielsch, K.; Hess, C.; Büchner, B.; Reinert, F.; Bentmann, H.; Oeckler, O.; Doert, T.; Ruck, M.; Isaeva, A. Chemical Aspects of the Candidate Antiferromagnetic Topological Insulator MnBi2Te4. Chem. Mater. 2019, 31 (8), 2795– 2806,  DOI: 10.1021/acs.chemmater.8b05017
      [ACS Full Text ACS Full Text], [CAS], Google Scholar
      5
      Chemical Aspects of the Candidate Antiferromagnetic Topological Insulator MnBi2Te4
      Zeugner, Alexander; Nietschke, Frederik; Wolter, Anja U. B.; Gass, Sebastian; Vidal, Raphael C.; Peixoto, Thiago R. F.; Pohl, Darius; Damm, Christine; Lubk, Axel; Hentrich, Richard; Moser, Simon K.; Fornari, Celso; Min, Chul Hee; Schatz, Sonja; Kissner, Katharina; Uenzelmann, Maximilian; Kaiser, Martin; Scaravaggi, Francesco; Rellinghaus, Bernd; Nielsch, Kornelius; Hess, Christian; Buechner, Bernd; Reinert, Friedrich; Bentmann, Hendrik; Oeckler, Oliver; Doert, Thomas; Ruck, Michael; Isaeva, Anna
      Chemistry of Materials (2019), 31 (8), 2795-2806CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      High-quality single crystals of MnBi2Te4 are grown for the first time by slow cooling within a narrow range between the m.ps. of Bi2Te3 (586 °C) and MnBi2Te4 (600 °C). Single-crystal X-ray diffraction and electron microscopy reveal ubiquitous antisite defects in both cation sites and, possibly, Mn vacancies (Mn0.85(3)Bi2.10(3)Te4). Thermochem. studies complemented with high-temp. X-ray diffraction establish a limited high-temp. range of phase stability and metastability at room temp. Nevertheless, the synthesis of MnBi2Te4 can be scaled-up as powders can be obtained at subsolidus temps. and quenched at room temp. Bulk samples exhibit long-range antiferromagnetic ordering below 24 K. The Mn(II) out-of-plane magnetic state is confirmed by the magnetization, X-ray photoemission, X-ray absorption, and linear dichroism measurements. The compd. shows a metallic type of resistivity in the range 4.5-300 K and is an n-type conductor that reaches a thermoelec. figure of merit up to ZT = 0.17. Angle-resolved photoemission expts. show a surface state forming a gapped Dirac cone, thus strengthening MnBi2Te4 as a promising candidate for the intrinsic magnetic topol. insulator, in accordance with theor. predictions. The developed synthetic protocols enable further exptl. studies of a crossover between magnetic ordering and nontrivial topol. in bulk MnBi2Te4.
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    6. 6
      Swatek, P.; Wu, Y.; Wang, L.-L.; Lee, K.; Schrunk, B.; Yan, J.; Kaminski, A. Gapless Dirac Surface States in the Antiferromagnetic Topological Insulator MnBi2Te4. Phys. Rev. B 2020, 101 (16), 161109,  DOI: 10.1103/PhysRevB.101.161109
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      6
      Gapless Dirac surface states in the antiferromagnetic topological insulator MnBi2Te4
      Swatek, Przemyslaw; Wu, Yun; Wang, Lin-Lin; Lee, Kyungchan; Schrunk, Benjamin; Yan, Jiaqiang; Kaminski, Adam
      Physical Review B (2020), 101 (16), 161109CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      A review. We used angle-resolved photoemission spectroscopy (ARPES) and d. functional theory calcns. to study the electronic properties of MnBi2Te4, a material that was predicted to be an intrinsic antiferromagnetic (AFM) topol. insulator. In striking contrast to earlier literature showing a full gap opening between two surface band manifolds on the (0001) surface, we obsd. a gapless Dirac surface state with a Dirac point sitting at EB=-280meV. Furthermore, our ARPES data revealed the existence of a second Dirac cone sitting closer to the Fermi level. Surprisingly, these surface states remain intact across the AFM transition. The presence of gapless Dirac states in this material may be caused by different ordering at the surface from the bulk or weaker magnetic coupling between the bulk and surface. Whereas the surface Dirac cones seem to be remarkably insensitive to the AFM ordering most likely due to weak coupling to magnetism, we did observe a splitting of the bulk band accompanying the AFM transition. With a moderately high ordering temp. and interesting gapless Dirac surface states, MnBi2Te4 provides a unique platform for studying the interplay between magnetic ordering and topol.
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    7. 7
      Hao, Y.-J.; Liu, P.; Feng, Y.; Ma, X.-M.; Schwier, E. F.; Arita, M.; Kumar, S.; Hu, C.; Lu, R.; Zeng, M.; Wang, Y.; Hao, Z.; Sun, H.-Y.; Zhang, K.; Mei, J.; Ni, N.; Wu, L.; Shimada, K.; Chen, C.; Liu, Q.; Liu, C. Gapless Surface Dirac Cone in Antiferromagnetic Topological Insulator MnBi2Te4. Phys. Rev. X 2019, 9 (4), 041038,  DOI: 10.1103/PhysRevX.9.041038
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      7
      Gapless Surface Dirac Cone in Antiferromagnetic Topological Insulator MnBi2Te4
      Hao, Yu-Jie; Liu, Pengfei; Feng, Yue; Ma, Xiao-Ming; Schwier, Eike F.; Arita, Masashi; Kumar, Shiv; Hu, Chaowei; Lu, Rui-e; Zeng, Meng; Wang, Yuan; Hao, Zhanyang; Sun, Hong-Yi; Zhang, Ke; Mei, Jiawei; Ni, Ni; Wu, Liusuo; Shimada, Kenya; Chen, Chaoyu; Liu, Qihang; Liu, Chang
      Physical Review X (2019), 9 (4), 041038CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
      The recently discovered antiferromagnetic topol. insulators in the Mn-Bi-Te family with intrinsic magnetic ordering have rapidly drawn broad interest since its cleaved surface state is believed to be gapped, hosting the unprecedented axion states with a half-integer quantum Hall effect. Here, however, we show unambiguously by using high-resoln. angle resolved photoemission spectroscopy that a gapless Dirac cone at the (0001) surface of MnBi2Te4 exists inside the bulk band gap. Such an unexpected surface state remains unchanged across the bulk N´eel temp., and is even robust against severe surface degrdn., indicating addnl. topol. protection. Through symmetry anal. and ab initio calcns. we consider different types of surface reconstruction of the magnetic moments as possible origins giving rise to such linear dispersion. Our results unveil the exptl. topol. properties of MnBi2Te4, revealing that the intrinsic magnetic topol. insulator hosts a rich platform to realize various topol. phases by tuning the magnetic or structural configurations, and thus push forward the comprehensive understanding of magnetic topol. materials.
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    8. 8
      Chen, Y. J.; Xu, L. X.; Li, J. H.; Li, Y. W.; Wang, H. Y.; Zhang, C. F.; Li, H.; Wu, Y.; Liang, A. J.; Chen, C.; Jung, S. W.; Cacho, C.; Mao, Y. H.; Liu, S.; Wang, M. X.; Guo, Y. F.; Xu, Y.; Liu, Z. K.; Yang, L. X.; Chen, Y. L. Topological Electronic Structure and Its Temperature Evolution in Antiferromagnetic Topological Insulator MnBi2Te4. Phys. Rev. X 2019, 9 (4), 041040,  DOI: 10.1103/PhysRevX.9.041040
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      8
      Topological Electronic Structure and Its Temperature Evolution in Antiferromagnetic Topological Insulator MnBi2Te4
      Chen, Y. J.; Xu, L. X.; Li, J. H.; Li, Y. W.; Wang, H. Y.; Zhang, C. F.; Li, H.; Wu, Y.; Liang, A. J.; Chen, C.; Jung, S. W.; Cacho, C.; Mao, Y. H.; Liu, S.; Wang, M. X.; Guo, Y. F.; Xu, Y.; Liu, Z. K.; Yang, L. X.; Chen, Y. L.
      Physical Review X (2019), 9 (4), 041040CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
      The intrinsic magnetic topol. insulator MnBi2Te4 exhibits rich topol. effects such as quantum anomalous Hall effect and axion electrodynamics. Here, by combining the use of synchrotron and laser light sources, we carry out comprehensive and high-resoln. angle-resolved photoemission spectroscopy studies on MnBi2Te4 and clearly identify its topol. electronic structure. In contrast to theor. predictions and previous studies, we observe topol. surface states with diminished gap forming a characteristic Dirac cone. We argue that the topol. surface states are mediated by multidomains of different magnetization orientations. In addn., the temp. evolution of the energy bands clearly reveals their interplay with the magnetic phase transition by showing interesting differences between the bulk and surface states, resp. The investigation of the detailed electronic structure of MnBi2Te4 and its temp. evolution provides important insight into not only the exotic properties of MnBi2Te4, but also the generic understanding of the interplay between magnetism and topol. electronic structure in magnetic topol. quantum materials.
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    9. 9
      Li, H.; Gao, S.-Y.; Duan, S.-F.; Xu, Y.-F.; Zhu, K.-J.; Tian, S.-J.; Gao, J.-C.; Fan, W.-H.; Rao, Z.-C.; Huang, J.-R.; Li, J.-J.; Yan, D.-Y.; Liu, Z.-T.; Liu, W.-L.; Huang, Y.-B.; Li, Y.-L.; Liu, Y.; Zhang, G.-B.; Zhang, P.; Kondo, T.; Shin, S.; Lei, H.-C.; Shi, Y.-G.; Zhang, W.-T.; Weng, H.-M.; Qian, T.; Ding, H. Dirac Surface States in Intrinsic Magnetic Topological Insulators EuSn2As2 and MnBi2nTe3n+1. Phys. Rev. X 2019, 9 (4), 041039,  DOI: 10.1103/PhysRevX.9.041039
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      9
      Dirac Surface States in Intrinsic Magnetic Topological Insulators EuSn2As2 and MnBi2nTe3n+1
      Li, Hang; Gao, Shun-Ye; Duan, Shao-Feng; Xu, Yuan-Feng; Zhu, Ke-Jia; Tian, Shang-Jie; Gao, Jia-Cheng; Fan, Wen-Hui; Rao, Zhi-Cheng; Huang, Jie-Rui; Li, Jia-Jun; Yan, Da-Yu; Liu, Zheng-Tai; Liu, Wan-Ling; Huang, Yao-Bo; Li, Yu-Liang; Liu, Yi; Zhang, Guo-Bin; Zhang, Peng; Kondo, Takeshi; Shin, Shik; Lei, He-Chang; Shi, You-Guo; Zhang, Wen-Tao; Weng, Hong-Ming; Qian, Tian; Ding, Hong
      Physical Review X (2019), 9 (4), 041039CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
      In magnetic topol. insulators (TIs), the interplay between magnetic order and nontrivial topol. can induce fascinating topol. quantum phenomena, such as the quantum anomalous Hall effect, chiral Majorana fermions, and axion electrodynamics. Recently, a great deal of attention has been focused on the intrinsic magnetic TIs, where disorder effects can be eliminated to a large extent, which is expected to facilitate the emergence of topol. quantum phenomena. Despite intensive efforts, exptl. evidence of the topol. surface states (SSs) remains elusive. Here, by combining first-principles calcns. and angle-resolved photoemission spectroscopy (ARPES) expts., we reveal that EuSn2As2 is an antiferromagnetic TI with the observation of Dirac SSs consistent with our prediction. We also observe nearly gapless Dirac SSs in antiferromagnetic TIs MnBi2nTe3n+1 (n=1 and 2), which are absent in previous ARPES results. These results provide clear evidence for nontrivial topol. of these intrinsic magnetic TIs. Furthermore, we find that the topol. SSs show no observable changes across the magnetic transition within the exptl. resoln., indicating that the magnetic order has a quite small effect on the topol. SSs, which can be attributed to weak hybridization between the localized magnetic moments, from either 4f or 3d orbitals, and the topol. electronic states. This finding provides insights for further research that the correlations between magnetism and topol. states need to be strengthened to induce larger gaps in the topol. SSs, which will facilitate the realization of topol. quantum phenomena at higher temps.
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    10. 10
      Vidal, R. C.; Bentmann, H.; Peixoto, T. R. F.; Zeugner, A.; Moser, S.; Min, C.-H.; Schatz, S.; Kißner, K.; Ünzelmann, M.; Fornari, C. I.; Vasili, H. B.; Valvidares, M.; Sakamoto, K.; Mondal, D.; Fujii, J.; Vobornik, I.; Jung, S.; Cacho, C.; Kim, T. K.; Koch, R. J.; Jozwiak, C.; Bostwick, A.; Denlinger, J. D.; Rotenberg, E.; Buck, J.; Hoesch, M.; Diekmann, F.; Rohlf, S.; Kalläne, M.; Rossnagel, K.; Otrokov, M. M.; Chulkov, E. V.; Ruck, M.; Isaeva, A.; Reinert, F. Surface States and Rashba-Type Spin Polarization in Antiferromagnetic MnBi2Te4 (0001). Phys. Rev. B 2019, 100 (12), 121104,  DOI: 10.1103/PhysRevB.100.121104
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      10
      Surface states and Rashba-type spin polarization in antiferromagnetic MnBi2Te4(0001)
      Vidal, R. C.; Bentmann, H.; Peixoto, T. R. F.; Zeugner, A.; Moser, S.; Min, C.-H.; Schatz, S.; Kissner, K.; Uenzelmann, M.; Fornari, C. I.; Vasili, H. B.; Valvidares, M.; Sakamoto, K.; Mondal, D.; Fujii, J.; Vobornik, I.; Jung, S.; Cacho, C.; Kim, T. K.; Koch, R. J.; Jozwiak, C.; Bostwick, A.; Denlinger, J. D.; Rotenberg, E.; Buck, J.; Hoesch, M.; Diekmann, F.; Rohlf, S.; Kallaene, M.; Rossnagel, K.; Otrokov, M. M.; Chulkov, E. V.; Ruck, M.; Isaeva, A.; Reinert, F.
      Physical Review B (2019), 100 (12), 121104CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      The layered van der Waals antiferromagnet MnBi2Te4 has been predicted to combine the band ordering of archetypical topol. insulators such as Bi2Te3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topol. states. We have systematically investigated the surface electronic structure of MnBi2Te4(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy expts. In line with theor. predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.
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    11. 11
      Nevola, D.; Li, H. X.; Yan, J.-Q.; Moore, R. G.; Lee, H.-N.; Miao, H.; Johnson, P. D. Coexistence of Surface Ferromagnetism and a Gapless Topological State in MnBi2Te4. Phys. Rev. Lett. 2020, 125 (11), 117205,  DOI: 10.1103/PhysRevLett.125.117205
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      11
      Coexistence of Surface Ferromagnetism and a Gapless Topological State in MnBi2Te4
      Nevola, D.; Li, H. X.; Yan, J.-Q.; Moore, R. G.; Lee, H.-N.; Miao, H.; Johnson, P. D.
      Physical Review Letters (2020), 125 (11), 117205CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
      Surface magnetism and its correlation with the electronic structure are crit. to understanding the topol. surface state in the intrinsic magnetic topol. insulator MnBi2Te4. Here, using static and time resolved angle-resolved photoemission spectroscopy (ARPES), we find a significant ARPES intensity change together with a gap opening on a Rashba-like conduction band. Comparison with a model simulation strongly indicates that the surface magnetism on cleaved MnBi2Te4 is the same as its bulk state. The inability of surface ferromagnetism to open a gap in the topol. surface state uncovers the novel complexity of MnBi2Te4 that may be responsible for the low quantum anomalous Hall temp. of exfoliated MnBi2Te4.
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    12. 12
      Chen, B.; Fei, F.; Zhang, D.; Zhang, B.; Liu, W.; Zhang, S.; Wang, P.; Wei, B.; Zhang, Y.; Zuo, Z.; Guo, J.; Liu, Q.; Wang, Z.; Wu, X.; Zong, J.; Xie, X.; Chen, W.; Sun, Z.; Wang, S.; Zhang, Y.; Zhang, M.; Wang, X.; Song, F.; Zhang, H.; Shen, D.; Wang, B. Intrinsic Magnetic Topological Insulator Phases in the Sb Doped MnBi2Te4 Bulks and Thin Flakes. Nat. Commun. 2019, 10, 4469,  DOI: 10.1038/s41467-019-12485-y
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      12
      Intrinsic magnetic topological insulator phases in the Sb doped MnBi2Te4 bulks and thin flakes
      Chen Bo; Fei Fucong; Zhang Dongqin; Zhang Shuai; Wei Boyuan; Zhang Yong; Zuo Zewen; Guo Jingwen; Liu Qianqian; Zong Junyu; Xie Xuedong; Chen Wang; Zhang Yi; Zhang Minhao; Song Fengqi; Zhang Haijun; Wang Baigeng; Chen Bo; Fei Fucong; Zhang Shuai; Wei Boyuan; Zhang Yong; Zuo Zewen; Guo Jingwen; Liu Qianqian; Zhang Minhao; Wang Xuefeng; Song Fengqi; Zhang Bo; Wang Pengdong; Sun Zhe; Liu Wanling; Liu Wanling; Shen Dawei; Liu Wanling; Shen Dawei; Wang Zilu; Wu Xuchuan; Wang Shancai; Wang Xuefeng
      Nature communications (2019), 10 (1), 4469 ISSN:.
      Magnetic topological insulators (MTIs) offer a combination of topologically nontrivial characteristics and magnetic order and show promise in terms of potentially interesting physical phenomena such as the quantum anomalous Hall (QAH) effect and topological axion insulating states. However, the understanding of their properties and potential applications have been limited due to a lack of suitable candidates for MTIs. Here, we grow two-dimensional single crystals of Mn(SbxBi(1-x))2Te4 bulk and exfoliate them into thin flakes in order to search for intrinsic MTIs. We perform angle-resolved photoemission spectroscopy, low-temperature transport measurements, and first-principles calculations to investigate the band structure, transport properties, and magnetism of this family of materials, as well as the evolution of their topological properties. We find that there exists an optimized MTI zone in the Mn(SbxBi(1-x))2Te4 phase diagram, which could possibly host a high-temperature QAH phase, offering a promising avenue for new device applications.
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    13. 13
      Otrokov, M. M.; Rusinov, I. P.; Blanco-Rey, M.; Hoffmann, M.; Vyazovskaya, A. Yu.; Eremeev, S. V.; Ernst, A.; Echenique, P. M.; Arnau, A.; Chulkov, E. V. Unique Thickness-Dependent Properties of the van Der Waals Interlayer Antiferromagnet MnBi2Te4 Films. Phys. Rev. Lett. 2019, 122 (10), 107202,  DOI: 10.1103/PhysRevLett.122.107202
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      13
      Unique Thickness-Dependent Properties of the van der Waals Interlayer Antiferromagnet MnBi2Te4 Films
      Otrokov, M. M.; Rusinov, I. P.; Blanco-Rey, M.; Hoffmann, M.; Vyazovskaya, A. Yu.; Eremeev, S. V.; Ernst, A.; Echenique, P. M.; Arnau, A.; Chulkov, E. V.
      Physical Review Letters (2019), 122 (10), 107202CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
      Using d. functional theory and Monte Carlo calcns., we study the thickness dependence of the magnetic and electronic properties of a van der Waals interlayer antiferromagnet in the two-dimensional limit. Considering MnBi2Te4 as a model material, we find it to demonstrate a remarkable set of thickness-dependent magnetic and topol. transitions. While a single septuple layer block of MnBi2Te4 is a topol. trivial ferromagnet, the thicker films made of an odd (even) no. of blocks are uncompensated (compensated) interlayer antiferromagnets, which show wide band gap quantum anomalous Hall (zero plateau quantum anomalous Hall) states. Thus, MnBi2Te4 is the first stoichiometric material predicted to realize the zero plateau quantum anomalous Hall state intrinsically. This state has been theor. shown to host the exotic axion insulator phase.
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    14. 14
      Deng, Y.; Yu, Y.; Shi, M. Z.; Guo, Z.; Xu, Z.; Wang, J.; Chen, X. H.; Zhang, Y. Quantum Anomalous Hall Effect in Intrinsic Magnetic Topological Insulator MnBi2Te4. Science 2020, 367 (6480), 895– 900,  DOI: 10.1126/science.aax8156
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      14
      Quantum anomalous Hall effect in intrinsic magnetic topological insulator MnBi2Te4
      Deng, Yujun; Yu, Yijun; Shi, Meng Zhu; Guo, Zhongxun; Xu, Zihan; Wang, Jing; Chen, Xian Hui; Zhang, Yuanbo
      Science (Washington, DC, United States) (2020), 367 (6480), 895-900CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      In a magnetic topol. insulator, nontrivial band topol. combines with magnetic order to produce exotic states of matter, such as quantum anomalous Hall (QAH) insulators and axion insulators. In this work, the authors probe quantum transport in MnBi2Te4 thin flakes-a topol. insulator with intrinsic magnetic order. In this layered van der Waals crystal, the ferromagnetic layers couple antiparallel to each other; atomically thin MnBi2Te4, however, becomes ferromagnetic when the sample has an odd no. of septuple layers. The authors observe a zero-field QAH effect in a five-septuple-layer specimen at 1.4 K, and an external magnetic field further raises the quantization temp. to 6.5 K by aligning all layers ferromagnetically. The results establish MnBi2Te4 as an ideal arena for further exploring various topol. phenomena with a spontaneously broken time-reversal symmetry.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXjs1Kntbs%253D&md5=0526852e5e682cfd4d4d20821e907e27
    15. 15
      Estyunin, D. A.; Klimovskikh, I. I.; Shikin, A. M.; Schwier, E. F.; Otrokov, M. M.; Kimura, A.; Kumar, S.; Filnov, S. O.; Aliev, Z. S.; Babanly, M. B.; Chulkov, E. V. Signatures of Temperature Driven Antiferromagnetic Transition in the Electronic Structure of Topological Insulator MnBi2Te4. APL Mater. 2020, 8 (2), 021105,  DOI: 10.1063/1.5142846
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      15
      Signatures of temperature driven antiferromagnetic transition in the electronic structure of topological insulator MnBi2Te4
      Estyunin, D. A.; Klimovskikh, I. I.; Shikin, A. M.; Schwier, E. F.; Otrokov, M. M.; Kimura, A.; Kumar, S.; Filnov, S. O.; Aliev, Z. S.; Babanly, M. B.; Chulkov, E. V.
      APL Materials (2020), 8 (2), 021105CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      In this work, we employed angle resolved photoemission spectroscopy (ARPES) to analyze the temp. dependent changes in the electronic structure of the first antiferromagnetic topol. insulator MnBi2Te4 upon crossing the Ne´el temp. TN ≈25 K. We obsd. an exchange splitting of the bulk conduction band, which has a power law dependence on temp. (1 - T/T0)2β with an onset temp. T0 well matching the measured bulk TN. We found a matching temp. evolution of the topol. surface states integrated spectral wt. in the vicinity of the Dirac point. Furthermore, we obsd. an addnl. quasi-2D state with Rashba-type splitting, which is also affected by the emerged magnetism and exhibits an opening of a gap, reminiscent of the effect of an out-of-plane magnetic field, below TN. All these findings point toward strong evidence of the interplay between emerged magnetism with bulk and topol. surface states. The obsd. temp.-dependent effects in MnBi2Te4 may be used as an exptl. fingerprint for the presence of magnetism and may guide the future anal. of ARPES spectra in magnetic topol. insulators. (c) 2020 American Institute of Physics.
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    16. 16
      Yan, C.; Fernandez-Mulligan, S.; Mei, R.; Lee, S. H.; Protic, N.; Fukumori, R.; Yan, B.; Liu, C.; Mao, Z.; Yang, S. Origins of Electronic Bands in the Antiferromagnetic Topological Insulator MnBi2Te4. Phys. Rev. B 2021, 104 (4), L041102,  DOI: 10.1103/PhysRevB.104.L041102
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      16
      Origins of electronic bands in the antiferromagnetic topological insulator MnBi2Te4
      Yan, Chenhui; Fernandez-Mulligan, Sebastian; Mei, Ruobing; Lee, Seng Huat; Protic, Nikola; Fukumori, Rikuto; Yan, Binghai; Liu, Chaoxing; Mao, Zhiqiang; Yang, Shuolong
      Physical Review B (2021), 104 (4), L041102CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Despite the rapid progress in understanding the first intrinsic magnetic topol. insulator MnBi2Te4, its electronic structure remains a topic under debates. Here we perform a thorough spectroscopic investigation into the electronic structure of MnBi2Te4 via laser-based angle-resolved photoemission spectroscopy. Through quant. anal., we est. an upper bound of 3 meV for the gap size of the topol. surface state. Furthermore, our CD measurements reveal band chiralities for both the topol. surface state and quasi-2D bands, which can be well reproduced in a band hybridization model. A numerical simulation of energy-momentum dispersions based on a four-band model with an addnl. step potential near the surface provides a promising explanation for the origin of the quasi-2D bands. Our study represents a solid step forward in reconciling the existing controversies in the electronic structure of MnBi2Te4, and provides an important framework to understand the electronic structures of other relevant topol. materials MnBi2nTe3n+1.
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    17. 17
      Hu, C.; Gordon, K. N.; Liu, P.; Liu, J.; Zhou, X.; Hao, P.; Narayan, D.; Emmanouilidou, E.; Sun, H.; Liu, Y.; Brawer, H.; Ramirez, A. P.; Ding, L.; Cao, H.; Liu, Q.; Dessau, D.; Ni, N. A van Der Waals Antiferromagnetic Topological Insulator with Weak Interlayer Magnetic Coupling. Nat. Commun. 2020, 11, 97,  DOI: 10.1038/s41467-019-13814-x
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      17
      A van der Waals antiferromagnetic topological insulator with weak interlayer magnetic coupling
      Hu, Chaowei; Gordon, Kyle N.; Liu, Pengfei; Liu, Jinyu; Zhou, Xiaoqing; Hao, Peipei; Narayan, Dushyant; Emmanouilidou, Eve; Sun, Hongyi; Liu, Yuntian; Brawer, Harlan; Ramirez, Arthur P.; Ding, Lei; Cao, Huibo; Liu, Qihang; Dessau, Dan; Ni, Ni
      Nature Communications (2020), 11 (1), 97CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Magnetic topol. insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential tech. revolutions. By realizing a bulk van der Waals material MnBi4Te7 with alternating septuple [MnBi2Te4] and quintuple [Bi2Te3] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane satn. field of ∼0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calcns. further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states assocd. with the [MnBi2Te4] or [Bi2Te3] termination, resp. Addnl., its superlattice nature may make various heterostructures of [MnBi2Te4] and [Bi2Te3] layers possible by exfoliation. Therefore, the low satn. field and the superlattice nature of MnBi4Te7 make it an ideal system to investigate rich emergent phenomena.
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    18. 18
      Wu, J.; Liu, F.; Sasase, M.; Ienaga, K.; Obata, Y.; Yukawa, R.; Horiba, K.; Kumigashira, H.; Okuma, S.; Inoshita, T.; Hosono, H. Natural van Der Waals Heterostructural Single Crystals with Both Magnetic and Topological Properties. Sci. Adv. 2019, 5 (11), eaax9989  DOI: 10.1126/sciadv.aax9989
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      There is no corresponding record for this reference.
    19. 19
      Vidal, R. C.; Zeugner, A.; Facio, J. I.; Ray, R.; Haghighi, M. H.; Wolter, A. U. B.; Corredor Bohorquez, L. T.; Caglieris, F.; Moser, S.; Figgemeier, T.; Peixoto, T. R. F.; Vasili, H. B.; Valvidares, M.; Jung, S.; Cacho, C.; Alfonsov, A.; Mehlawat, K.; Kataev, V.; Hess, C.; Richter, M.; Büchner, B.; van den Brink, J.; Ruck, M.; Reinert, F.; Bentmann, H.; Isaeva, A. Topological Electronic Structure and Intrinsic Magnetization in MnBi4Te7: A Bi2Te3 Derivative with a Periodic Mn Sublattice. Phys. Rev. X 2019, 9 (4), 041065,  DOI: 10.1103/PhysRevX.9.041065
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      19
      Topological Electronic Structure and Intrinsic Magnetization in MnBi4Te7: A Bi2Te3 Derivative with a Periodic Mn Sublattice
      Vidal, Raphael C.; Zeugner, Alexander; Facio, Jorge I.; Ray, Rajyavardhan; Haghighi, M. Hossein; Wolter, Anja U. B.; Corredor Bohorquez, Laura T.; Caglieris, Federico; Moser, Simon; Figgemeier, Tim; Peixoto, Thiago R. F.; Vasili, Hari Babu; Valvidares, Manuel; Jung, Sungwon; Cacho, Cephise; Alfonsov, Alexey; Mehlawat, Kavita; Kataev, Vladislav; Hess, Christian; Richter, Manuel; Buechner, Bernd; van den Brink, Jeroen; Ruck, Michael; Reinert, Friedrich; Bentmann, Hendrik; Isaeva, Anna
      Physical Review X (2019), 9 (4), 041065CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
      Combinations of nontrivial band topol. and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topol. magnetoelec. effect. Following theor. advances, material candidates are emerging. Yet, so far a compd. that combines a band-inverted electronic structure with an intrinsic net magnetization remains unrealized. MnBi2Te4 has been established as the first antiferromagnetic topol. insulator and constitutes the progenitor of a modular (Bi2Te3)n(MnBi2Te4) series. Here, for n=1, we confirm a nonstoichiometric compn. proximate to MnBi4Te7. We establish an antiferromagnetic state below 13 K followed by a state with a net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission expts. and d.-functional calcns. reveal a topol. nontrivial surface state on the MnBi4Te7(0001) surface, analogous to the nonmagnetic parent compd. Bi2Te3. Our results establish MnBi4Te7 as the first band-inverted compd. with intrinsic net magnetization providing a versatile platform for the realization of magnetic topol. states of matter.
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    20. 20
      Shi, M. Z.; Lei, B.; Zhu, C. S.; Ma, D. H.; Cui, J. H.; Sun, Z. L.; Ying, J. J.; Chen, X. H. Magnetic and Transport Properties in the Magnetic Topological Insulators MnB2Te4(Bi2Te3)n (N = 1,2). Phys. Rev. B 2019, 100 (15), 155144,  DOI: 10.1103/PhysRevB.100.155144
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      20
      Magnetic and transport properties in the magnetic topological insulators MnBi2Te4(Bi2Te3)n (n=1,2)
      Shi, M. Z.; Lei, B.; Zhu, C. S.; Ma, D. H.; Cui, J. H.; Sun, Z. L.; Ying, J. J.; Chen, X. H.
      Physical Review B (2019), 100 (15), 155144CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      The observation of quantized anomalous Hall conductance in the forced ferromagnetic state of MnBi2Te4 thin flakes has attracted much attention. However, a strong magnetic field is needed to fully polarize the magnetic moments due to the large antiferromagnetic interlayer exchange coupling. Here, we reported the magnetic and elec. transport properties of the magnetic van der Waals MnBi2Te4(Bi2Te3)n (n=1,2) single crystals, in which the interlayer antiferromagnetic exchange coupling is greatly suppressed with the increase of the sepn. layers Bi2Te3. MnBi4Te7 and MnBi6Te10 show weak antiferromagnetic transition at 12.3 and 10.5 K, resp. The ferromagnetic hysteresis was obsd. at low temp. for both of the crystals, which is quite crucial for realizing the quantum anomalous Hall effect without external magnetic field. Our work indicates that MnBi2Te4(Bi2Te3)n (n=1,2) provides an ideal platform to investigate the rich topol. phases with their two-dimensional limits.
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    21. 21
      Tian, S.; Gao, S.; Nie, S.; Qian, Y.; Gong, C.; Fu, Y.; Li, H.; Fan, W.; Zhang, P.; Kondo, T.; Shin, S.; Adell, J.; Fedderwitz, H.; Ding, H.; Wang, Z.; Qian, T.; Lei, H. Magnetic Topological Insulator MnBi6Te10with a Zero-Field Ferromagnetic State and Gapped Dirac Surface States. Phys. Rev. B 2020, 102 (3), 035144,  DOI: 10.1103/PhysRevB.102.035144
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      21
      Magnetic topological insulator MnBi6Te10 with a zero-field ferromagnetic state and gapped Dirac surface states
      Tian, Shangjie; Gao, Shunye; Nie, Simin; Qian, Yuting; Gong, Chunsheng; Fu, Yang; Li, Hang; Fan, Wenhui; Zhang, Peng; Kondo, Takesh; Shin, Shik; Adell, Johan; Fedderwitz, Hanna; Ding, Hong; Wang, Zhijun; Qian, Tian; Lei, Hechang
      Physical Review B (2020), 102 (3), 035144CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Magnetic topol. insulators (TIs) with nontrivial topol. electronic structure and broken time-reversal symmetry exhibit various exotic topol. quantum phenomena. The realization of such exotic phenomena at high temp. is one of the central topics in this area. We reveal that MnBi6Te10 is a magnetic TI with an antiferromagnetic ground state below 10.8 K whose nontrivial topol. is manifested by Dirac-like surface states. The ferromagnetic axion insulator state with Z4=2 emerges once spins are polarized at a field as low as 0.1 T, accompanied with satd. anomalous Hall resistivity up to 10 K. Such a ferromagnetic state is preserved even with an external field down to zero at 2 K. Theor. calcns. indicate that the few-layer ferromagnetic MnBi6Te10 is also topol. nontrivial with a nonzero Chern no. Angle-resolved photoemission spectroscopy expts. further reveal three types of Dirac surface states arising from different terminations on the cleavage surfaces, one of which has insulating behavior with an energy gap of ~ 28 meV at the Dirac point. These outstanding features suggest that MnBi6Te10 is a promising system to realize various topol. quantum effects at zero field and high temp.
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    22. 22
      Hu, Y.; Xu, L.; Shi, M.; Luo, A.; Peng, S.; Wang, Z. Y.; Ying, J. J.; Wu, T.; Liu, Z. K.; Zhang, C. F.; Chen, Y. L.; Xu, G.; Chen, X.-H.; He, J.-F. Universal Gapless Dirac Cone and Tunable Topological States in (MnBi2Te4)m(Bi2Te3)n Heterostructures. Phys. Rev. B 2020, 101 (16), 161113,  DOI: 10.1103/PhysRevB.101.161113
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      22
      Universal gapless Dirac cone and tunable topological states in (MnBi2Te4)m(Bi2Te3)n heterostructures
      Hu, Yong; Xu, Lixuan; Shi, Mengzhu; Luo, Aiyun; Peng, Shuting; Wang, Z. Y.; Ying, J. J.; Wu, T.; Liu, Z. K.; Zhang, C. F.; Chen, Y. L.; Xu, G.; Chen, X.-H.; He, J.-F.
      Physical Review B (2020), 101 (16), 161113CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      A review The newly discovered magnetic topol. insulators (MnBi2Te4)m(Bi2Te3)n are predicted to be a versatile platform for exploring novel topol. states. Here, we report angle-resolved photoemission spectroscopy studies on a series of (MnBi2Te4)m(Bi2Te3)n heterostructures. An unexpected but universal gapless Dirac cone is obsd. on the (MnBi2Te4) terminated (0001) surfaces in all systems, indicating an altered magnetic structure near the surface. The specific band dispersion of the surface states, presumably dominated by the top surface, is found to be sensitive to different stackings of the underlying MnBi2Te4 and Bi2Te3 layers. Our results suggest the high tunability of both magnetic and electronic structures of the topol. surface states in (MnBi2Te4)m(Bi2Te3)n heterostructures, which is essential in realizing and manipulating various topol. states.
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    23. 23
      Klimovskikh, I. I.; Otrokov, M. M.; Estyunin, D.; Eremeev, S. V.; Filnov, S. O.; Koroleva, A.; Shevchenko, E.; Voroshnin, V.; Rybkin, A. G.; Rusinov, I. P.; Blanco-Rey, M.; Hoffmann, M.; Aliev, Z. S.; Babanly, M. B.; Amiraslanov, I. R.; Abdullayev, N. A.; Zverev, V. N.; Kimura, A.; Tereshchenko, O. E.; Kokh, K. A.; Petaccia, L.; Di Santo, G.; Ernst, A.; Echenique, P. M.; Mamedov, N. T.; Shikin, A. M.; Chulkov, E. V. Tunable 3D/2D Magnetism in the (MnBi2Te4)(Bi2Te3)m Topological Insulators Family. Npj Quantum Mater. 2020, 5 (1), 54,  DOI: 10.1038/s41535-020-00255-9
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      23
      Tunable 3D/2D magnetism in the (MnBi2Te4)(Bi2Te3)m topological insulators family
      Klimovskikh, Ilya I.; Otrokov, Mikhail M.; Estyunin, Dmitry; Eremeev, Sergey V.; Filnov, Sergey O.; Koroleva, Alexandra; Shevchenko, Eugene; Voroshnin, Vladimir; Rybkin, Artem G.; Rusinov, Igor P.; Blanco-Rey, Maria; Hoffmann, Martin; Aliev, Ziya S.; Babanly, Mahammad B.; Amiraslanov, Imamaddin R.; Abdullayev, Nadir A.; Zverev, Vladimir N.; Kimura, Akio; Tereshchenko, Oleg E.; Kokh, Konstantin A.; Petaccia, Luca; Di Santo, Giovanni; Ernst, Arthur; Echenique, Pedro M.; Mamedov, Nazim T.; Shikin, Alexander M.; Chulkov, Eugene V.
      npj Quantum Materials (2020), 5 (1), 54CODEN: QMUADP; ISSN:2397-4648. (Nature Research)
      Feasibility of many emergent phenomena that intrinsic magnetic topol. insulators (TIs) may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic structures. Here we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compds. (MnBi2Te4)(Bi2Te3)m with m = 0, ···, 6. Magnetic, electronic and, consequently, topol. properties of these materials depend strongly on the m value and are thus highly tunable. The antiferromagnetic (AFM) coupling between the neighboring Mn layers strongly weakens on moving from MnBi2Te4 (m = 0) to MnBi4Te7 (m = 1) and MnBi6Te10 (m = 2). Further increase in m leads to change of the overall magnetic behavior to ferromagnetic (FM) one for (m = 3), while the interlayer coupling almost disappears. In this way, the AFM and FM TI states are, resp., realized in the m = 0, 1, 2 and m = 3 cases. For large m nos. a hitherto-unknown topol. nontrivial phase can be created, in which below the corresponding crit. temp. the magnetizations of the non-interacting 2D ferromagnets, formed by the MnBi2Te4 building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi2Te4)(Bi2Te3)m allows efficient engineering of functional van der Waals heterostructures for topol. quantum computation, as well as antiferromagnetic and 2D spintronics.
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    24. 24
      Lu, R.; Sun, H.; Kumar, S.; Wang, Y.; Gu, M.; Zeng, M.; Hao, Y.-J.; Li, J.; Shao, J.; Ma, X.-M.; Hao, Z.; Zhang, K.; Mansuer, W.; Mei, J.; Zhao, Y.; Liu, C.; Deng, K.; Huang, W.; Shen, B.; Shimada, K.; Schwier, E. F.; Liu, C.; Liu, Q.; Chen, C. Half-Magnetic Topological Insulator with Magnetization-Induced Dirac Gap at a Selected Surface. Phys. Rev. X 2021, 11 (1), 011039,  DOI: 10.1103/PhysRevX.11.011039
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      24
      Half-Magnetic Topological Insulator with Magnetization-Induced Dirac Gap at a Selected Surface
      Lu, Ruie; Sun, Hongyi; Kumar, Shiv; Wang, Yuan; Gu, Mingqiang; Zeng, Meng; Hao, Yu-Jie; Li, Jiayu; Shao, Jifeng; Ma, Xiao-Ming; Hao, Zhanyang; Zhang, Ke; Mansuer, Wumiti; Mei, Jiawei; Zhao, Yue; Liu, Cai; Deng, Ke; Huang, Wen; Shen, Bing; Shimada, Kenya; Schwier, Eike F.; Liu, Chang; Liu, Qihang; Chen, Chaoyu
      Physical Review X (2021), 11 (1), 011039CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
      Topol. magnets are a new family of quantum materials providing great potential to realize emergent phenomena, such as the quantum anomalous Hall effect and the axion-insulator state. Here, we present our discovery that the stoichiometric ferromagnet MnBi8Te13 with natural heterostructure MnBi2Te4/(Bi2Te3)3 is an unprecedented "half-magnetic topol. insulator," with the magnetization existing at the MnBi2Te4 surface but not at the opposite surface terminated by triple Bi2Te3 layers. Our angle-resolved photoemission spectroscopy measurements unveil a massive Dirac gap at the MnBi2Te4 surface and a gapless Dirac cone on the other side. Remarkably, the Dirac gap (about 28 meV) at the MnBi2Te4 surface decreases monotonically with increasing temp. and closes right at the Curie temp., thereby representing the first smoking-gun spectroscopic evidence of a magnetization-induced topol. surface gap among all known magnetic topol. materials. We further demonstrate theor. that the half-magnetic topol. insulator is desirable to realize the surface anomalous Hall effect, which serves as direct proof of the general concept of axion electrodynamics in condensed matter systems.
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    25. 25
      Hu, C.; Ding, L.; Gordon, K. N.; Ghosh, B.; Tien, H.-J.; Li, H.; Linn, A. G.; Lien, S.-W.; Huang, C.-Y.; Mackey, S.; Liu, J.; Reddy, P. V. S.; Singh, B.; Agarwal, A.; Bansil, A.; Song, M.; Li, D.; Xu, S.-Y.; Lin, H.; Cao, H.; Chang, T.-R.; Dessau, D.; Ni, N. Realization of an Intrinsic Ferromagnetic Topological State in MnBi8Te13. Sci. Adv. 2020, 6 (30), eaba4275  DOI: 10.1126/sciadv.aba4275
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      There is no corresponding record for this reference.
    26. 26
      Souchay, D.; Nentwig, M.; Günther, D.; Keilholz, S.; de Boor, J.; Zeugner, A.; Isaeva, A.; Ruck, M.; Wolter, A. U.; Büchner, B.; Oeckler, O. Layered Manganese Bismuth Tellurides with GeBi4Te7 and GeBi6Te10 Type Structures: Towards Multifunctional Materials. J. Mater. Chem. C 2019, 7 (32), 9939– 9953,  DOI: 10.1039/C9TC00979E
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      26
      Layered manganese bismuth tellurides with GeBi4Te7- and GeBi6Te10-type structures: towards multifunctional materials
      Souchay, Daniel; Nentwig, Markus; Guenther, Daniel; Keilholz, Simon; de Boor, Johannes; Zeugner, Alexander; Isaeva, Anna; Ruck, Michael; Wolter, Anja U. B.; Buechner, Bernd; Oeckler, Oliver
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2019), 7 (32), 9939-9953CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      The crystal structures of new layered manganese bismuth tellurides with the compns. Mn0.85(3)Bi4.10(2)Te7 and Mn0.73(4)Bi6.18(2)Te10 were detd. by single-crystal X-ray diffraction, including the use of microfocused synchrotron radiation. These analyses reveal that the layered structures deviate from the idealized stoichiometry of the 12P-GeBi4Te7 (space group P‾3m1) and 51R-GeBi6Te10 (space group R‾3m) structure types they adopt. Modified compns. Mn1-xBi4+2x/3Te7 (x = 0.15-0.2) and Mn1-xBi6+2x/3Te10 (x = 0.19-0.26) assume cation vacancies and lead to homogenous bulk samples as confirmed by Rietveld refinements. Electron diffraction patterns exhibit no diffuse streaks that would indicate stacking disorder. The alternating quintuple-layer [M2Te3] and septuple-layer [M3Te4] slabs (M = mixed occupied by Bi and Mn) with 1:1 sequence (12P stacking) in Mn0.85Bi4.10Te7 and 2:1 sequence (51R stacking) in Mn0.81Bi6.13Te10 were also obsd. in HRTEM images. Temp.-dependent powder diffraction and differential scanning calorimetry show that the compds. are high-temp. phases, which are metastable at ambient temp. Magnetization measurements are in accordance with a MnII oxidn. state and point at predominantly ferromagnetic coupling in both compds. The thermoelec. figures of merit of n-type conducting Mn0.85Bi4.10Te7 and Mn0.81Bi6.13Te10 reach zT = 0.25 at 375° and zT = 0.28 at 325°, resp. Although the compds. are metastable, compact ingots exhibit still ≤80% of the main phases after thermoelec. measurements ≤400°.
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    27. 27
      Shao, J.; Liu, Y.; Zeng, M.; Li, J.; Wu, X.; Ma, X.-M.; Jin, F.; Lu, R.; Sun, Y.; Gu, M.; Wang, K.; Wu, W.; Wu, L.; Liu, C.; Liu, Q.; Zhao, Y. Pressure-Tuned Intralayer Exchange in Superlattice-Like MnBi2Te4/(Bi2Te3)n Topological Insulators. Nano Lett. 2021, 21 (13), 5874– 5880,  DOI: 10.1021/acs.nanolett.1c01874
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      27
      Pressure-Tuned Intralayer Exchange in Superlattice-Like MnBi2Te4/(Bi2Te3)n Topological Insulators
      Shao, Jifeng; Liu, Yuntian; Zeng, Meng; Li, Jingyuan; Wu, Xuefeng; Ma, Xiao-Ming; Jin, Feng; Lu, Ruie; Sun, Yichen; Gu, Mingqiang; Wang, Kedong; Wu, Wenbin; Wu, Liusuo; Liu, Chang; Liu, Qihang; Zhao, Yue
      Nano Letters (2021), 21 (13), 5874-5880CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The magnetic structures of MnBi2Te4(Bi2Te3)n can be manipulated by tuning the interlayer coupling via the no. of Bi2Te3 spacer layers n, while the intralayer ferromagnetic (FM) exchange coupling is considered too robust to control. By applying hydrostatic pressure up to 3.5 GPa, we discover opposite responses of magnetic properties for n = 1 and 2. MnBi4Te7 stays at A-type antiferromagnetic (AFM) phase with a decreasing Neel temp. and an increasing satn. field. In sharp contrast, MnBi6Te10 experiences a phase transition from A-type AFM to a quasi-two-dimensional FM state with a suppressed satn. field under pressure. First-principles calcns. reveal the essential role of intralayer exchange coupling from lattice compression in detg. these magnetic properties. Such magnetic phase transition is also obsd. in 20% Sb-doped MnBi6Te10 because of the in-plane lattice compression.
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    28. 28
      Liu, Y.; Wang, L.-L.; Zheng, Q.; Huang, Z.; Wang, X.; Chi, M.; Wu, Y.; Chakoumakos, B. C.; McGuire, M. A.; Sales, B. C.; Wu, W.; Yan, J. Site Mixing for Engineering Magnetic Topological Insulators. Phys. Rev. X 2021, 11 (2), 021033,  DOI: 10.1103/PhysRevX.11.021033
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      28
      Site Mixing for Engineering Magnetic Topological Insulators
      Liu, Yaohua; Wang, Lin-Lin; Zheng, Qiang; Huang, Zengle; Wang, Xiaoping; Chi, Miaofang; Wu, Yan; Chakoumakos, Bryan C.; McGuire, Michael A.; Sales, Brian C.; Wu, Weida; Yan, Jiaqiang
      Physical Review X (2021), 11 (2), 021033CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
      The van der Waals compd., MnBi2Te4, is the first intrinsic magnetic topol. insulator, providing a materials platform for exploring exotic quantum phenomena such as the axion insulator state and the quantum anomalous Hall effect. However, intrinsic structural imperfections lead to bulk cond., and the roles of magnetic defects are still unknown. With higher concns. of the same types of magnetic defects, the isostructural compd. MnSb2Te4 is a better model system for a systematic investigation of the connections among magnetism, topol., and lattice defects. In this work, the impact of antisite defects on the magnetism and electronic structure is studied in MnSb2Te4. Mn-Sb site mixing leads to complex magnetic structures and tunes the interlayer magnetic coupling between antiferromagnetic and ferromagnetic. The detailed nonstoichiometry and site mixing of MnSb2Te4 crystals depend on the growth parameters, which can lead to ≈40% of Mn sites occupied by Sb and ≈15% of Sb sites by Mn in as-grown crystals. Single-crystal neutron diffraction and electron microscopy studies show nearly random distribution of the antisite defects. Band structure calcns. suggest that the Mn-Sb site mixing favors a ferromagnetic interlayer coupling, consistent with exptl. observation, but is detrimental to the band inversion required for a nontrivial topol. Our results suggest a long-range magnetic order of Mn ions sitting on Bi sites in MnBi2Te4. The effects of site mixing should be considered in all layered heterostructures that consist of alternating magnetic and topol. layers, including the entire family of MnTe(Bi2Te3)n, its Sb analogs, and their solid soln.
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    29. 29
      Murakami, T.; Nambu, Y.; Koretsune, T.; Xiangyu, G.; Yamamoto, T.; Brown, C. M.; Kageyama, H. Realization of Interlayer Ferromagnetic Interaction in MnSb2Te4 toward the Magnetic Weyl Semimetal State. Phys. Rev. B 2019, 100 (19), 195103,  DOI: 10.1103/PhysRevB.100.195103
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      29
      Realization of interlayer ferromagnetic interaction in MnSb2Te4 toward the magnetic Weyl semimetal state
      Murakami, Taito; Nambu, Yusuke; Koretsune, Takashi; Gu, Xiangyu; Yamamoto, Takafumi; Brown, Craig M.; Kageyama, Hiroshi
      Physical Review B (2019), 100 (19), 195103CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Magnetic properties of MnSb2Te4 were examd. through magnetic susceptibility, specific-heat, and neutron-diffraction measurements. As opposed to isostructural MnBi2Te4 with the antiferromagnetic ground state, MnSb2Te4 develops a spontaneous magnetization below 25 K. From our first-principles calcns. on the material in a ferromagnetic state, the state could be interpreted as a type-II Weyl semimetal state with broken time-reversal symmetry. Detailed structural refinements using x-ray-diffraction and neutron-diffraction data reveal the presence of site mixing between Mn and Sb sites, leading to the ferrimagnetic ground state. With theor. calcns., we found that the presence of site mixing plays an important role for the interlayer Mn-Mn ferromagnetic interactions.
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    30. 30
      Hou, F.; Yao, Q.; Zhou, C.-S.; Ma, X.-M.; Han, M.; Hao, Y.-J.; Wu, X.; Zhang, Y.; Sun, H.; Liu, C.; Zhao, Y.; Liu, Q.; Lin, J. Te-Vacancy-Induced Surface Collapse and Reconstruction in Antiferromagnetic Topological Insulator MnBi2Te4. ACS Nano 2020, 14 (9), 11262– 11272,  DOI: 10.1021/acsnano.0c03149
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      30
      Te-Vacancy-Induced Surface Collapse and Reconstruction in Antiferromagnetic Topological Insulator MnBi2Te4
      Hou, Fuchen; Yao, Qiushi; Zhou, Chun-Sheng; Ma, Xiao-Ming; Han, Mengjiao; Hao, Yu-Jie; Wu, Xuefeng; Zhang, Yu; Sun, Hongyi; Liu, Chang; Zhao, Yue; Liu, Qihang; Lin, Junhao
      ACS Nano (2020), 14 (9), 11262-11272CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      MnBi2Te4 is an antiferromagnetic topol. insulator that has stimulated intense interest due to its exotic quantum phenomena and promising device applications. The surface structure is a determinant factor to understand the magnetic and topol. behavior of MnBi2Te4, yet its precise at. structure remains elusive. Here the authors discovered a surface collapse and reconstruction of few-layer MnBi2Te4 exfoliated under delicate protection. Instead of the ideal septuple-layer structure in the bulk, the collapsed surface is shown to reconstruct as a Mn-doped Bi2Te3 quintuple layer and a MnxBiyTe double layer with a clear van der Waals gap in between. Combined with 1st-principles calcns., such surface collapse is attributed to the abundant intrinsic Mn-Bi antisite defects and the Te vacancy in the exfoliated surface, which is further supported by in situ annealing and electron irradn. expts. Results shed light on the understanding of the intricate surface-bulk correspondence of MnBi2Te4 and provide an insightful perspective on the surface-related quantum measurements in MnBi2Te4 few-layer devices.
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    31. 31
      Huang, Z.; Du, M.-H.; Yan, J.; Wu, W. Native Defects in Antiferromagnetic Topological Insulator MnBi2Te4. Phys. Rev. Mater. 2020, 4 (12), 121202,  DOI: 10.1103/PhysRevMaterials.4.121202
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      31
      Native defects in antiferromagnetic topological insulator MnBi2Te4
      Huang, Zengle; Du, Mao-Hua; Yan, Jiaqiang; Wu, Weida
      Physical Review Materials (2020), 4 (12), 121202CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
      Using scanning tunneling microscopy and spectroscopy, we visualized the native defects in antiferromagnetic topol. insulator MnBi2Te4. Two native defects, MnBi and BiTe antisites, can be well resolved in the topog. images. MnBi tend to suppress the d. of states at the conduction band edge. Spectroscopy imaging reveals a localized peaklike local d. of state at ∼80 meV below the Fermi energy. A careful inspection of topog. and spectroscopic images, combined with d. functional theory calcn., suggests this results from BiMn antisites at Mn sites. The random distribution of MnBi and BiMn antisites results in spatial fluctuation of local d. of states near the Fermi level in MnBi2Te4.
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    32. 32
      Lai, Y.; Ke, L.; Yan, J.; McDonald, R. D.; McQueeney, R. J. Defect-Driven Ferrimagnetism and Hidden Magnetization in MnBi2Te4. Phys. Rev. B 2021, 103 (18), 184429,  DOI: 10.1103/PhysRevB.103.184429
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      32
      Defect-driven ferrimagnetism and hidden magnetization in MnBi2Te4
      Lai, You; Ke, Liqin; Yan, Jiaqiang; McDonald, Ross D.; McQueeney, Robert J.
      Physical Review B (2021), 103 (18), 184429CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      MnBi2Te4 (MBT) materials are promising antiferromagnetic topol. insulators in which field-driven ferromagnetism is predicted to cause a transition between axion insulator and Weyl semimetallic states. However, the presence of antiferromagnetic coupling between Mn/Bi antisite defects and the main Mn layer can reduce the low-field magnetization, and it has been shown that such defects are more prevalent in the structurally identical magnetic insulator MnSb2Te4 (MST). We use high-field magnetization measurements to show that the magnetization of MBT and MST occur in stages and full satn. requires fields of ∼60 T. As a consequence, the low-field magnetization plateau state in MBT, where many detns. of the quantum anomalous Hall state are studied, actually consists of ferrimagnetic septuple blocks contg. both uniform and staggered magnetization components.
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    33. 33
      Du, M.-H.; Yan, J.; Cooper, V. R.; Eisenbach, M. Tuning Fermi Levels in Intrinsic Antiferromagnetic Topological Insulators MnBi2Te4 and MnBi4Te7 by Defect Engineering and Chemical Doping. Adv. Funct. Mater. 2021, 31 (3), 2006516,  DOI: 10.1002/adfm.202006516
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      33
      Tuning Fermi Levels in Intrinsic Antiferromagnetic Topological Insulators MnBi2Te4 and MnBi4Te7 by Defect Engineering and Chemical Doping
      Du, Mao-Hua; Yan, Jiaqiang; Cooper, Valentino R.; Eisenbach, Markus
      Advanced Functional Materials (2021), 31 (3), 2006516CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      MnBi2Te4 and MnBi4Te7 are intrinsic antiferromagnetic topol. insulators, offering a promising materials platform for realizing exotic topol. quantum states. However, high densities of intrinsic defects in these materials not only cause bulk metallic cond., preventing the measurement of quantum transport in surface states, but may also affect magnetism and topol. properties. In this paper, systematic d. functional theory calcns. reveal specific material chem. and growth conditions that det. the defect formation and dopant incorporation in MnBi2Te4 and MnBi4Te7. The large strain induced by the internal heterostructure promotes the formation of large-size-mismatched antisite defects and substitutional dopants. The results here show that the abundance of antisite defects is responsible for the obsd. n-type metallic cond. A Te-rich growth condition is predicted to reduce the bulk free electron d., which is confirmed by exptl. synthesis and transport measurements in MnBi2Te4. Furthermore, Na doping is proposed to be an effective acceptor dopant to pin the Fermi level within the bulk band gap to enable the observation of surface quantum transport. The defect engineering and doping strategies proposed here should stimulate further studies for improving synthesis and for manipulating magnetic and topol. properties in MnBi2Te4, MnBi4Te7, and related magnetic topol. insulators.
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    34. 34
      Hu, C.; Lien, S.-W.; Feng, E.; Mackey, S.; Tien, H.-J.; Mazin, I. I.; Cao, H.; Chang, T.-R.; Ni, N. Tuning Magnetism and Band Topology through Antisite Defects in Sb-Doped MnBi4Te7. Phys. Rev. B 2021, 104 (5), 054422,  DOI: 10.1103/PhysRevB.104.054422
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      34
      Tuning magnetism and band topology through antisite defects in Sb-doped MnBi4Te7
      Hu, Chaowei; Lien, Shang-Wei; Feng, Erxi; Mackey, Scott; Tien, Hung-Ju; Mazin, Igor I.; Cao, Huibo; Chang, Tay-Rong; Ni, Ni
      Physical Review B (2021), 104 (5), 054422CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      The fine control of magnetism and electronic structure in a magnetic topol. insulator is crucial in order to realize various novel magnetic topol. states including axion insulators, magnetic Weyl semimetals, Chern insulators, etc. Through crystal growth, transport, thermodn., neutron diffraction measurements, we show that under Sb doping the newly discovered intrinsic antiferromagnetic (AFM) topol. insulator MnBi4Te7 evolves from AFM to ferromagnetic (FM) and then ferrimagnetic. We attribute this to the formation of Mn(Bi,Sb) antisites upon doping, which results in addnl. Mn sublattices that modify the delicate interlayer magnetic interactions and cause the dominant Mn sublattice to go from AFM to FM. We further investigate the effect of antisites on the band topol. using the first-principles calcns. Without considering antisites, the series evolves from AFM topol. insulator (x = 0) to FM axion insulators. In the exaggerated case of 16.7% of periodic antisites, the band topol. is modified and a type-I magnetic Weyl semimetal phase can be realized at intermediate dopings. Therefore, this doping series provides a fruitful platform with continuously tunable magnetism and topol. for investigating emergent phenomena, including quantum anomalous Hall effect, Fermi arc states, etc.
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    35. 35
      Chen, L.; Wang, D.; Shi, C.; Jiang, C.; Liu, H.; Cui, G.; Zhang, X.; Li, X. Electronic Structure and Magnetism of MnSb2Te4. J. Mater. Sci. 2020, 55 (29), 14292– 14300,  DOI: 10.1007/s10853-020-05005-7
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      35
      Electronic structure and magnetism of MnSb2Te4
      Chen, Li; Wang, Dongchao; Shi, Changmin; Jiang, Chuan; Liu, Hongmei; Cui, Guangliang; Zhang, Xiaoming; Li, Xiaolong
      Journal of Materials Science (2020), 55 (29), 14292-14300CODEN: JMTSAS; ISSN:0022-2461. (Springer)
      MnSb2Te4 has the same crystal structure as MnBi2Te4. Whether it is an intrinsic antiferromagnetic TI, quantum anomalous Hall insulator or axion insulator like MnBi2Te4 [CHIN. PHYS. LETT. 36, (2019) 076801] has not been reported yet. The electronic structure and magnetism of MnSb2Te4 have been studied using first-principles calcns. The results show that the MnSb2Te4 is an antiferromagnetic semiconductor with a trivial energy gap (∼ 0.132 eV). The band gap decreases to 0.057 eV under the tensile strain (1.03a0, 1.03c0). The feature of Weyl semimetal could be presented in MnSb2Te4 with ferromagnetic phase under strain 3%. Thin films (011) are metals with antiferromagnetic order and also metals with ferromagnetic order. Thin film (111) with alternate of thick (1 septuple layer-7 septuple layers) is an intrinsic magnetic semiconductor with a trivial energy gap (0.002-0.344 eV) rather than an intrinsic quantum anomalous Hall insulator or axion insulator.
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    36. 36
      Wimmer, S.; Sánchez-Barriga, J.; Küppers, P.; Ney, A.; Schierle, E.; Freyse, F.; Caha, O.; Michalička, J.; Liebmann, M.; Primetzhofer, D.; Hoffman, M.; Ernst, A.; Otrokov, M. M.; Bihlmayer, G.; Weschke, E.; Lake, B.; Chulkov, E. V.; Morgenstern, M.; Bauer, G.; Springholz, G.; Rader, O. Mn-Rich MnSb2Te4: A Topological Insulator with Magnetic Gap Closing at High Curie Temperatures of 45–50 K. Adv. Mater. 2021, 33 (42), 2102935,  DOI: 10.1002/adma.202102935
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      36
      Mn-Rich MnSb2Te4: A Topological Insulator with Magnetic Gap Closing at High Curie Temperatures of 45-50 K
      Wimmer, Stefan; Sanchez-Barriga, Jaime; Kueppers, Philipp; Ney, Andreas; Schierle, Enrico; Freyse, Friedrich; Caha, Ondrej; Michalicka, Jan; Liebmann, Marcus; Primetzhofer, Daniel; Hoffman, Martin; Ernst, Arthur; Otrokov, Mikhail M.; Bihlmayer, Gustav; Weschke, Eugen; Lake, Bella; Chulkov, Evgueni V.; Morgenstern, Markus; Bauer, Guenther; Springholz, Gunther; Rader, Oliver
      Advanced Materials (Weinheim, Germany) (2021), 33 (42), 2102935CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Ferromagnetic topol. insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high-precision metrol., edge channel spintronics, and topol. qubits. The stable 2+ state of Mn enables intrinsic magnetic topol. insulators. MnBi2Te4 is, however, antiferromagnetic with 25 K Neel temp. and is strongly n-doped. In this work, p-type MnSb2Te4, previously considered topol. trivial, is shown to be a ferromagnetic topol. insulator for a few percent Mn excess. (i) Ferromagnetic hysteresis with record Curie temp. of 45-50 K, (ii) out-of-plane magnetic anisotropy, (iii) a 2D Dirac cone with the Dirac point close to the Fermi level, (iv) out-of-plane spin polarization as revealed by photoelectron spectroscopy, and (v) a magnetically induced bandgap closing at the Curie temp., demonstrated by scanning tunneling spectroscopy (STS), are shown. Moreover, a crit. exponent of the magnetization β ≈ 1 is found, indicating the vicinity of a quantum crit. point. Ab initio calcns. reveal that Mn-Sb site exchange provides the ferromagnetic interlayer coupling and the slight excess of Mn nearly doubles the Curie temp. Remaining deviations from the ferromagnetic order open the inverted bulk bandgap and render MnSb2Te4 a robust topol. insulator and new benchmark for magnetic topol. insulators.
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    37. 37
      Guan, Y. D.; Yan, C. H.; Lee, S. H.; Gui, X.; Ning, W.; Ning, J. L.; Zhu, Y. L.; Kothakonda, M.; Xu, C. Q.; Ke, X. L.; Sun, J. W.; Xie, W. W.; Yang, S. L.; Mao, Z. Q. Ferromagnetic MnBi4Te7 Obtained with Low-Concentration Sb Doping: A Promising Platform for Exploring Topological Quantum States. Phys. Rev. Mater. 2022, 6 (5), 054203,  DOI: 10.1103/PhysRevMaterials.6.054203
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      37
      Ferromagnetic MnBi4Te7 obtained with low-concentration Sb doping: A promising platform for exploring topological quantum states
      Guan, Y. D.; Yan, C. H.; Lee, S. H.; Gui, X.; Ning, W.; Ning, J. L.; Zhu, Y. L.; Kothakonda, M.; Xu, C. Q.; Ke, X. L.; Sun, J. W.; Xie, W. W.; Yang, S. L.; Mao, Z. Q.
      Physical Review Materials (2022), 6 (5), 054203CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
      The tuning of the magnetic phase, chem. potential, and structure is crucial to observe diverse exotic topol. quantum states in MnBi2Te4(Bi2Te3)m(m=0-3). Here we show a ferromagnetic (FM) phase with a chiral crystal structure in Mn(Bi1-xSbx)4Te7, obtained via tuning the growth conditions and Sb concn. Unlike previously reported Mn(Bi1-xSbx)4Te7, which exhibits FM transitions only at high Sb doping levels, our samples show FM transitions (TC=13.5K) at 15%-27% doping levels. Furthermore, our single-crystal x-ray-diffraction structure refinements find Sb doping leads to a chiral structure with the space group of P3-, contrasted with the centrosym. P3‾ m1 crystal structure of the parent compd. MnBi4Te7. Through angle-resolved photoemission spectroscopy measurements, we also demonstrated that the nontrivial band topol. is preserved in the Sb-doped FM samples. Given that the nontrivial band topol. of this system remains robust for low Sb doping levels, our success in making FM Mn(Bi1-xSbx)4Te7 with x=0.15, 0.175, 0.2, and 0.27 paves the way for realizing the predicted topol. quantum states, such as the axion insulator and Weyl semimetals. Addnl., we also obsd. magnetic glassy behavior in both antiferromagnetic MnBi4Te7 and FM Mn(Bi1-xSbx)4Te7 samples, which we believe originates from cluster spin-glass phases coexisting with long-range antiferromagnetic/FM orders. We have also discussed how the antisite Mn ions impact the interlayer magnetic coupling and how FM interlayer coupling is stabilized in this system.
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    38. 38
      Xie, H.; Wang, D.; Cai, Z.; Chen, B.; Guo, J.; Naveed, M.; Zhang, S.; Zhang, M.; Wang, X.; Fei, F.; Zhang, H.; Song, F. The Mechanism Exploration for Zero-Field Ferromagnetism in Intrinsic Topological Insulator MnBi2Te4 by Bi2Te3 Intercalations. Appl. Phys. Lett. 2020, 116 (22), 221902,  DOI: 10.1063/5.0009085
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      38
      The mechanism exploration for zero-field ferromagnetism in intrinsic topological insulator MnBi2Te4 by Bi2Te3 intercalations
      Xie, Hangkai; Wang, Dinghui; Cai, Zixiu; Chen, Bo; Guo, Jingwen; Naveed, Muhammad; Zhang, Shuai; Zhang, Minhao; Wang, Xuefeng; Fei, Fucong; Zhang, Haijun; Song, Fengqi
      Applied Physics Letters (2020), 116 (22), 221902CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Recent research on intrinsic magnetic topol. insulators (MTIs), MnBi2Te4, sheds new light on the observation of a long-expected high-temp. quantum anomalous Hall effect (QAHE). However, the strong interlayered anti-ferromagnetic (AFM) coupling hinders the practical applications without applying a magnetic field. Thus, how to adjust the magnetism of this compd. under zero field is essential. Here, we theor. and exptl. study the magnetic properties of two new promising intrinsic MTI candidates MnBi4Te7 and MnBi6Te10, formed by intercalating the Bi2Te3 layer into MnBi2Te4. The first-principles calcns. reveal that the relative energy between ferromagnetic (FM) and AFM states is greatly reduced by Bi2Te3 intercalations. The calcd. energy barriers for the spin flipping process also point out that the metastable FM state is more easily retained by intercalation. Meanwhile, we also exptl. carry out magnetic and transport measurements on these materials. By increasing Bi2Te3 intercalations, the AFM coupling becomes weaker, and an almost fully polarized FM state can be preserved in MnBi6Te10 at low temps., which are consistent with our calcns. (c) 2020 American Institute of Physics.
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    39. 39
      Aliev, Z. S.; Amiraslanov, I. R.; Nasonova, D. I.; Shevelkov, A. V.; Abdullayev, N. A.; Jahangirli, Z. A.; Orujlu, E. N.; Otrokov, M. M.; Mamedov, N. T.; Babanly, M. B.; Chulkov, E. V. Novel Ternary Layered Manganese Bismuth Tellurides of the MnTe-Bi2Te3 System: Synthesis and Crystal Structure. J. Alloys Compd. 2019, 789, 443– 450,  DOI: 10.1016/j.jallcom.2019.03.030
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      39
      Novel ternary layered manganese bismuth tellurides of the MnTe-Bi2Te3 system: Synthesis and crystal structure
      Aliev, Ziya S.; Amiraslanov, Imamaddin R.; Nasonova, Daria I.; Shevelkov, Andrei V.; Abdullayev, Nadir A.; Jahangirli, Zakir A.; Orujlu, Elnur N.; Otrokov, Mikhail M.; Mamedov, Nazim T.; Babanly, Mahammad B.; Chulkov, Evgueni V.
      Journal of Alloys and Compounds (2019), 789 (), 443-450CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      It is shown that MnTe-Bi2Te3 system is quasi-binary and in fact hosts three intermediate phases. Along with already known MnBi2Te4 phase, another two, MnBi4Te7 and MnBi6Te10 have been found to exist. All the phases melt incongruently in a very narrow temp. range of 577-590°C via peritectic reactions. Directional crystal growth results in hetero-phase ingots due to the narrow compositional range and narrow primary crystn. fields. The crystal structure of each phase is a derivation of the prototype tetradymit-type layered structure and the phases constitute a new homologous series with the chem. formula (MnTe)·n(Bi2Te3). X-ray diffraction patterns and Raman spectroscopy of the sorted-out single phase samples show that different phases have different no. of the seven (7)- and five (5)-layer blocks and their different stacking manner in the unit cell. In particular, MnBi2Te4 exhibits the -7-7-7-, MnBi4Te7 -5-7-5-7-, and MnBi6Te10 -5-5-7-5-5-7- sequence of the blocks. Thus, these structures are the first derivs. of Bi2Te3 structure to contain a transition metal cation Mn2+.
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    40. 40
      Ma, X.-M.; Chen, Z.; Schwier, E. F.; Zhang, Y.; Hao, Y.-J.; Kumar, S.; Lu, R.; Shao, J.; Jin, Y.; Zeng, M.; Liu, X.-R.; Hao, Z.; Zhang, K.; Mansuer, W.; Song, C.; Wang, Y.; Zhao, B.; Liu, C.; Deng, K.; Mei, J.; Shimada, K.; Zhao, Y.; Zhou, X.; Shen, B.; Huang, W.; Liu, C.; Xu, H.; Chen, C. Hybridization-Induced Gapped and Gapless States on the Surface of Magnetic Topological Insulators. Phys. Rev. B 2020, 102 (24), 245136,  DOI: 10.1103/PhysRevB.102.245136
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      40
      Hybridization-induced gapped and gapless states on the surface of magnetic topological insulators
      Ma, Xiao-Ming; Chen, Zhongjia; Schwier, Eike F.; Zhang, Yang; Hao, Yu-Jie; Kumar, Shiv; Lu, Ruie; Shao, Jifeng; Jin, Yuanjun; Zeng, Meng; Liu, Xiang-Rui; Hao, Zhanyang; Zhang, Ke; Mansuer, Wumiti; Song, Chunyao; Wang, Yuan; Zhao, Boyan; Liu, Cai; Deng, Ke; Mei, Jiawei; Shimada, Kenya; Zhao, Yue; Zhou, Xingjiang; Shen, Bing; Huang, Wen; Liu, Chang; Xu, Hu; Chen, Chaoyu
      Physical Review B (2020), 102 (24), 245136CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      The layered MnBi2nTe3n+1 family represents the first intrinsic antiferromagnetic (AFM) topol. insulator (protected by a combination symmetry S) ever discovered, providing an ideal platform to explore novel areas of physics such as the quantum anomalous Hall effect at elevated temp. and axion electrodynamics. Some of the recent angle-resolved photoemission spectroscopy (ARPES) expts. on this family have revealed that all terminations exhibit (nearly) gapless topol. surface states (TSSs) in the AFM state. The gapless behavior is inconsistent with the theor. expectation, as the surfaces being studied are S-breaking and shall therefore open a gap. Here we explain this curious paradox using a surface-bulk band hybridization picture. Combining CD ARPES and first-principles calcns. on MnBi6Te10, we prove that gaplike features are induced through hybridization between TSSs and certain bulk bands with Rashba character. The obsd. (nearly) gapless features are consistently reproduced by tight-binding simulations where TSSs are coupled to a pair of Rashba-split bands (RSBs). The Dirac-cone-like spectral features actually originate from the RSBs. Our findings highlight the role of band hybridization, superior to magnetism in this case, in shaping the general surface band structure in this family of magnetic topol. materials.
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    41. 41
      Hu, C.; Tanatar, M. A.; Prozorov, R.; Ni, N. Unusual Dynamic Susceptibility Arising from Soft Ferromagnetic Domains in MnBi 8 Te 13 and Sb-Doped MnBi2nTe3n+1(n = 2, 3). J. Phys. Appl. Phys. 2022, 55 (5), 054003,  DOI: 10.1088/1361-6463/ac3032
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      41
      Unusual dynamic susceptibility arising from soft ferromagnetic domains in MnBi8Te13 and Sb-doped MnBi2nTe3n + 1 (n = 2, 3)
      Hu, Chaowei; Tanatar, Makariy A.; Prozorov, Ruslan; Ni, Ni
      Journal of Physics D: Applied Physics (2022), 55 (5), 054003CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
      MnBi2nTe3n + 1 (MBT) is the first intrinsic magnetic topol. insulator and is promising to host emergent phenomena such as quantum anomalous Hall effect. They can be made ferromagnetic by having n ≥ 4 or with Sb doping. We studied the magnetic dynamics in a few selected ferromagnetic (FM) MBT compds., including MnBi8Te13 and Sb doped MnBi2nTe3n + 1 (n = 2, 3) using AC susceptibility and magneto-optical imaging. Slow relaxation behavior is obsd. in all three compds., suggesting its universality among FM MBT. We attribute the origin of the relaxation behavior to the irreversible domain movements since they only appear below the satn. fields when ferromagnetic domains form. The very soft ferromagnetic domain nature is revealed by the low-field fine-structured domains and high-field sea-urchin-shaped remanent-state domains imaged via our magneto-optical measurements. Finally, we ascribe the rare 'double-peak' behavior obsd. in the AC susceptibility under small DC bias fields to the very soft ferromagnetic domain formations.
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    42. 42
      Wu, X.; Li, J.; Ma, X.-M.; Zhang, Y.; Liu, Y.; Zhou, C.-S.; Shao, J.; Wang, Q.; Hao, Y.-J.; Feng, Y.; Schwier, E. F.; Kumar, S.; Sun, H.; Liu, P.; Shimada, K.; Miyamoto, K.; Okuda, T.; Wang, K.; Xie, M.; Chen, C.; Liu, Q.; Liu, C.; Zhao, Y. Distinct Topological Surface States on the Two Terminations of MnBi4Te7. Phys. Rev. X 2020, 10 (3), 031013,  DOI: 10.1103/PhysRevX.10.031013
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      42
      Distinct Topological Surface States on the Two Terminations of MnBi4Te7
      Wu, Xuefeng; Li, Jiayu; Ma, Xiao-Ming; Zhang, Yu; Liu, Yuntian; Zhou, Chun-Sheng; Shao, Jifeng; Wang, Qiaoming; Hao, Yu-Jie; Feng, Yue; Schwier, Eike F.; Kumar, Shiv; Sun, Hongyi; Liu, Pengfei; Shimada, Kenya; Miyamoto, Koji; Okuda, Taichi; Wang, Kedong; Xie, Maohai; Chen, Chaoyu; Liu, Qihang; Liu, Chang; Zhao, Yue
      Physical Review X (2020), 10 (3), 031013CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
      Intrinsic magnetic topol. insulator MnBi2Te4 has got unusual success in hosting emergent phenomena such as quantum anomalous Hall effect and axion insulator states. By using scanning tunneling microscopy and angle-resolved photoemission spectroscopy techniques, we unambiguously assign the two distinct surface states of MnBi4Te7 (n=1) to the quintuple-layer (QL) Bi2Te3 termination and the septuple-layer (SL) MnBi2Te4 termination, resp. A comparison of the exptl. observations with theor. calcns. reveals diverging topol. behaviors, esp. the hybridization effect between the QL and SL, on the two terminations. We identify a gap on the QL termination, originating from the hybridization between the topol. surface states of the QL and the bands of the SL beneath, and a gapless Dirac-cone band structure on the SL termination with time-reversal symmetry. The quasiparticle interference patterns further confirm the topol. nature of the surface states for both terminations, continuing far above the Fermi energy. The QL termination carries a spin-helical Dirac state with hexagonal warping, while at the SL termination, a strongly canted helical state from the surface lies between a pair of Rashba-like splitting bands from its neighboring layer. Our work elucidates an unprecedented hybridization effect between the building blocks of the topol. surface states and also reveals the termination-dependent time-reversal symmetry breaking in a magnetic topol. insulator.
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    43. 43
      Vidal, R. C.; Bentmann, H.; Facio, J. I.; Heider, T.; Kagerer, P.; Fornari, C. I.; Peixoto, T. R. F.; Figgemeier, T.; Jung, S.; Cacho, C.; Büchner, B.; van den Brink, J.; Schneider, C. M.; Plucinski, L.; Schwier, E. F.; Shimada, K.; Richter, M.; Isaeva, A.; Reinert, F. Orbital Complexity in Intrinsic Magnetic Topological Insulator MnBi4Te7 and MnBi6Te10. Phys. Rev. Lett. 2021, 126 (17), 176403,  DOI: 10.1103/PhysRevLett.126.176403
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      43
      Orbital Complexity in Intrinsic Magnetic Topological Insulators MnBi4Te7 and MnBi6Te10
      Vidal, R. C.; Bentmann, H.; Facio, J. I.; Heider, T.; Kagerer, P.; Fornari, C. I.; Peixoto, T. R. F.; Figgemeier, T.; Jung, S.; Cacho, C.; Buchner, B.; van den Brink, J.; Schneider, C. M.; Plucinski, L.; Schwier, E. F.; Shimada, K.; Richter, M.; Isaeva, A.; Reinert, F.
      Physical Review Letters (2021), 126 (17), 176403CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
      Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compds. MnBi4Te7 and MnBi6Te10, the n=1 and 2 members of a modular (Bi2Te3)n(MnBi2Te4) series, which have attracted recent interest as intrinsic magnetic topol. insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calcns. based on d. functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topol. from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for Bi2Te3-terminated surfaces but remains preserved for MnBi2Te4-terminated surfaces. Our results firmly establish the topol. nontrivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall cond. depends on surface termination.
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    44. 44
      Jo, N. H.; Wang, L.-L.; Slager, R.-J.; Yan, J.; Wu, Y.; Lee, K.; Schrunk, B.; Vishwanath, A.; Kaminski, A. Intrinsic Axion Insulating Behavior in Antiferromagnetic MnBi6Te10. Phys. Rev. B 2020, 102 (4), 045130,  DOI: 10.1103/PhysRevB.102.045130
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      44
      Intrinsic axion insulating behavior in antiferromagnetic MnBi6Te10
      Jo, Na Hyun; Wang, Lin-Lin; Slager, Robert-Jan; Yan, Jiaqiang; Wu, Yun; Lee, Kyungchan; Schrunk, Benjamin; Vishwanath, Ashvin; Kaminski, Adam
      Physical Review B (2020), 102 (4), 045130CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      A striking feature of time-reversal symmetry (TRS) protected topol. insulators (TIs) is that they are characterized by a half integer quantum Hall effect on the boundary when the surface states are gapped by time-reversal breaking perturbations. While TRS-protected TIs have become increasingly under control, magnetic analogs are still a largely unexplored territory with novel rich structures. In particular, magnetic topol. insulators can also host a quantized axion term in the presence of lattice symmetries. Since these symmetries are naturally broken on the boundary, the surface states can develop a gap without external manipulation. In this paper, we combine theor. anal., d.-functional calcns. and exptl. evidence to reveal intrinsic axion insulating behavior in MnBi6Te10. The quantized axion term arises from the simplest possible mechanism in the antiferromagnetic regime where it is protected by inversion symmetry and the product of a fractional translation and TRS. The anticipated gapping of the Dirac surface state at the edge is subsequently exptl. established using angle resolved photoemission spectroscopy (ARPES). As a result, this system provides the magnetic analog of the simplest TRS-protected TI with a single, gapped Dirac cone at the surface.
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    45. 45
      Gordon, K. N.; Sun, H.; Hu, C.; Linn, A. G.; Li, H.; Liu, Y.; Liu, P.; Mackey, S.; Liu, Q.; Ni, N.; Dessau, D. Strongly Gapped Topological Surface States on Protected Surfaces of Antiferromagnetic MnBi4Te7 and MnBi6Te10. 2019, 1910.13943. arXiv.  DOI: 10.48550/arXiv.1910.13943 (accessed June 24, 2022).
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      Tan, H.; Yan, B. Distinct Magnetic Gaps between Antiferromagnetic and Ferromagnetic Orders Driven by Surface Defects in the Topological Magnet MnBi2Te4. 2022, 2207.13511. arXiv.  DOI: 10.48550/arXiv.2207.13511 (accessed September 08, 2022).
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    47. 47
      Garnica, M.; Otrokov, M. M.; Aguilar, P. C.; Klimovskikh, I. I.; Estyunin, D.; Aliev, Z. S.; Amiraslanov, I. R.; Abdullayev, N. A.; Zverev, V. N.; Babanly, M. B.; Mamedov, N. T.; Shikin, A. M.; Arnau, A.; de Parga, A. L. V.; Chulkov, E. V.; Miranda, R. Native Point Defects and Their Implications for the Dirac Point Gap at MnBi2Te4(0001). Npj Quantum Mater. 2022, 7 (1), 1– 9,  DOI: 10.1038/s41535-021-00414-6
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    48. 48
      Shikin, A. M.; Estyunin, D. A.; Klimovskikh, I. I.; Filnov, S. O.; Schwier, E. F.; Kumar, S.; Miyamoto, K.; Okuda, T.; Kimura, A.; Kuroda, K.; Yaji, K.; Shin, S.; Takeda, Y.; Saitoh, Y.; Aliev, Z. S.; Mamedov, N. T.; Amiraslanov, I. R.; Babanly, M. B.; Otrokov, M. M.; Eremeev, S. V.; Chulkov, E. V. Nature of the Dirac Gap Modulation and Surface Magnetic Interaction in Axion Antiferromagnetic Topological Insulator MnBi2Te4. Sci. Rep. 2020, 10 (1), 13226,  DOI: 10.1038/s41598-020-70089-9
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      48
      Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator MnBi2Te4
      Shikin, A. M.; Estyunin, D. A.; Klimovskikh, I. I.; Filnov, S. O.; Schwier, E. F.; Kumar, S.; Miyamoto, K.; Okuda, T.; Kimura, A.; Kuroda, K.; Yaji, K.; Shin, S.; Takeda, Y.; Saitoh, Y.; Aliev, Z. S.; Mamedov, N. T.; Amiraslanov, I. R.; Babanly, M. B.; Otrokov, M. M.; Eremeev, S. V.; Chulkov, E. V.
      Scientific Reports (2020), 10 (1), 13226CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
      Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topol. insulator MnBi2Te4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temps. (9-35 K), light polarizations and photon energies. We have distinguished both large (60-70 meV) and reduced (< 20meV) gaps at the DP in the ARPES dispersions, which remain open above the Neel temp. (TN = 24.5K). We propose that the gap above TN remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and CD ARPES measurements show a surface ferromagnetic ordering for the "large gap" sample and apparently significantly reduced effective magnetic moment for the "reduced gap" sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calcns. surface structural modification can result in a significant modulation of the DP gap.
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    49. 49
      Ma, X.-M.; Zhao, Y.; Zhang, K.; Kumar, S.; Lu, R.; Li, J.; Yao, Q.; Shao, J.; Hou, F.; Wu, X.; Zeng, M.; Hao, Y.-J.; Hao, Z.; Wang, Y.; Liu, X.-R.; Shen, H.; Sun, H.; Mei, J.; Miyamoto, K.; Okuda, T.; Arita, M.; Schwier, E. F.; Shimada, K.; Deng, K.; Liu, C.; Lin, J.; Zhao, Y.; Chen, C.; Liu, Q.; Liu, C. Realization of a Tunable Surface Dirac Gap in Sb-Doped MnBi2Te4. Phys. Rev. B 2021, 103 (12), L121112,  DOI: 10.1103/PhysRevB.103.L121112
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      49
      Realization of a tunable surface Dirac gap in Sb-doped MnBi2Te4
      Ma, Xiao-Ming; Zhao, Yufei; Zhang, Ke; Kumar, Shiv; Lu, Ruie; Li, Jiayu; Yao, Qiushi; Shao, Jifeng; Hou, Fuchen; Wu, Xuefeng; Zeng, Meng; Hao, Yu-Jie; Hao, Zhanyang; Wang, Yuan; Liu, Xiang-Rui; Shen, Huiwen; Sun, Hongyi; Mei, Jiawei; Miyamoto, Koji; Okuda, Taichi; Arita, Masashi; Schwier, Eike F.; Shimada, Kenya; Deng, Ke; Liu, Cai; Lin, Junhao; Zhao, Yue; Chen, Chaoyu; Liu, Qihang; Liu, Chang
      Physical Review B (2021), 103 (12), L121112CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Signatures of both the quantum anomalous Hall effect and axion electrodynamics have been recently obsd. to exist in thin films of MnBi2Te4, a stoichiometric antiferromagnetic topol. insulator. Direct evidence of the bulk topol. magnetoelec. response in an axion insulator requires an energy gap at its topol. surface state (TSS). However, independent spectroscopic expts. revealed that such a surface gap is much smaller than previously thought. Here we utilize angle resolved photoemission spectroscopy and d. functional theory calcns. to demonstrate that a sizable TSS gap unexpectedly exists in Sb-doped MnBi2Te4 where the bulk system remains topol. nontrivial. This gap is found to be insensitive to the bulk antiferromagnetic-paramagnetic transition, while it enlarges along with increasing Sb concn., enabling simultaneous tunability of the Fermi level and the TSS gap size (up to >100 meV). Our work shows that Sb dopants in MnBi2Te4 can not only control the Fermi level but also induce a tunable surface gap, providing a potential platform to observe the key features of the high-temp. axion-insulator phase.
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    50. 50
      Shikin, A. M.; Estyunin, D. A.; Zaitsev, N. L.; Glazkova, D.; Klimovskikh, I. I.; Filnov, S. O.; Rybkin, A. G.; Schwier, E. F.; Kumar, S.; Kimura, A.; Mamedov, N.; Aliev, Z.; Babanly, M. B.; Kokh, K.; Tereshchenko, O. E.; Otrokov, M. M.; Chulkov, E. V.; Zvezdin, K. A.; Zvezdin, A. K. Sample-Dependent Dirac-Point Gap in MnBi2Te4 and Its Response to Applied Surface Charge: A Combined Photoemission and Ab Initio Study. Phys. Rev. B 2021, 104 (11), 115168,  DOI: 10.1103/PhysRevB.104.115168
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      50
      Sample-dependent Dirac-point gap in MnBi2Te4 and its response to applied surface charge: A combined photoemission and ab initio study
      Shikin, A. M.; Estyunin, D. A.; Zaitsev, N. L.; Glazkova, D.; Klimovskikh, I. I.; Filnov, S. O.; Rybkin, A. G.; Schwier, E. F.; Kumar, S.; Kimura, A.; Mamedov, N.; Aliev, Z.; Babanly, M. B.; Kokh, K.; Tereshchenko, O. E.; Otrokov, M. M.; Chulkov, E. V.; Zvezdin, K. A.; Zvezdin, A. K.
      Physical Review B (2021), 104 (11), 115168CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Recently discovered intrinsic antiferromagnetic topol. insulator MnBi2Te4 presents an exciting platform for realization of the quantum anomalous Hall effect and a no. of related phenomena at elevated temps. An important characteristic making this material attractive for applications is its predicted large magnetic gap at the Dirac point (DP). However, while the early exptl. measurements reported on large DP gaps, a no. of recent studies claimed to observe a gapless dispersion of the MnBi2Te4 Dirac cone. Here, using micro(μ)-laser angle-resolved photoemission spectroscopy, we study the electronic structure of 15 different MnBi2Te4 samples, grown by two different chemists groups. Based on the careful energy distribution curves anal., the DP gaps between 15 and 65 meV are obsd., as measured below the Neel temp. at about 10-16 K. At that, roughly half of the studied samples show the DP gap of about 30 meV, while for a quarter of the samples the gaps are in the 50 to 60 meV range. Summarizing the results of both our and other groups, in the currently available MnBi2Te4 samples the DP gap can acquire an arbitrary value between a few and several tens of meV. Furthermore, based on the d. functional theory, we discuss a possible factor that might contribute to the redn. of the DP gap size, which is the excess surface charge that can appear due to various defects in surface region. We demonstrate that the DP gap is influenced by the applied surface charge and even can be closed, which can be taken advantage of to tune the MnBi2Te4 DP gap size.
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    51. 51
      Hor, Y. S.; Roushan, P.; Beidenkopf, H.; Seo, J.; Qu, D.; Checkelsky, J. G.; Wray, L. A.; Hsieh, D.; Xia, Y.; Xu, S.-Y.; Qian, D.; Hasan, M. Z.; Ong, N. P.; Yazdani, A.; Cava, R. J. Development of Ferromagnetism in the Doped Topological Insulator Bi2Te3. Phys. Rev. B 2010, 81 (19), 195203,  DOI: 10.1103/PhysRevB.81.195203
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      51
      Development of ferromagnetism in the doped topological insulator Bi2-xMnxTe3
      Hor, Y. S.; Roushan, P.; Beidenkopf, H.; Seo, J.; Qu, D.; Checkelsky, J. G.; Wray, L. A.; Hsieh, D.; Xia, Y.; Xu, S.-Y.; Qian, D.; Hasan, M. Z.; Ong, N. P.; Yazdani, A.; Cava, R. J.
      Physical Review B: Condensed Matter and Materials Physics (2010), 81 (19), 195203/1-195203/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The development of ferromagnetism in Mn-doped Bi2Te3 is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy anal. shows that the Mn substitutes on the Bi sites, forming compds. of the type Bi2-xMnxTe3, and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compns. Bi1.96Mn0.04Te3 and Bi1.91Mn0.09Te3, a second-order ferromagnetic transition is obsd., with TC ∼9-12 K. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the Bi2Te3 basal plane. Thermoelec. power and Hall effect measurements show that the Mn-doped Bi2Te3 crystals are p-type. Angle-resolved photoemission spectroscopy measurements show that the topol. surface states that are present in pristine Bi2Te3 are also present at 15 K in ferromagnetic Mn-doped Bi2-xMnxTe3 and that the dispersion relations of the surface states are changed in a subtle fashion.
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    52. 52
      Lee, J. S.; Richardella, A.; Rench, D. W.; Fraleigh, R. D.; Flanagan, T. C.; Borchers, J. A.; Tao, J.; Samarth, N. Ferromagnetism and Spin-Dependent Transport in n-Type Mn-Doped Bismuth Telluride Thin Films. Phys. Rev. B 2014, 89 (17), 174425,  DOI: 10.1103/PhysRevB.89.174425
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      52
      Ferromagnetism and spin-dependent transport in n-type Mn-doped bismuth telluride thin films
      Lee, Joon Sue; Richardella, Anthony; Rench, David W.; Fraleigh, Robert D.; Flanagan, Thomas C.; Borchers, Julie A.; Tao, Jing; Samarth, Nitin
      Physical Review B: Condensed Matter and Materials Physics (2014), 89 (17), 174425/1-174425/8, 8 pp.CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We describe a detailed study of the structural, magnetic and magnetotransport properties of single-crystal, n-type, Mn-doped bismuth telluride thin films grown by mol. beam epitaxy. With increasing Mn concn., the crystal structure changes from the tetradymite structure of the Bi2Te3 parent crystal at low Mn concns. towards a BiTe phase in the (Bi2Te3)m(Bi2)n homologous series. Magnetization measurements reveal the onset of ferromagnetism with a Curie temp. in the range 13.8-17 K in films with ∼2%-∼10% Mn concn. Magnetization hysteresis loops reveal that the magnetic easy axis is along the c axis of the crystal (perpendicular to the plane). Polarized neutron reflectivity measurements of a 68-nm-thick sample show that the magnetization is uniform through the film. The presence of ferromagnetism is also manifest in a strong anomalous Hall effect and a hysteretic magnetoresistance arising from domain-wall scattering. Ordinary Hall effect measurements show that the carrier d. is n type, increases with Mn doping and is high enough (≥2.8 × 1013 cm-2) to place the chem. potential in the conduction band. Thus the obsd. ferromagnetism is likely assocd. with both bulk and surface states. Surprisingly, the Curie temp. does not show any clear dependence on the carrier d. but does increase with Mn concn. Our results suggest that the ferromagnetism probed in these Mn-doped bismuth telluride films is not mediated by carriers in the conduction band or in an impurity band.
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    53. 53
      Vobornik, I.; Panaccione, G.; Fujii, J.; Zhu, Z.-H.; Offi, F.; Salles, B. R.; Borgatti, F.; Torelli, P.; Rueff, J. P.; Ceolin, D.; Artioli, A.; Unnikrishnan, M.; Levy, G.; Marangolo, M.; Eddrief, M.; Krizmancic, D.; Ji, H.; Damascelli, A.; van der Laan, G.; Egdell, R. G.; Cava, R. J. Observation of Distinct Bulk and Surface Chemical Environments in a Topological Insulator under Magnetic Doping. J. Phys. Chem. C 2014, 118 (23), 12333– 12339,  DOI: 10.1021/jp502729u
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      53
      Observation of Distinct Bulk and Surface Chemical Environments in a Topological Insulator under Magnetic Doping
      Vobornik, Ivana; Panaccione, Giancarlo; Fujii, Jun; Zhu, Zhi-Huai; Offi, Francesco; Salles, Benjamin R.; Borgatti, Francesco; Torelli, Piero; Rueff, Jean Pascal; Ceolin, Denis; Artioli, Alberto; Unnikrishnan, Manju; Levy, Giorgio; Marangolo, Massimiliano; Eddrief, Mamhoud; Krizmancic, Damjan; Ji, Huiwen; Damascelli, Andrea; van der Laan, Gerrit; Egdell, Russell G.; Cava, Robert J.
      Journal of Physical Chemistry C (2014), 118 (23), 12333-12339CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The influence of magnetic dopants on the electronic and chem. environments in topol. insulators (TIs) is a key factor when considering possible spintronic applications based on topol. surface state properties. Here the authors provide spectroscopic evidence for the presence of distinct chem. and electronic behavior for surface and bulk magnetic doping of Bi2Te3. The inclusion of Mn in the bulk of Bi2Te3 induces a genuine dil. ferromagnetic state, with redn. of the bulk band gap as the Mn content is increased. Deposition of Fe on the Bi2Te3 surface, however, favors the formation of iron telluride already at coverage ≥0.07 monolayer, as a consequence of the reactivity of the Te-rich surface. The authors' results identify the factors that need to be controlled in the realization of magnetic nanosystems and interfaces based on TIs.
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      Teng, J.; Liu, N.; Li, Y. Mn-Doped Topological Insulators: A Review. J. Semicond. 2019, 40 (8), 081507,  DOI: 10.1088/1674-4926/40/8/081507
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      Mn-doped topological insulators: a review
      Teng, Jing; Liu, Nan; Li, Yongqing
      Journal of Semiconductors (2019), 40 (8), 081507CODEN: JSOEB4; ISSN:1674-4926. (IOP Publishing Ltd.)
      A review. Topol. insulators (TIs) host robust edge or surface states protected by time-reversal symmetry (TRS), which makes them prime candidates for applications in spintronic devices. A promising avenue of research for the development of functional TI devices has involved doping of three-dimensional (3D) TI thin film and bulk materials with magnetic elements. This approach aims to break the TRS and open a surface band gap near the Dirac point. Utilizing this gapped surface state allows for a wide range of novel phys. effects to be obsd., paving a way for applications in spintronics and quantum computation. This review focuses on the research of 3D TIs doped with manganese (Mn). We summarize major progress in the study of Mn doped chalcogenide TIs, including Bi2Se3, Bi2Te3, and Bi2(Te,Se)3. The transport properties, in particular the anomalous Hall effect, of the Mn-doped Bi2Se3 are discussed in detail. Finally, we conclude with future prospects and challenges in further studies of Mn doped TIs.
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    55. 55
      Chen, B.; Wang, D.; Jiang, Z.; Zhang, B.; Cui, S.; Guo, J.; Xie, H.; Zhang, Y.; Naveed, M.; Du, Y.; Wang, X.; Zhang, H.; Fei, F.; Shen, D.; Sun, Z.; Song, F. Coexistence of Ferromagnetism and Topology by Charge Carrier Engineering in the Intrinsic Magnetic Topological Insulator Mn Bi4Te7. Phys. Rev. B 2021, 104 (7), 075134,  DOI: 10.1103/PhysRevB.104.075134
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      55
      Coexistence of ferromagnetism and topology by charge carrier engineering in the intrinsic magnetic topological insulator MnBi4Te7
      Chen, Bo; Wang, Dinghui; Jiang, Zhicheng; Zhang, Bo; Cui, Shengtao; Guo, Jingwen; Xie, Hangkai; Zhang, Yong; Naveed, Muhammad; Du, Yu; Wang, Xuefeng; Zhang, Haijun; Fei, Fucong; Shen, Dawei; Sun, Zhe; Song, Fengqi
      Physical Review B (2021), 104 (7), 075134CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Intrinsic magnetic topol. insulators (MTIs) MnBi2Te4 and MnBi2Te4/(Bi2Te3)n are expected to realize the high-temp. quantum anomalous Hall effect and dissipationless elec. transport. However, there is still a lack of ideal MTI candidates with magnetic ordering of the ferromagnetic (FM) ground state. Here, we show a MTI sample of Mn(Bi0.7Sb0.3)4Te7 which holds the coexistence of a FM behavior state and topol. nontriviality. The dramatic modulation of the magnetism is induced by a charge carrier engineering process via the Sb substitution in the MnBi4Te7 matrix with antiferromagnetic ordering. The evolution of magnetism in Mn(Bi1-xSbx)4Te7 is systematically investigated by our magnetic measurements and theor. calcns. The clear topol. surface states of the FM sample of Mn(Bi0.7Sb0.3)4Te7 are further verified by angle-resolved photoemission spectroscopy. The demonstration of the intrinsic FM-MTI of Mn(Bi0.7Sb0.3)4Te7 in this paper sheds light on further material optimization of intrinsic MTIs and paves the way for further studies to clarify the relationships between topol., magnetism, and charge carriers in topol. materials.
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    56. 56
      Sitnicka, J.; Park, K.; Skupinski, P.ł; Grasza, K.; Reszka, A.; Sobczak, K.; Borysiuk, J.; Adamus, Z.; Tokarczyk, M.; Avdonin, A.; Fedorchenko, I.; Abaloszewa, I.; Turczyniak-Surdacka, S.; Olszowska, N.; Kołodziej, J.; Kowalski, B. J; Deng, H.; Konczykowski, M.; Krusin-Elbaum, L.; Wołos, A. Systemic Consequences of Disorder in Magnetically Self-Organized Topological MnBi2Te4/(Bi2Te3) n Superlattices. 2D Mater. 2022, 9 (1), 015026,  DOI: 10.1088/2053-1583/ac3cc6
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      56
      Systemic consequences of disorder in magnetically self-organized topological MnBi2Te4/(Bi2Te3)n superlattices
      Sitnicka, Joanna; Park, Kyungwha; Skupinski, Pawel; Grasza, Krzysztof; Reszka, Anna; Sobczak, Kamil; Borysiuk, Jolanta; Adamus, Zbigniew; Tokarczyk, Mateusz; Avdonin, Andrei; Fedorchenko, Irina; Abaloszewa, Irina; Turczyniak-Surdacka, Sylwia; Olszowska, Natalia; Kolodziej, Jacek; Kowalski, Bogdan J.; Deng, Haiming; Konczykowski, Marcin; Krusin-Elbaum, Lia; Wolos, Agnieszka
      2D Materials (2022), 9 (1), 015026CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      MnBi2Te4/(Bi2Te3)n materials system has recently generated strong interest as a natural platform for the realization of the quantum anomalous Hall (QAH) state. The system is magnetically much better ordered than substitutionally doped materials, however, the detrimental effects of certain disorders are becoming increasingly acknowledged. Here, from compiling structural, compositional, and magnetic metrics of disorder in ferromagnetic (FM) MnBi2Te4/(Bi2Te3)n it is found that migration of Mn between MnBi2Te4 septuple layers (SLs) and otherwise non-magnetic Bi2Te3 quintuple layers (QLs) has systemic consequences-it induces FM coupling of Mn-depleted SLs with Mn-doped QLs, seen in ferromagnetic resonance as an acoustic and optical resonance mode of the two coupled spin subsystems. Even for a large SL sepn. (n ⪆ 4 QLs) the structure cannot be considered as a stack of uncoupled two-dimensional layers. Angle-resolved photoemission spectroscopy and d. functional theory studies show that Mn disorder within an SL causes delocalization of electron wave functions and a change of the surface band structure as compared to the ideal MnBi2Te4/(Bi2Te3)n. These findings highlight the crit. importance of inter- and intra-SL disorder towards achieving new QAH platforms as well as exploring novel axion physics in intrinsic topol. magnets.
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    57. 57
      Yan, C.; Green, E.; Fukumori, R.; Protic, N.; Lee, S. H.; Fernandez-Mulligan, S.; Raja, R.; Erdakos, R.; Mao, Z.; Yang, S. An Integrated Quantum Material Testbed with Multi-Resolution Photoemission Spectroscopy. Rev. Sci. Instrum. 2021, 92 (11), 113907,  DOI: 10.1063/5.0072979
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      57
      An integrated quantum material testbed with multi-resolution photoemission spectroscopy
      Yan, Chenhui; Green, Emanuel; Fukumori, Riku; Protic, Nikola; Lee, Seng Huat; Fernandez-Mulligan, Sebastian; Raja, Rahim; Erdakos, Robin; Mao, Zhiqiang; Yang, Shuolong
      Review of Scientific Instruments (2021), 92 (11), 113907CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
      The development of a multi-resoln. photoemission spectroscopy (MRPES) setup, which probes quantum materials in energy, momentum, space, and time is presented. This versatile setup integrates 3 light sources in 1 photoelectron setup and can conveniently switch between traditional angle-resolved photoelectron spectroscopy (ARPES), time-resolved ARPES (trARPES), and micrometer-scale spatially resolved ARPES. It provides a 1st-time all-in-one soln. to achieve an energy resoln. of <4 meV, a time resoln. of <35 fs, and a spatial resoln. of ∼10μm in photoelectron spectroscopy. The shortest time resoln. among the trARPES setups was obtained using solid-state nonlinear crystals for frequency upconversion. This MRPES setup is integrated with a shadow-mask assisted MBE system, which transforms the traditional photoemission spectroscopy into a quantum device characterization instrument. The functionalities of this novel quantum material testbed were demonstrated using FeSe/SrTiO3 films and MnBi4Te7 magnetic topol. insulators. (c) 2021 American Institute of Physics.
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    • Defects and doping, circular dichroism in the ARPES spectra of ferromagnetic MnBi6Te10, side-by-side comparison of FM and AFM MnBi6Te10 using the temperature evolutions of the TSS’s on the MBT termination, electronic structure of antiferromagnetic MnBi6Te10, circular dichroism in the ARPES spectra of antiferromagnetic MnBi6Te10, selected area electron diffraction (SAED) patterns of MnBi6Te10, high-field magnetization measurements of AFM and FM MnBi6Te10, estimates of the chemical concentrations of MnBi6Te10 based on the scanning transmission electron microscopy–energy dispersive X-ray (STEM-EDX) analysis, magnetic interactions, and the determination of the Mn density ratio between different layers (PDF)


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