Magnetic Properties and Electronic Structure of Magnetic Topological Insulator MnBi$_2$Se$_4$
Tiancong Zhu, Alexander J. Bishop, Tong Zhou, Menglin Zhu, Dante J. O'Hara, Alexander A. Baker, Shuyu Cheng, Robert C. Walko, Jacob J. Repicky, Jay A. Gupta, Ch… 展开作者
Tiancong Zhu, Alexander J. Bishop, Tong Zhou, Menglin Zhu, Dante J. O'Hara, Alexander A. Baker, Shuyu Cheng, Robert C. Walko, Jacob J. Repicky, Jay A. Gupta, Chris M. Jozwiak, Eli Rotenberg, Jinwoo Hwang, Igor Žutić, Roland K. Kawakami
The intrinsic magnetic topological insulators MnBi$_2$X$_4$ (X = Se, Te) are promising candidates in realizing various novel topological states related to symmetry breaking by magnetic order. Although much progress had been made in MnBi$_2$Te$_4$, the study of MnBi$_2$Se$_4$ has been lacking due to the difficulty of material synthesis of the desired trigonal phase. Here, we report the synthesis of multilayer trigonal MnBi$_2$Se$_4$ with alternating-layer molecular beam epitaxy. Atomic-resolution scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM) identify a well-ordered multilayer van der Waals (vdW) crystal with septuple-layer base units in agreement with the trigonal structure. Systematic thickness-dependent magnetometry studies illustrate the layered antiferromagnetic ordering as predicted by theory. Angle-resolved photoemission spectroscopy (ARPES) reveals the gapless Dirac-like surface state of MnBi$_2$Se$_4$, which demonstrates that MnBi$_2$Se$_4$ is a topological insulator above the magnetic ordering temperature. These systematic studies show that MnBi$_2$Se$_4$ is a promising candidate for exploring the rich topological phases of layered antiferromagnetic topological insulators.
In a magnetic topological insulator, nontrivial band topology conspires with magnetic order to produce exotic states of matter that are best exemplified by quantum anomalous Hall (QAH) insulators and axion insulators. Up till now, such magnetic topological insulators are obtained by doping topological insulators with magnetic atoms. The random magnetic dopants, however, inevitably introduce disorders that hinder further exploration of quantum effects in the material. Here, we resolve this dilemma by probing quantum transport in MnBi$_2$Te$_4$ thin flake - a topological insulator with intrinsic…
Realizing axion insulator state with a uniform magnetization considerably facilitates experimental explorations of the intriguing topological magnetoelectric effect, a hallmark of three-dimensional (3D) topological insulators (TIs). Through density functional theory calculations and four-band model studies, we find that magnetic ions Cr3+ in monolayer CrI3 and Mn2+ in septuple-layer MnBi2Se4 have opposite exchange couplings to the topological surface states of 3D TI Bi2Se3. As an exciting result of such opposite exchange couplings, axion insulator state is realized by a uniform magnetization i…
Intrinsic magnetic topological insulators offer low disorder and large magnetic bandgaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the Quantum Anomalous Hall (QAH) effect and axion insulator phases have been realised. These observations occur at temperatures significantly lower than the Neel temperature of bulk MnBi2Te4, and measurement of the magnetic energy gap at the Dirac point in ultra-thin MnBi2Te4 has yet to be achieved. Critical to achieving the promise of this system is a direct measurement o…
Breaking time reversal symmetry in a topological insulator may lead to quantum anomalous Hall effect and axion insulator phase. MnBi4Te7 is a recently discovered antiferromagnetic topological insulator with TN ~12.5 K, which is constituted of alternatively stacked magnetic layer (MnBi2Te4) and non-magnetic layer (Bi2Te3). By means of scanning tunneling spectroscopy, we clearly observe the electronic state present at a step edge of a magnetic MnBi2Te4 layer but absent at non-magnetic Bi2Te3 layers at 4.5 K. Furthermore, we find that as the temperature rises above TN, the edge state vanishes, wh…
MnBi2Te4 and MnBi4Te7 are intrinsic antiferromagnetic topological insulators, offering a promising materials platform for realizing exotic topological quantum states. However, high densities of intrinsic defects in these materials not only cause bulk metallic conductivity, preventing the measurement of quantum transport in surface states, but may also affect magnetism and topological properties. In this paper, we show by density functional theory calculations that the strain induced by the internal heterostructure promotes the formation of large-size-mismatched antisite defect BiMn in MnBi2Te4…
The layered topological insulator MnBi$_2$Te$_4$ has attracted great interest recently due to its intrinsic antiferromagnetic order, potentially hosting various topological phases. By temperature-dependent infrared spectroscopy over a broad frequency range, we studied the changes in the optical conductivity of MnBi$_2$Te$_4$ at the magnetic ordering temperature. The temperature dependence of several optical parameters reveals an anomaly at the magnetic phase transition, which suggests the correlation between the bulk electronic band structure and the magnetism. We relate our findings to recent…
Magnetic topological quantum materials (TQMs) provide a fertile ground for the emergence of fascinating topological magneto-electric effects. Recently, the discovery of intrinsic antiferromagnetic (AFM) topological insulator MnBi2Te4 that could realize quantized anomalous Hall effect and axion insulator phase ignited intensive study on this family of TQM compounds. Here, we investigated the AFM compound MnBi4Te7 where Bi2Te3 and MnBi2Te4 layers alternate to form a superlattice. Using spatial- and angle-resolved photoemission spectroscopy, we identified ubiquitous (albeit termination dependent)…
MnBi$_8$Te$_{13}$ is an intrinsic ferromagnetic (FM) topological insulator with different complex surface terminations. Resolving the electronic structures of different termination surfaces and manipulation of the electronic state are important. Here, by using micrometer spot time- and angle-resolved photoemission spectroscopy ($μ$-TrARPES), we resolve the electronic structures and reveal the ultrafast dynamics upon photoexcitation. Photoinduced filling of the surface state hybridization gap is observed for the Bi$_2$Te$_3$ quintuple layer directly above MnBi$_2$Te$_4$ accompanied by a nontriv…
Using scanning tunneling microscopy and spectroscopy, we visualized the native defects in antiferromagnetic topological insulator $\mathrm{MnBi_2Te_4}$. Two native defects $\mathrm{Mn_{Bi}}$ and $\mathrm{Bi_{Te}}$ antisites can be well resolved in the topographic images. $\mathrm{Mn_{Bi}}$ tend to suppress the density of states at conduction band edge. Spectroscopy imaging reveals a localized peak-like local density of state at $\sim80$~meV below the Fermi energy. A careful inspection of topographic and spectroscopic images, combined with density functional theory calculation, suggests this re…
Despite the rapid progress in understanding the first intrinsic magnetic topological insulator MnBi$_2$Te$_4$, its electronic structure remains a topic under debates. Here we perform a thorough spectroscopic investigation into the electronic structure of MnBi$_2$Te$_4$ via laser-based angle-resolved photoemission spectroscopy. Through quantitative analysis, we estimate an upper bound of 3 meV for the gap size of the topological surface state. Furthermore, our circular dichroism measurements reveal band chiralities for both the topological surface state and quasi-2D bands, which can be well rep…
MnBi$_2$Te$_4$ is an antiferromagnetic topological insulator which stimulates intense interests due to the exotic quantum phenomena and promising device applications. Surface structure is a determinant factor to understand the novel magnetic and topological behavior of MnBi2Te4, yet its precise atomic structure remains elusive. Here, we discovered a spontaneous surface collapse and reconstruction in 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 Mn-doped Bi$_2$Te$_3$ quintuple-…
The magnetic structures of MnBi2Te4(Bi2Te3)n can be manipulated by tuning the interlayer coupling via the number 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 Néel temperature and an increasing saturation field. In sharp contrast, MnBi6Te10 experiences a phase transition from A-type AFM to a quasi-two-dimensional FM state with a s…
Further to the structure of the intrinsic magnetic topological insulators MnBi2Te4 n(Bi2Te3) with n<4, where index n is the number of quintuple Te-Bi-Te-Bi-Te building blocks inserted between the neighboring septuple Te-Bi-Te-Mn-Te-Bi-Te building blocks, the structure of the members with n=4, 5 and 6 was studied using X-ray powder diffraction. The unit cell parameters and atomic positions were calculated. The obtained and available structural data were summarized to show that the crystal structure of all members of MnBi2Te4 n(Bi2Te3) follows the cubic close packing principle, independently of …
The metastable layered compound MnBi2Te4 is the first experimentally realized intrinsic antiferromagnetic topological insulator, predicted to host the quantum anomalous Hall effect at high temperatures upon exfoliation to atomically thin layers. While its magnetic ordering and topological properties have generated intensive interest, the mechanism behind its metastability and the ideal crystal synthesis conditions have remained elusive. Here, using a combined first-principles-based approach that considers lattice, charge, and spin degrees of freedom, we investigate the metastability of MnBi2Te…
A new kind of intrinsic magnetic topological insulators (MTI) MnBi2Te4 family have shed light on the observation of novel topological quantum effect such as quantum anomalous Hall effect (QAHE). However, the strong anti-ferromagnetic (AFM) coupling and high carrier concentration in the bulk hinder the practical applications. In closely related materials MnBi4Te7 and MnBi6Te10, the interlayer magnetic coupling is greatly suppressed by Bi2Te3 layer intercalation. However, AFM is still the ground state in these compounds. Here by magnetic and transport measurements, we demonstrate that Sb substit…
Magnetic topological insulators (MTIs) have received a lot of attention due to the existence of various quantum phenomena such as quantum anomalous Hall effect (QAHE) and topological magnetoelectric effect, etc. The intrinsic superlattice-like layered MTIs, MnBi2Te4/(Bi2Te3)n, have been extensively investigated mainly through the transport measurements, while the direct investigation of their superlattice-sensitive magnetic behaviors is relatively rare. Here, we report a microscopic real-space investigation of magnetic phase behaviors in MnBi4Te7 using cryogenic magnetic force microscopy (MFM)…
Intrinsic magnetic topological insulator ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ is believed to be an axion insulator in its antiferromagnetic ground state. However, direct identification of axion insulators remains experimentally elusive because the observed vanishing Hall resistance, while indicating the onset of the axion field, is inadequate to distinguish the system from a trivial normal insulator. Using numerical Green's functions, we theoretically demonstrate the quantized magnetoelectric current in a tunnel junction of atomically thin ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ sandwiched be…
Intrinsic magnetic topological insulators (MTIs) MnBi2Te4 and MnBi2Te4/(Bi2Te3)n are expected to realize the high-temperature quantum anomalous Hall effect (QAHE) and dissipationless electrical transport. Extensive efforts have been made on this field but there is still lack of ideal MTI candidate with magnetic ordering of ferromagnetic (FM) ground state. Here, we demonstrate a MTI sample of Mn(Bi0.7Sb0.3)4Te7 which holds the coexistence of FM ground state and topological non-triviality. The dramatic modulation of the magnetism is induced by a charge carrier engineering process by the way of S…
$\mathrm{MnBi_2Te_4}$ has recently been established as an intrinsic antiferromagnetic (AFM) topological insulator and predicted to be an ideal platform to realize quantum anomalous Hall (QAH) insulator and axion insulator states. We performed comprehensive studies on the structure, nontrivial surface state and magnetotransport properties of this material. Our results reveal an intrinsic anomalous Hall effect arising from a non-collinear spin structure for the magnetic field parallel to the $c$-axis. We also observed remarkable negative magnetoresistance under arbitrary field orientation below …
Crystal growth of MnBi$_{2}$Te$_{4}$ has delivered the first experimental corroboration of the 3D antiferromagnetic topological insulator state. Our present results confirm that the synthesis of MnBi$_{2}$Te$_{4}$ can be scaled-up and strengthen it as a promising experimental platform for studies of a crossover between magnetic ordering and non-trivial topology. High-quality single crystals of MnBi$_{2}$Te$_{4}$ are grown by slow cooling within a narrow range between the melting points of Bi$_{2}$Te$_{3}$ (586 °C) and MnBi$_{2}$Te$_{4}$ (600 °C). Single crystal X-ray diffraction and electron m…
Quantum anomalous Hall effect (QAHE) has been experimentally realized in magnetically-doped topological insulators or intrinsic magnetic topological insulator MnBi$_2$Te$_4$ by applying an external magnetic field. However, either the low observation temperature or the unexpected external magnetic field (tuning all MnBi$_2$Te$_4$ layers to be ferromagnetic) still hinders further application of QAHE. Here, we theoretically demonstrate that proper stacking of MnBi$_2$Te$_4$ and Sb$_2$Te$_3$ layers is able to produce intrinsically ferromagnetic van der Waals heterostructures to realize the high-te…
Understanding the effects of interfacial modification to the functional properties of magnetic topological insulator thin films is crucial for developing novel technological applications from spintronics to quantum computing. Here, we report that a large electronic and magnetic response is induced in the intrinsic magnetic topological insulator MnBi2Te4 by controlling the propagation of surface oxidation. We show that the formation of the surface oxide layer is confined to the top 1-2 unit cells but drives large changes in the overall magnetic response. Specifically, we observe a dramatic reve…
Through a thorough magneto-transport study of antiferromagnetic topological insulator MnBi2Te4 (MBT) thick films, a positive linear magnetoresistance (LMR) with a two-dimensional (2D) character is found in high perpendicular magnetic fields and temperatures up to at least 260 K. The nonlinear Hall effect further reveals the existence of high-mobility surface states in addition to the bulk states in MBT. We ascribe the 2D LMR to the high-mobility surface states of MBT, thus unveiling a transport signature of surface states in thick MBT films. A suppression of LMR near the Neel temperature of MB…
Ultrathin films of intrinsic magnetic topological insulator MnBi2Te4 exhibit fascinating quantum properties such as quantum anomalous Hall effect and axion insulator state. In this work, we systematically investigate the evolution of the electronic structure of MnBi2Te4 thin films. With increasing film thickness, the electronic structure changes from an insulator-type with a large energy gap to one with in-gap topological surface states, which is, however, still drastically different from the bulk material. By surface doping of alkali-metal atoms, a Rashba split band gradually emerges and hybr…
The intrinsic antiferromagnetic topological insulator MnBi2Te4 provides an ideal platform for exploring exotic topological quantum phenomena. Recently, the Chern insulator and axion insulator phases have been realized in few-layer MnBi2Te4 devices at low magnetic field regime. However, the fate of MnBi2Te4 in high magnetic field has never been explored in experiment. In this work, we report transport studies of exfoliated MnBi2Te4 flakes in pulsed magnetic fields up to 61.5 T. In the high-field limit, the Chern insulator phase with Chern number C = -1 evolves into a robust zero Hall resistance…
The recent discovered antiferromagnetic topological insulators in 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 half-integer quantum Hall effect. Here, however, we show unambiguously by using high-resolution angle-resolved photoemission spectroscopy that a gapless Dirac cone at the (0001) surface of MnBi$_2$Te$_4$ exists between the bulk band gap. Such unexpected surface state remains unchanged across the bulk Néel temperature, and is even robust against se…
Using single crystal neutron diffraction, we present a systematic investigation of the crystal structure and magnetism of van der Waals topological insulators MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$, where rich topological quantum states have been recently predicted and observed. Structural refinements reveal that considerable Bi atoms occupied on the Mn sites in both materials, distinct from the previously reported antisite disorder. We show unambiguously that MnBi$_{2}$Te$_{4}$ orders antiferromagnetically below 24 K featured by a magnetic symmetry $R_I$-${3c}$ while MnBi$_{4}$Te$_{7}$ is antiferr…
Topological quantum materials coupled with magnetism can provide a platform for realizing rich exotic physical phenomena, including quantum anomalous Hall effect, axion electrodynamics and Majorana fermions. However, these unusual effects typically require extreme experimental conditions such as ultralow temperature or sophisticate material growth and fabrication. Recently, new intrinsic magnetic topological insulators were proposed in MnBi2Te4-family compounds - on which rich topological effects could be realized under much relaxed experimental conditions. However, despite the exciting progre…
Topological magnets are a new family of quantum materials providing great potential to realize emergent phenomena, such as quantum anomalous Hall effect and axion-insulator state. Here we present our discovery that stoichiometric ferromagnet MnBi8Te13 with natural heterostructure MnBi2Te4-(Bi2Te3)3 is an unprecedented half-magnetic topological 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 gapl…
The recent discovery of antiferromagnetic (AFM) topological insulator (TI) MnBi$_2$Te$_4$ has triggered great research efforts on exploring novel magnetic topological physics. Based on first-principles calculations, we find that the manipulation of magnetic orientation and order not only significantly affects material symmetries and orbital hybridizations, but also results in variant new magnetic topological phases in MnBi$_2$Te$_4$. We thus predict a series of unusual topological quantum phase transitions that are magnetically controllable in the material, including phase transitions from AFM…
The discovery of an intrinsic antiferromagnetic topological insulator (AFM-TI) in MnBi$_2$Te$_4$ has attracted intense attention, most of which lies on its electrical properties. In this paper, we report electronic, thermal, and thermoelectric transport studies of this newly found AFM-TI. The temperature and magnetic field dependence of its resistivity, thermal conductivity, and Seebeck coefficient indicate strong coupling between charge, lattice, and spin degrees of freedom in this system. Furthermore, MnBi$_2$Te$_4$ exhibits a large anomalous Nernst signal, which is associated with non-zero …
MnBi$_2$Te$_4$ (MBT) materials are promising antiferromagnetic topological insulators where 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 trivial magnetic insulator MnSb$_2$Te$_4$ (MST). We use high-field magnetization measurements to show that the magnetization of MBT and MST occur in stages a…
Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$, the $n=$~1 and 2 members of a modular (Bi$_2$Te$_3$)$_n$(MnBi$_2$Te$_4$) series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disent…
Recently, MnBi2Te4 has been discovered as the first intrinsic antiferromagnetic topological insulator (AFM TI), and will become a promising material to discover exotic topological quantum phenomena. In this work, we have realized the successful synthesis of high-quality MnBi2Te4 single crystals by solid-state reactions. The as-grown MnBi2Te4 single crystal exhibits a van der Waals layered structure, which is composed of septuple Te-Bi-Te-Mn-Te-Bi-Te sequences as determined by powder X-ray diffraction (PXRD) and high-resolution high-angle annular dark field scanning transmission electron micros…
Even-Odd Layer-Dependent Exchange Bias Effect in MnBi2Te4 Chern Insulator Devices
The Pennsylvania State University（宾夕法尼亚州立大学）；Nanjing University（南京大学）；Nankai University（南开大学）；National Institute for Materials Science（国立材料研究所）；Oak Ridge National Laboratory（橡树岭国家实验室）；University of California, Riverside（加州大学河滨分校）
Axion, first postulated as a hypothetical particle in high-energy physics, is now extended to describe a novel topological magnetoelectric effect derived from the Chern-Simons theory in condensed matter systems. The recent discovered intrinsic magnetic topological insulators MnBi2Te4 and its derivatives have attracted great attention because of their potential as a material platform to realize such a quantized axion field. Since the magnetic exchange gap can bring the half-quantized anomalous Hall effect at the surface, an axion insulator manifests as quantum anomalous Hall and zero Hall plate…
Here we present microscopic evidence of the persistence of uniaxial A-type antiferromagnetic order to the surface layers of MnBi$_2$Te$_4$ single crystals using magnetic force microscopy. Our results reveal termination-dependent magnetic contrast across both surface step edges and domain walls, which can be screened by thin layers of soft magnetism. The robust surface A-type order is further corroborated by the observation of termination-dependent surface spin-flop transitions, which have been theoretically proposed decades ago. Our results not only provide key ingredients for understanding th…
Recently, the intrinsic magnetic topological insulator MnBi2Te4 has attracted enormous research interest due to the great success in realizing exotic topological quantum states, such as the quantum anomalous Hall effect (QAHE), axion insulator state, high-Chern-number and high-temperature Chern insulator states. One key issue in this field is to effectively manipulate these states and control topological phase transitions. Here, by systematic angle-dependent transport measurements, we reveal a magnetization-tuned topological quantum phase transition from Chern insulator to magnetic insulator w…
Modification of the gap at the Dirac point (DP) in antiferromagnetic (AFM) axion topological insulator MnBi$_2$Te$_4$ and its electronic and spin structure has been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation with variation of temperature (9-35~K), light polarization and photon energy. We have distinguished both a large (62-67~meV) and a reduced (15-18~meV) gap at the DP in the ARPES dispersions, which remains open above the Néel temperature ($T_\mathrm{N}=24.5$~K). We propose that the gap above $T_\mathrm{N}$ remains open due to short-range ma…
Recently, natural van der Waals heterostructures of (MnBi2Te4)m(Bi2Te3)n have been theoretically predicted and experimentally shown to host tunable magnetic properties and topologically nontrivial surface states. In this work, we systematically investigate both the structural and electronic responses of MnBi2Te4 and MnBi4Te7 to external pressure. In addition to the suppression of antiferromagnetic order, MnBi2Te4 is found to undergo a metal-semiconductor-metal transition upon compression. The resistivity of MnBi4Te7 changes dramatically under high pressure and a non-monotonic evolution of \r{h…
Optical study of the charge dynamics evolution in the topological insulators MnBi$_2$Te$_4$ and Mn(Bi$_{0.74}$Sb$_{0.26}$)$_2$Te$_4$ under high pressure
The observation of quantized anomalous Hall conductance in the forced ferromagnetic state of MnBi2Te4 thin flakes has attracted much attentions. However, 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 electrical 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 separation layers Bi2Te3. MnBi4Te7 and MnBi6Te10 show weak anti…
In the newly discovered magnetic topological insulator MnBi$_2$Te$_4$, both axion insulator state and quantized anomalous Hall effect (QAHE) have been observed by tuning the magnetic structure. The related (MnBi$_2$Te$_4$)$_m$(Bi$_2$Te$_3$)$_n$ heterostructures with increased tuning knobs, are predicted to be a more versatile platform for exotic topological states. Here, we report angle-resolved photoemission spectroscopy (ARPES) studies on a series of the heterostructures (MnBi$_2$Te$_4$, MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$). A universal gapless Dirac cone is observed at the MnBi$_2$Te$_4$ t…
The combination of topology and magnetism is attractive to produce exotic quantum matters, such as the quantum anomalous Hall state, axion insulators and the magnetic Weyl semimetals. MnBi2Te4, as an intrinsic magnetic topological insulator, provides a platform for the realization of various topological phases. Here we report the intermediate Hall steps in the magnetic hysteresis of MnBi2Te4, where four distinguishable magnetic memory states at zero magnetic field are revealed. The gate and temperature dependence of the magnetic intermediate states indicates the noncollinear spin structure in …
We use high-resolution, tunable angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the electronic properties of single crystals of MnBi2Te4, a material that was predicted to be the first intrinsic antiferromagnetic (AFM) topological insulator. We observe both bulk and surface bands in the electronic spectra, in reasonable agreement with the DFT calculations results. In striking contrast to the earlier literatures showing a full gap opening between two surface band manifolds along (0001) direction, we observed a gapless Dirac cone remain …
As the first intrinsic antiferromagnetic topological insulator, MnBi$_2$Te$_4$ has provided a platform to investigate the interplay of band topology and magnetism as well as the emergent phenomena arising from such an interplay. Here we report the chemical-vapor-transport (CVT) growth and characterization of MnBi$_2$Te$_4$, as well as the observation of the field-induced quantized Hall conductance in 6-layer devices. Through comparative studies between our CVT-grown and flux-grown MnBi$_2$Te$_4$ via magnetic, transport, scanning tunneling microscopy, and angle-resolved photoemission spectrosco…
The topological electronic structure plays a central role in the non-trivial physical properties in topological quantum materials. A minimal, hydrogen-atom-like topological electronic structure is desired for researches. In this work, we demonstrate an effort towards the realization of such a system in the intrinsic magnetic topological insulator MnBi2Te4, by manipulating the topological surface state (TSS) via surface modification. Using high resolution laser- and synchrotron-based angle-resolved photoemission spectroscopy (ARPES), we found the TSS in MnBi2Te4 is heavily hybridized with a tri…
Recently, intrinsic antiferromagnetic topological insulator MnBi2Te4 has drawn intense research interest and leads to plenty of significant progress in physics and materials science by hosting quantum anomalous Hall effect, axion insulator state, and other quantum phases. An essential ingredient to realize these quantum states is the magnetic gap in the topological surface states induced by the out-of-plane ferromagnetism on the surface of MnBi2Te4. However, the experimental observations of the surface gap remain controversial. Here, we report the observation of the surface gap via the point c…
The layered van der Waals (vdW) material MnBi$_2$Te$_4$ is an intrinsic magnetic topological insulator with various topological phases such as quantum anomalous Hall effect (QAHE) and axion states. However, both the zero-field and high-temperature QAHE are not easy to realize. It is theoretically proposed that the exchange bias can be introduced in the MnBi2Te4/ferromagnetic (FM) insulator heterostructures and thus opens the surface states gap, making it easier to realize the zero-field or high-temperature QAHE. Here we report the electrically tunable exchange bias in the van der Waals MnBi$_2…
The magnetic topological insulator, MnBi$_2$Te$_4$, shows metallic behavior at zero magnetic fields (antiferromagnetic phase, AFM) in thin film transport, which coincides with gapless surface states observed by angle-resolved photoemission spectroscopy, while it can become a Chern insulator at field larger than 6 T (ferromagnetic phase, FM). Thus, the zero-field surface magnetism was once speculated to be different from the bulk AFM phase. However, recent magnetic force microscopy refutes this assumption by detecting persistent AFM order on the surface. In this work, we propose a mechanism rel…
