MnBi$_2$Te$_4$ has recently been the subject of intensive study, due to the prediction of axion insulator, Weyl semimetal, and quantum anomalous Hall insulator phases, depending on the structure and magnetic ordering. Experimental results have confirmed some aspects of this picture, but several experiments have seen zero-gap surfaces states at low temperature, in conflict with expectations. In this work, we develop a first-principles-based tight-binding model that allows for arbitrary control of the local spin direction and spin-orbit coupling, enabling us to accurately treat large unit-cells. Using this model, we examine the behavior of the topological surface state as a function of temperature, finding a gap closure only above the Néel temperature. In addition, we examine the effect of magnetic domains on the electronic structure, and we find that the domain wall zero-gap states extend over many unit-cells. These domain wall states can appear similar to the high temperature topological surface state when many domain sizes are averaged, potentially reconciling theoretical results with experiments.
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…
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…
Surface magnetism and its correlation with the electronic structure are critical to understand the gapless topological surface state in the intrinsic magnetic topological insulator MnBi$_2$Te$_4$. 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 MnBi$_2$Te$_4$ is the same as its bulk state. The coexistence of surface ferromagnetism and a gapless TSS uncovers t…
We predict a fast domain wall (DW) motion induced by a thermal gradient across a nanoscopic ferromagnetic stripe of MnBi. The driving mechanism is an exchange torque fueled by magnon accumulation at the DWs. Depending on the thickness of the sample, both hot-to-cold and cold-to-hot DW motion directions are possible. The finding unveils an energy efficient way to manipulate DWs as an essential element in magnetic information processing such as racetrack memory.
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 …
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…
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…
Combinations of non-trivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances material candidates are emerging. Yet, a compound with a band-inverted electronic structure and an intrinsic net magnetization remains unrealized. MnBi$_2$Te$_4$ is a candidate for the first antiferromagnetic topological insulator and the progenitor of a modular (Bi$_2$Te$_3$)$_n$(MnBi$_2$Te$_4$) series. For $n$ = 1, we confirm a non-stoichiometric…
Magnetic topological insulators (TIs) with nontrivial topological electronic structure and broken time-reversal symmetry exhibit various exotic topological quantum phenomena. The realization of such exotic phenomena at high temperature is one of central topics in this area. We reveal that MnBi6Te10 is a magnetic TI with an antiferromagnetic ground state below 10.8 K whose nontrivial topology is manifested by Dirac-like surface states. The ferromagnetic axion insulator state with Z4 = 2 emerges once spins polarized at field as low as 0.1 T, accompanied with saturated anomalous Hall resistivity …
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 …
Intrinsic magnetic topological insulators from MnBi2Te4 family promise realization of axion insulator, a theoretically predicted state of matter capable to mutually convert electric and magnetic signals, and whose properties are reminiscent of cosmological axion dark matter. Here we report the first experimental realization of room-temperature axionic single-spin switch in topological metamaterial MnBi2Te3. The metamaterial was in situ created in the scanning tunneling microscopy (STM) experiment by removing Te atomic layer from MnBi2Te4 surface using STM tip. Room temperature STM study of MnB…
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…
Motivated by fine tuning of the magnetic and topological properties of MnBi$_2$Te$_4$ via defect engineering, in this work, we report the crystal growth of MnBi$_2$Te$_4$ and related compounds using vapor transport method and characterization of vapor transported crystals by measuring elemental analysis, magnetic and transport properties, and scanning tunneling microscopy. For the growth of MnBi$_2$Te$_4$ single crystals, I$_2$, MnI$_2$ , MnCl$_2$, TeCl$_4$, or MoCl$_5$ are all effective transport agents; chemical transportation occurs faster in the presence of iodides than chlorides. MnBi$_2$…
Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall (QAH) effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical 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 saturation field of ~ 0.22 T at 2 K. Our angle-resolved photoemission spect…
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…
Observation of the quantum anomalous Hall effect (QAHE) in MnBi$_2$Te$_4$ flakes is one of the most exciting results in the study of the intrinsic magnetic topological insulator MnBi$_2$Te$_4$ and related compounds. This fascinating result is yet to be reproduced two years after its first report. The quality of starting MnBi$_2$Te$_4$ single crystals is believed to be the key factor. An interesting and important question to address is what is the right quality to enable the QAHE. In this perspective, we present possible approaches to tuning the magnetic and topological properties of MnBi$_2$Te…
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…
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…
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…
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…
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)…
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…
With infrared spectroscopy we studied the bulk electronic properties of the topological antiferromagnet MnBi$_2$Te$_4$ with $T_N \simeq 25~\mathrm{K}$. With the support of band structure calculations, we assign the intra- and interband excitations and determine the band gap of $E_g \approx$ 0.17 eV. We also obtain evidence for two types of conduction bands with light and very heavy carriers. The multiband free carrier response gives rise to an unusually strong increase of the combined plasma frequency, $ω_{\mathrm{pl}}$, below 300 K. The band reconstruction below $T_N$, yields an additional in…
Magnetism in topological materials creates phases exhibiting quantized transport phenomena with applications in spintronics and quantum information. The emergence of such phases relies on strong interaction between localized spins and itinerant states comprising the topological bands, and the subsequent formation of an exchange gap. However, this interaction has never been measured in any intrinsic magnetic topological material. Using a multimodal approach, this exchange interaction is measured in MnBi2Te4, the first realized intrinsic magnetic topological insulator. Interrogating nonequilibri…
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…
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…
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…
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-…
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…
We propose MnBi$_{2n}$Te$_{3n+1}$ as a magnetically tunable platform for realizing various symmetry-protected higher-order topology. Its canted antiferromagnetic phase can host exotic topological surface states with a Möbius twist that are protected by nonsymmorphic symmetry. Moreover, opposite surfaces hosting Möbius fermions are connected by one-dimensional chiral hinge modes, which offers the first material candidate of a higher-order topological Möbius insulator. We uncover a general mechanism to feasibly induce this exotic physics by applying a small in-plane magnetic field to the antifer…
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…
Two-dimensional van der Waals MnBi$_{2n}$Te$_{3n+1}$ (n = 1, 2, 3, 4) compounds have been recently found to be intrinsic magnetic topological insulators rendering quantum anomalous Hall effect and diverse topological states. Here, we summarize and compare the crystal and magnetic structures of this family, and discuss the effects of chemical composition on their magnetism. We found that a considerable fraction of Bi occupies at the Mn sites in MnBi$_{2n}$Te$_{3n+1}$ (n = 1, 2, 3, 4) while Mn is no detectable at the non-magnetic atomic sites within the resolution of neutron diffraction experime…
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…
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…
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…
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 MnBi$_6$Te$_{10}$. In the ferromagnetic phase, an energy gap of 15 meV is resolved at the Dirac point on the MnBi$_2$Te$_4$ termination. In contrast, an…
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 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…
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)…
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…
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…
MnBi2nTe3n+1(MBT) is the first intrinsic magnetic topological insulator and is promising to hostemergent 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 ferromag-netic (FM) MBT compounds, including MnBi8Te13and Sb doped MnBi2nTe3n+1(n= 2,3) usingAC susceptibility and magneto-optical imaging. Slow relaxation behavior is observed in all threecompounds, suggesting its universality among FM MBT. We attribute the origin of the relaxationbehavior to the irreversible domain movements…
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…
In magnetic topological insulators (TIs), the interplay between magnetic order and nontrivial topology can induce fascinating topological 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 topological quantum phenomena. In despite of intensive efforts, experimental evidence of the topological surface states (SSs) remains elusive. Here, by com…
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 …
Topological surface states with intrinsic magnetic ordering in the MnBi$_2$Te$_4$(Bi$_2$Te$_3$)$_n$ compounds have been predicted to host rich topological phenomena including quantized anomalous Hall effect and axion insulator state. Here we use scanning tunneling microscopy to image the surface Dirac fermions in MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$. We have determined the energy dispersion and helical spin texture of the surface states through quasiparticle interference patterns far above Dirac energy, which confirms its topological nature. Approaching the Dirac point, the native defects in the …
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…
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…
