Session P59: MnBi2Te4: Spectroscopic Properties

Spin waves in the antiferromagnetic topological insulator MnBi2Te4

MnBi₂Te₄ is proposed to be the first antiferromagnetic (AF) topological insulator (TI). The natural intergrowth of magnetic and TI layers, and the ground state tunability via a magnetic field provide a unique platform for studying the interplay between magnetism and topological electronic states, which give rise to the quantum anomalous Hall effect and axion electrodynamics. Here we present results from our inelastic neutron scattering (INS) study on MnBi₂Te₄ single crystals, where we determined the strength of the magnetic interactions and anisotropy in the AF state (T_N = 24 K). By comparing our INS data to the dynamic spin susceptibility calculated using a linear-response density functional theory, we find that magnetic interactions extending further than the Mn nearest-neighbor distance are necessary to understand the observed spin wave dispersion.   

Electronic states and magnetic response of MnBi2Te4 surface by scanning tunneling microscopy and spectroscopy

The newly discovered intrinsic magnetic topological insulator, MnBi₂Te₄, has demonstrated novel quantum phenomena. Quantized Hall conductance and topological axion state have been realized in its thin flakes. However, the measured band structures show deviations from calculation results and the reason is still in debate. In our studies, low-temperature scanning tunneling microscopy experiment is carried out to investigate its surface states and magnetic response. Unexpected electronic structures are obtained from quasiparticle interference patterns on the topmost layer, which show deviations from the prediction of out-of-plane A-type antiferromagnetic structure. Such differences presumably originate from the re-orientation of magnetic moments near the surface. Moreover, 5% of Mn substitutions at Bi sites are observed. They not only give rise to fluctuating electronic structures, but also affect the magnetism of the material. Our findings shed new lights on the magnetic property of MnBi₂Te₄ and the design of magnetic topological insulators.   

Gapless surface Dirac cone and its possible origin in antiferromagnetic topological insulator MnBi2Te4

The recent discovered antiferromagnetic topological insulators in the Mn-Bi-Te family have rapidly drawn broad interest since its cleaved surface state is believed to be gapped, hosting the unprecedented axion states. However, we show by ARPES that a gapless Dirac cone at the (0001) surface of MnBi₂Te₄ exists inside the bulk band gap. Such unexpected surface state remains unchanged across the bulk Néel temperature, and is robust against severe surface degradation, indicating additional topological protection. Through symmetry analysis and ab initio calculations we consider different types of surface magneitc reconstruction as possible origins giving rise to such linear dispersion. Our results reveal that the intrinsic magnetic topological insulator hosts a rich platform to realize various topological phases, and thus pushed forward the comprehensive understanding of magnetic topological materials.   

MBE Synthesis and Characterization of Intrinsic Magnetic Topological Insulator MnBi2Se4

Intrinsic magnetic topological insulators are stoichiometric materials with both magnetic ordering and topological electronic states. Due to its inherent magnetism, this class of materials processes rich topological quantum phases that induced by symmetry breaking, while maintaining good crystallinity and high purity of the material. We report the MBE synthesis and characterization of an intrinsic magnetic topological insulator MnBi₂Se₄. The MnBi₂Se₄ thin films are deposited with alternating layer growth of Bi₂Se₃ and a-MnSe(111). The high quality of the material is shown with cross-sectional TEM. Dirac surface state is demonstrated with angle-resolved photoemission spectroscopy above its magnetic ordering temperature. SQUID magnetometry measurement on films with different thicknesses shows layered-antiferromagnetic ordering in this material. Low-field SQUID measurement also shows a hysteresis loop, with magnetic moments preferred to stay within the lattice plane. The ferromagnetic signal is attributed to uncompensated magnetic moments at the surface. Its magnetic characterization is also confirmed with XMCD measurement. Furthermore, magnetometry measurement also shows that the magnetic ordering in MnBi₂Se₄ persists down to the monolayer limit.   

Scanning tunneling microscopy and spectroscopy investigation of the antiferromagnetic topological insulator MnBi2Te4

The interplay of topology and magnetism within the same material can give rise to exotic electronic phases, such as the quantum anomalous Hall state and the axion insulator state. Recently discovered MnBi₂Te₄ is believed to be the first intrinsic antiferromagnetic topological insulator. Using low temperature scanning tunneling microscopy and spectroscopy, we study the surface of high-quality single crystals of MnBi₂Te₄. From STM topographs, we visualize the spatial distribution of accidental Mn substitutions in the topmost Te layer. Moreover, by imaging the scattering of electrons on the surface, we track the evolution of the electronic band structure and compare it with angle-resolved photoemission spectroscopy experiments. Our measurements provide a nanoscale insight into the chemical inhomogeneity and the electronic structure of this novel system.   

Strongly Gapped Topological Surface States in MnBi2Te4(Bi2Te3)n Family

The MnBi₂Te₄(Bi₂Te₃)_n family of materials are potential candidates for magnetic topological insulators (MTIs) that may show the Quantum Anomalous Hall Effect (QAHE). This class of materials are heterostructures of MnBi₂Te₄ and Bi₂Te₃ layers that are stacked together via van der Waals forces, with n number of Bi₂Te₃ layers interstitially placed between MnBi₂Te₄ layers. In this talk, we present Angular Resolved Photoemission Spectroscopy (ARPES) experiments on MnBi₄Te₇, MnBi₆Te₁₀, and MnBi₈Te₁₃, demonstrating the topological nature of each material. Furthermore, we deconvolve the surface spectra due to each possible surface termination for each stacking arrangement, demonstrating a rich transition from the gapless MnBi₂Te₄ surface, through a gapped Bi₂Te₃-like surface, and ending with an approximate pure Bi₂Te₃ surface.   

Competing Interactions in the Antiferromagnetic State of the Axion Insulator Candidate MnBi2Te4

A new class of materials, antiferromagnetic topological insulators, has been predicted to realize the bulk axion insulator with a large and in some cases quantized magneto-electric coefficient. Hexagonal MnBi₂Te₄ is a candidate material and here we report neutron scattering experiments conducted at the NIST Center for Neutron Research to understand its magnetism. We confirm A-type antiferromagnetic order (alternating FM basal planes with spins along the hexagonal axis) with T_N=24 K. Inelastic scattering reveals an excitation spectrum with a bandwidth of 4.5 meV and gap △<0.6 meV. Modelling these data using spin-wave theory, we obtain an effective spin Hamiltonian for MnBi₂Te₄ that features competing ferro- and antiferromagnetic interactions within the basal plane.   

Investigation on the topological surface states of a superlattice-like magnetic topological insulator MnBi4Te7

The recently verified intrinsic magnetic topological insulator MnBi₂Te₄ has attracted tremendous research interest, as it is considered to be a promising platform for emergent quantum phenomena such as the quantum anomalous Hall effect, and the axion insulator state, etc. In a superlattice-like bulk of alternating MnBi₂Te₄ and Bi₂Te₃ layers, the antiferromagnetic exchanged coupling can be effectively weakened. Here, we investigate the surface states of MnBi₄Te₇ crystal via angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) techniques. Two different band structures are observed, both showing the Dirac point located at ~280mV below the Fermi level and gapless feature at Dirac point. The two different band structures can be associated with the MnBi₂Te₄-terminated and Bi₂Te₃-terminated surfaces by STM study. We further analyze the quasi-particle interference scattering at various energies, which exhibit distinct Γ-M features corresponding to the scattering of topological surface states, showing good correspondence to ARPES data. Our results can help to understand the electronic structure and related phenomena in the emerging intrinsic magnetic topological insulators.   

Dynamic magnetic properties of a magnetic topological insulator material MnBi4Te7

A van der Waals compound MnBi₄Te₇ belongs to the family of (Bi₂Te₃)_n(MnBi₂Te₄), (n = 0, 1, 2) heterostructures and is a candidate magnetic topological insulator [1]. It is the first magnetic material that features both, the intrinsic net magnetization and a band inversion. Static magnetic susceptibility (χ) and magnetization (M) measurements as a function of the applied field (H) on MnBi₄Te₇ single-crystals show an antiferromagnetic state at T_N = 13 K and a ferromagnetic-like hysteresis occurring upon cooling below 5 K [1]. We performed electron spin resonance (ESR) spectroscopy measurements in wide frequency and temperature ranges to explore the dynamic magnetic properties of MnBi₄Te₇. From high-frequency ESR measurements, we obtain evidence that MnBi₄Te₇ is an easy-axis type ferromagnet and ferromagnetic spin correlations persist up to T = 30 K on the time scale of an ESR experiment (10^-10 - 10^-11 s).

[1] Raphael C. Vidal et. al, Topological electronic structure and intrinsic magnetization in MnBi₄Te₇: a Bi₂Te₃-derivative with a periodic Mn sublattice, arXiv:1906.08394.   

Topological Electronic Structure and Its Temperature Evolution in Antiferromagnetic Topological Insulators

Magnetic topological insulators represent a novel state of topological quantum materials with unique blends of non-trivial band topology and magnetism. Recently, Intrinsic magnetic topological insulator MnBi₂Te₄ is shown to exhibit rich topological effects such as quantum anomalous Hall effect and axion electrodynamics, which attracts tremendous research interests. Here, we carried out comprehensive and high-resolution angle-resolved photoemission spectroscopy studies on MnBi₂Te₄, and identified its topological electronic structure. In contrast to theoretical predictions and previous studies, we observe topological surface states with a diminished gap forming a characteristic Dirac cone. In addition, the temperature evolution of the energy bands reveals their interplay with the magnetic phase transition by showing interesting differences between the bulk and surface states, respectively. Our results provide important insights into not only the exotic properties of MnBi₂Te₄, but also the generic understanding of the interplay between magnetism and topological electronic structure in magnetic TQMs.   

Characterization of the Layered Antiferromagnetic Topological Insulator MnBi2Se4 Using Scanning Tunneling Microscopy

Recently, topological insulators have been an area of interest due to the existence of symmetry protected states. Those with broken time reversal symmetry are particularly interesting because they have been predicted to host several intriguing phenomena such as the quantum anomalous hall effect, magneto-electric effects, and topologically protected surface and edge states. MnBi2Se4 (MBS) is a predicted topological insulator formed of van der Waals separated septuple layers (SL) with a layered anti-ferromagnetic structure, which breaks time-reversal symmetry and could therefore potentially realize some of those phenomena. For this study, a Se capped 20SL MBS film was grown on a sapphire substrate using molecular beam epitaxy. After other measurements were taken ex-situ the sample was then de-capped and transferred via vacuum suitcase to a low-temperature (5K) scanning tunneling microscope (STM). In our STM images, we observed a triangular lattice with an average lattice constant of 3.85Å. Additionally, we see a semiconducting nature in scanning tunneling spectroscopy with a band gap of approximately 700meV.   

Engineering MnBi2Te4 thin films by chemical dopants and tailoring structure

As the first realized instrisic magnetic topological insulators, MnBi₂Te₄ (MBT) has been extensivly studied and shows rich novel trasnport properties and magnetic properties. As MBT is layered material with antifferomagnetic coupling between adjacent layers, it is a challenge to realize the quantum anomalous Hall effect in MBT. With molecular beam epitaxy technique,we prepared MBT thin films and tried to modify the magnetic coupling by constructing multilayer structures and co-doping method. In this work we systematically studied the magneto-electrical transport properties with the bottom gate tuning method. In the multilayer films stacked with other magnetically doped topological insulators, we observed a diversity behaviors of magnetresistance and Hall effect in different samples, which are likely due to the varible magnetic orders. Besides, we also discovered an evident evolution in Sb-doped MBT films with varying the ratio of Bi/Sb. Our study indicates that the magnetic order can be tuned in MBT based films and it paves a promissing way to control different exotic topological states.   

Surface magnetism of antiferromagnetic topological insulator MnBi2Te4

MnBi₂Te₄ is a promising candidate of antiferromagnetic (AFM) topological insulator which might host fascinating quantized anomalous Hall effect or axion insulator state in few molecular layers^1,2. These quantum phenomena are induced by the mass gap on Dirac surface states opened by the A-type AFM order (alternating ferromagnetic layers)³ with uniaxial anisotropy in MnBi₂Te₄. However, recent high resolution angle resolved photoemission spectroscopy studies reveal gapless surface states, indicating paramagnetic or non-uniaxial A-type AFM orders^4–7. Yet there is no direct evidence of such surface magnetism. Recently we demonstrated magnetic imaging of AFM domain walls in MnBi2Te4 family using our homemade cryogenic magnetic force microscope (MFM)⁸. In this talk, we will present MFM studies of MnBi2Te4 single crystals to address the nature of surface magnetism.

References
1. Otrokov, M. M. et al. PRL 122, 107202 (2019).
2. Gong, Y. et al. CPL 36, 076801 (2019).
3. Yan, J.-Q. et al. PRM 3, 064202 (2019).
4. Li, H. et al. arXiv:1907.06491 (2019).
5. Swatek, P. et al. arXiv:1907.09596 (2019).
6. Hao, Y.-J. et al. arXiv:1907.03722 (2019).
7. Chen, Y. J. et al. arXiv:1907.05119 (2019).
8. Sass, P. M. et al. arXiv:1910.06488 (2019).   

Quantum transport in a dual-gated antiferromagnetic topological insulator

Magnetic topological insulator provides an exciting arena to investigate the interplay between magnetism and spin-momentum-locked Dirac surface state [1]. The recent discovery of the intrinsic antiferromagnetic topological insulators, MnBi₂Te₄, MnBi₄Te₇, and MnBi₆Te₁₀ has enabled us to study the quantum anomalous Hall effect and axion insulator in exfoliated crystals [2,3]. Especially interesting is the electric-field effect on the crystals due to the expected finite topological magnetoelectric effect. We here report the quantum transport properties of dual-gated antiferromagnetic topological insulator devices, which allow the independent control of the carrier density and the electric field.

[1] Y. Tokura, K. Yasuda, A. Tsukazaki, Nat. Rev. Phys. 1, 126–143 (2019) [2] Y. Deng et al., arXiv 1904.11468 [3] C. Liu et al., arXiv 1905.00715   

Realizing intrinsic magnetic topological insulators with gapped topological surface states in MnBi2-xSbxTe4

The interplay between magnetism and topology of materials could create a variety of exotic topological quantum states, including the quantum anomalous Hall (QAH) effect showing dissipationless chiral edge states, the topological axion states displaying quantized magnetoelectric effects, and Majorana fermions, obeying non-Abelian statistics. All of these quantum states are hopefully realizable in MnBi₂Te₄. However, the Fermi level is usually in the bulk conduction band of the materials, which is not ideal for revealing these topological properties in transport. Here we show that by doping MnBi2Te4 with Sb, the Fermi level can be shifted while the non-trivial topology is maintained. Scanning tunneling microscopy (STM) is utilized to observe electronic band structure by quasi-particle interference, which reveals surface states in the bulk band gap and a surface band gap at the Dirac point. The transport from multiprobe-STM further displays a surface dominant transport. Our results show that the MnBi_2-xSb_xTe₄ is an intrinsic magnetic topological insulator that would offer an ideal platform to observe various exotic topological phenomena.