Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session M72: Mn-Te Magnetic Topology IIIFocus Recordings Available
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Sponsoring Units: DMP GMAG DCMP Chair: Max Hirschberger, University of Tokyo; Jun Xiao, University of Wisconsin - Madison Room: Hyatt Regency Hotel -Jackson Park D |
Wednesday, March 16, 2022 8:00AM - 8:36AM |
M72.00001: High Chern Number Quantum Anomalous Hall Effect in Magnetic Topological Insulator Multilayers Invited Speaker: Yifan Zhao The quantum anomalous Hall (QAH) effect is a two-dimensional topological insulating state that has quantized Hall resistance of h/Ce2 and vanishing longitudinal resistance under zero external magnetic field, where C is called the Chern number. The resistance-free chiral edge current of the QAH insulators has been predicted to have potential applications for next-generation energy-efficient electronics and spintronics as well as topological quantum computation. The QAH effect was first realized in chromium(Cr)- doped topological insulator (TI) thin films in 2013 [1] and soon after in vanadium (V)-doped TI thin films [2]. More recently, the QAH effect has also been realized in exfoliated intrinsic magnetic TI, MnBi2Te4 flakes with odd number layers [3], and manually assembled moiré graphene [4] or transition metal dichalcogenides [5]. In my talk, I will introduce our recent progress on the high Chern number QAH effect in magnetic TI multilayers. We used molecular beam epitaxy (MBE) to grow magnetic TI/TI multilayers and realized the QAH effect with tunable Chern number C from 1 to 5. We found that the Chern number C can be tuned by varying either the magnetic doping concentration or the thickness of the interior magnetic TI layers [6]. We leveraged this progress to further synthesize a series of magnetic TI/TI penta-layer heterostructures with different Cr doping concentrations and observed a plateau-to-plateau phase transition between the C = 1 to C = 2 QAH insulators under zero magnetic field [7]. |
Wednesday, March 16, 2022 8:36AM - 8:48AM |
M72.00002: Quantum Imaging of Magnetic Phase Transitions in Intrinsic Magnetic Topological Nanoflakes Mengqi Huang, Chaowei Hu, Nathan J McLaughlin, Shu Zhang, Hanyi Lu, Hailong Wang, Yaroslav Tserkovnyak, Ni Ni, Chunhui R Du Recently, the family of intrinsic topological magnet MnBi2Te4(Bi2Te3)n has emerged as an attractive material platform to explore topologically protected quantum spin and charge transport behaviors in a broad magnetic field and temperature regime. Examples include the high-temperature quantum anomalous Hall effect, layer Hall effect, axion insulator phase, and many others. Establishment of a detailed knowledge of the local magnetic properties of MnBi2Te4(Bi2Te3)n contributes to a comprehensive understanding of the observed exotic quantum transport behaviors. Here, we introduce nitrogen-vacancy (NV) centers as single-spin sensors to directly image the local static and dynamic spin properties of MnBi2Te4(Bi2Te3)n nanoflakes, revealing the detailed process of the magnetic phase transition in nanometer length scale. We also reveal the intriguing physics underlying the spin transport in MnBi2Te4(Bi2Te3)n. Our results illustrate the unique capability enabled by NV centers for investigating the interplay between magnetism and topology potentially in broad range of quantum materials. |
Wednesday, March 16, 2022 8:48AM - 9:00AM |
M72.00003: Electronic structure and magnetism in MnBi2Te4: a diffusion Monte Carlo and DFT Study Michael C Bennett, Panchapakesan Ganesh, Anh Pham, Jaron T Krogel, Guangming Wang The layered vdW MnBi2Te4 (MBT) material has attracted substantial attention recently due to claims that it is the first observed Z2 AFM topological insulator. Experimental measurements show that MBT has A-type AFM magnetic structure with intralayer FM order and interlayer AFM order. Some magnetic and electronic properties, however, remain unclear, for instance, measured local magnetic moments on the Mn site are widely scattered (4-6 μB) and calculated DFT band gaps are sensitive to model parameters and therefore require tuning to experiment. In this work, we use the benchmarking fixed-node diffusion Monte Carlo (FNDMC) method to shed light on these properties. By integrating angular averages of the FNDMC spin-densities at the Mn sites with optimized nodes, we provide predictive insights into the local moment. Additionally, we exploit the variational property of FNDMC to parameterize DFT models that require no experimental inputs to estimate an ab initio band gap that agrees with measured values for this topological material. |
Wednesday, March 16, 2022 9:00AM - 9:12AM |
M72.00004: Electric control of a canted-antiferromagnetic Chern insulator Jiaqi Cai, Dmitry Ovchinnikov, Zaiyao Fei, Minhao He, Tiancheng Song, Zhong Lin, Chong Wang, David H Cobden, Jiun-Haw Chu, Yongtao Cui, Cui-Zu Chang, Di Xiao, Jiaqiang Yan, Xiaodong Xu Nontrivial band topology and magnetic order meet in a topological magnet to produce novel topological phase of matters, such as quantum anomalous Hall insulator. While prior works have focused on ferromagnetic systems, little is known about band topology and its manipulation in antiferromagnets. Here, we report that MnBi2Te4 realizes a canted-antiferromagnetic (cAFM) Chern insulator with electrical control. We show that the Chern insulator state with C = 1 appears as soon as the AFM to canted-AFM phase transition happens. Near the phase boundary of cAFM and AFM phases, an electric field can switch the transport between dissipative or dissipationless transport. Our dual-gated RMCD measurement rules out magnetoelectric effects as the tuning mechanism and suggests a picture that electric field controls the relative alignment of the magnetic exchange gap between top and bottom surfaces. |
Wednesday, March 16, 2022 9:12AM - 9:24AM |
M72.00005: Achieving charge neutrality in the intrinsic topological antiferromagnet by hydrogenation. Entela Buzi, Ayesha Lakra, Haiming Deng, Lukas Zhao, Xiaxin Ding, Jiaqiang Yan, Lia Krusin-Elbaum Magnetism combined with the nontrivial band topology by breaking time reversal symmetry will gap out otherwise gapless topological surface states, allowing novel topological quantum phases, such as quantum anomalous Hall or axion insulator, to emerge. The newly discovered intrinsic topological magnets in the MnBi2Te4 class are potentially able to host new topological phases at higher temperatures, however bulk conduction in these materials is high (in the 1020 /cc range). Alloying Bi with Sb can provide charge compensation but this process is known to alter the bandstructure of MnBi2-xSbxTe4. Here we show that charge compensation can be achieved continuously by hydrogenation without any changes in the bandstructure. We demonstrate ambipolar conduction and reversible tuning of the Fermi level EF across the charge neutral point (CNP) in the 80 nm thick antiferromagnetic MnBi2-xSbxTe4 using ionic hydrogen [1]. Near the CNP we explore surface/edge transport by finetuning EF using electrostatic gating — transport under various hydrogenation conditions will be discussed. The technique shows promise for harnessing emergent topological quantum phenomena, such as axion physics, as well as higher-order topological orders. |
Wednesday, March 16, 2022 9:24AM - 9:36AM |
M72.00006: Layer Hall effect in a 2D topological axion antiferromagnet Anyuan Gao Whereas ferromagnets have been known and used for millennia, antiferromagnets were only discovered in the 1930. At a large scale, antiferromagnets may seem to behave like any non-magnetic material. At the microscopic level, however, the opposite alignment of spins forms a rich internal structure. In topological antiferromagnets, this internal structure leads to the possibility that the property known as the Berry phase can acquire distinct spatial textures. Here we show this possibility in an antiferromagnetic axion insulator—even-layered, two-dimensional MnBi2Te4—in which spatial degrees of freedom correspond to different layers. We observe a type of Hall effect—the layer Hall effect—in which electrons from the top and bottom layers spontaneously deflect in opposite directions. Specifically, under zero electric field, even-layered MnBi2Te4 shows no anomalous Hall effect. However, applying an electric field leads to the emergence of a large, layer-polarized anomalous Hall effect of about 0.5e2/h (where e is the electron charge and h is Planck’s constant). This layer Hall effect uncovers an unusual layer-locked Berry curvature, which serves to characterize the axion insulator state. Moreover, we find that the layer-locked Berry curvature can be manipulated by the axion field, E·B. Our results offer new pathways to detect and manipulate the internal spatial structure of fully compensated topological antiferromagnets. |
Wednesday, March 16, 2022 9:36AM - 9:48AM |
M72.00007: First-Principles Investigation of Disorder in an Antiferromagnetic Topological Insulator Amoolya Grandhi Topological insulators host protected edge and surface states due to the topology of the bulk band structure, and because of this are considered robust to disorder. They have great potential for improving the function and energy efficiency of applications ranging from microelectronics to quantum computing. The classification of topological materials has strongly been influenced by the presence of crystalline symmetries, however recent work suggests that both disordered and amorphous materials can host topological phases. Using first-principles calculations and model Hamiltonians we explore the effects of systematically breaking crystal symmetries on the magnetic, electronic, and topological structure of the antiferromagnetic topological insulator (AFTI) MnBi2Te4. Finally, we will discuss how disorder can be used to tune different phases in magnetic topological materials, with a focus on AFTIs. |
Wednesday, March 16, 2022 9:48AM - 10:00AM |
M72.00008: Cryogenic Raman Spectroscopy with 0.5 cm-1 Resolution of Few-Layer MnBi2Te4 and MnBi4Te7 Madeline K Taylor, Jin Ho Kang, Yujin Cho, Chee Wei Wong, Liangbo Liang, Ni Ni, Chaowei Hu, Alexander A Puretzky MnBi2Te4 is the first experimentally demonstrated intrinsic antiferromagnetic topological insulator. Its layered structure enables us to tune the physical properties by inserting an additional Bi2Te3 layer between adjacent MnBi2Te4 layers to create the heterostructure MnBi4Te7. High resolution Raman spectroscopy allows us to observe the inter- and intra-layer phonon modes of MnBi2Te4 and MnBi4Te7. In this study, we discuss the detailed phonon modes of few-layer MnBi2nTe3n+1 (n = 1,2) at cryogenic temperatures and different polarization configurations, through Raman spectroscopy with a resolution less than 0.5 cm-1. The few-layer MnBi2nTe3n+1 topological heterostructures are prepared through exfoliation, and cross-verified through scanning probe microscopy. Via cross- and co-polarized pump excitation, the family of dynamical modes including the Eg3, A1g1, A1g2, and A1g3 resonances among others are identified with high-resolution. Together with first-principles density functional theory computations, peak positions and linewidths are observed, as well as interlayer Davydov splitting of the A1g3 peak of the few-layer MnBi4Te7. Comparing the few-layer and bulk samples, the Raman intensities of the thin sample are significantly higher (up to ~6x) than that of the bulk samples enabling us to see the changes in phonon modes with polarization and temperature dependencies such as the Davydov splitting. Additionally, temperature dependent measurements from 4K to 294K of the few-layer sample demonstrate a trend of increasing intensities and left-shift of the Raman peaks. |
Wednesday, March 16, 2022 10:00AM - 10:12AM |
M72.00009: Spin dynamics in the antiferromagnetic topological insulator MnBi4Te7 Bing Li, Simon X Riberolles, Daniel M Pajerowski, Jiaqiang Yan, Robert J McQueeney MnBi2+2nTe4+3n are promising topological insulators (TI) where the natural intergrowth of magnetic layers and TI layers provides a unique platform for studying the interplay between magnetism and topological electronic states. Here we present results from our inelastic neutron scattering study on single crystals MnBi4Te7. The spin waves at base temperature can be described by a Heisenberg model with intra-layer interactions and the single-ion anisotropy. Through comparison to the spin waves in MnBi2Te4, we discuss the changes in the intra- and inter-layer magnetic interactions and the single-ion anisotropy across the Mn-Bi-Te series. At higher temperatures, MnBi4Te7 shows quasielastic scattering at magnetic peak positions below and above Neel temperature TN, which indicates the existence of two-dimensional magnetic correlations. We extract the correlation lengths and the damping parameters at various temperatures and discuss the influence on magnetism due to the reduced dimensions. |
Wednesday, March 16, 2022 10:12AM - 10:24AM |
M72.00010: Evidence of a ferromagnetic Weyl semimetal in Mn(Bi,Sb)4Te7 Yingdong Guan, Seng Huat Lee, Zhiqiang Mao Abstract: |
Wednesday, March 16, 2022 10:24AM - 10:36AM |
M72.00011: Direct visualization of surface spin flip transition in MnBi4Te7 Wenbo Ge, Jinwoong Kim, David Vanderbilt, Jiaqiang Yan, Weida Wu The intrinsic antiferromagnetic topological insulator family MnBi2nTe3n+1 (MBT) provides an ideal platform to realize exotic quantum transport such as QAHE and Axion insulator state1,2. Here, we focus to the n=2 member, MnBi4Te7, which consists of alternating MnBi2Te4 and Bi2Te3 layers with smaller interlayer coupling. Using cryogenic magnetic force microscopy, we observed termination-dependent magnetic contrast across both the domain wall and the step confirming the persistence of A-type order at the surface. Furthermore, we discovered a first-order surface spin flip transition in MnBi4Te7, which has never been reported in natural antiferromagnet. Our observation can be explained by a revised Mills’ Model with reduced exchange interaction and enhanced surface magnetization. Direct visualization of surface spin-flip transition not only initiates the exploration of surface transition in potential functional antiferromagnets, but also pave the way for realizing quantized transport in ultra-thin films of MnBi4Te7 and other MBT superlattices. |
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