Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session K43: Transport and Edge States in Magnetic Topological InsulatorsFocus

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Sponsoring Units: DMP Chair: Kin Fai Mak, Cornell University Room: Room 317 
Tuesday, March 7, 2023 3:00PM  3:12PM 
K43.00001: HighEfficiency SpinOrbit Torque in Antiferromagnetic Topological Insulator MnBi2Te4 Junyu Tang, Ran Cheng We formulate and quantify the spinorbit torque (SOT) in intrinsic antiferromagnetic topological insulator MnBi2Te4 of a few septuplelayer (SL) thick, which arises from electric fields (or voltages) rather than charge currents and exhibits a conspicuous evenodd SLnumber pattern. The predicted SOT is demonstrated in the SOTdriven magnetic resonance, where we identify a peculiar exchange mode in the triSL case that is blind to microwave electromagnetic fields but can only be excited by the SOT. As the reciprocal effect of the SOT, topological charge pumping generates an adiabatic current devoid of Joule heating with the onset of exchange resonance. Thanks to the absence of charge currents, MnBi2Te4 can be operated as an adiabatic quantum motor through pure voltage drives, achieving a theoretical mechanical efficiency of 100%. 
Tuesday, March 7, 2023 3:12PM  3:24PM 
K43.00002: Fieldlike and Dampinglike SpinOrbit Torque Efficiencies at an Interface Between NiFe and Bi_{2}Se_{3} Xiaohang Zhang, Connie H Li, Jisoo Moon, Enrique Cobas, Mehmet Noyan, Mark I Lohmann, Berend T Jonker, Olaf M van 't Erve An electrical current driven through a topological insulator can simultaneously induce a fieldlike spinorbit torque (SOT) and a dampinglike SOT in a proximitized ferromagnetic layer. Depending on the magnetic anisotropy, the fieldlike or dampinglike SOT may further lead to an inplane or outofplane magnetization reversal in the ferromagnetic layer. Therefore, it is an important aspect to determine the efficiencies of the two SOTs in order to realize magnetization manipulation in a ferromagnet/topologicalinsulator bilayer structure. In this work, we systematically studied the second harmonic Hall effect, magnetoresistance, ferromagnetic resonance of a NiFe/Bi_{2}Se_{3} sample. Taking the current shunting effect due to the metallic ferromagnetic layer into account, we obtained a fieldlike SOT efficiency of ~ 0.38 and a dampinglike SOT of ~ 0.06 for the bilayer structure. 
Tuesday, March 7, 2023 3:24PM  3:36PM 
K43.00003: SurfaceDriven Evolution of the Anomalous Hall Effect in MnBi2Te4 Thin Films: Implications for the Quantum Anomalous Hall Effect Matthew Brahlek, Alessandro R Mazza, Jason M Lapano, Harry M Meyer, Christopher T Nelson, YunYi Pai, Benjamin J Lawrie, Timothy R Charlton, Robert G Moore, Zac Ward, MaoHua Du, Gyula Eres Understanding interfacial modification of functional properties of magnetic topological insulator thin films is crucial for designing and manipulating properties and ultimately technological applications. This is especially critical for Atype antiferromagnets such as MnBi2Te4 where the magnetic compensation is dependent upon the thickness. In this talk I will discuss the large electronic and magnetic response that is induced in MnBi2Te4 by controlling the propagation of surface oxidation. This oxide is strongly confined to the top layer yet drives a dramatic reversal of the sign of the anomalous Hall effect driven by finite thickness magnetism, which indicates that the film splits into distinct magnetic layers each with a unique electronic signature. This suggests that magnetic perturbations resulting from surface oxidation may play a nontrivial role in the stabilization of the quantum anomalous Hall effect in this system and that understanding targeted modifications to the surface may open new routes for engineering novel topological and magnetic responses in this fascinating material. 
Tuesday, March 7, 2023 3:36PM  4:12PM 
K43.00004: Electrical Switching of the Edge Current Chirality in Quantum Anomalous Hall Insulators Invited Speaker: CuiZu Chang A quantum anomalous Hall (QAH) insulator is a topological state of matter, in which the interior is insulating but electrical current flows along the edges of the sample, in either a clockwise (righthanded) or counterclockwise (lefthanded) direction dictated by the spontaneous magnetization orientation. Such chiral edge current (CEC) eliminates any backscattering, giving rise to quantized Hall resistance and zero longitudinal resistance. In this work, we fabricate mesoscopic QAH sandwich (i.e. magnetic topological insulator (TI)/TI/magnetic TI) Hall bar devices and succeed in switching the CEC chirality through spinorbit torque (SOT) by applying a current pulse under a suitably controlled gate voltage. The wellquantized QAH states before and after SOT switching with opposite CEC chiralities are demonstrated through four and threeterminal measurements. Our theoretical calculations show that the SOT that enables the magnetization switching can be generated by both bulk and surface carriers in QAH insulators, in good agreement with experimental observations. Finally, I will briefly talk about the SOT switchinginduced topological phase transition between the QAH and axion insulator states. 
Tuesday, March 7, 2023 4:12PM  4:24PM 
K43.00005: Topological current divider in a Chern insulator junction Dmitry Ovchinnikov, Jiaqi Cai, Zhong Lin, Zaiyao Fei, Zhaoyu Liu, Yongtao Cui, David H Cobden, JiunHaw Chu, CuiZu Chang, Di Xiao, Jiaqiang Yan, Xiaodong Xu A Chern insulator is a twodimensional material that hosts chiral edge states produced by the combination of topology with timereversal symmetry breaking. Such edge states are perfect onedimensional conductors, which may exist not only on sample edges but on any boundary between two materials with distinct topological invariants (or Chern numbers). MnBi_{2}Te_{4}, a recently discovered van der Waals topological magnet, offers rich opportunities for Chern number engineering by means of combining Chern insulator and quantum Hall physics. In this talk, I will report a chiral edgecurrent divider based on Chern insulator junctions formed within the layered topological magnet MnBi_{2}Te_{4}. In a device containing a boundary between regions of different thicknesses, topological domains with different Chern numbers can coexist. At the domain boundary, a Chern insulator junction forms, where we identify a chiral edge mode along the junction interface. I will further demonstrate how this mode can be used to construct topological circuits in which the chiral edge current can be split, rerouted, or switched off by controlling the Chern numbers of the individual domains. Our results demonstrate MnBi_{2}Te_{4} as an emerging platform for topological circuit design. 
Tuesday, March 7, 2023 4:24PM  4:36PM 
K43.00006: High Chern number van der Waals magnetic topological insulator MnBi_{2}Te_{4}/hBN Mihovil Bosnar, Alexandra Y Vyazovskaya, Evgeniy K Petrov, Evgueni V Chulkov, Mikhail M Otrokov The Chern insulators are materials featuring unidirectional metallic surface states due to their nontrivial band topology [1]. As these surface states conduct electricity without dissipation, these materials were proposed for the construction of highefficiency interconnects in future miniaturized electronic devices [2]. However, the number of the surface states is determined by a topological invariant of the band structure called Chern number, which usually turns out to be 1. Therefore, the conductance of realistic interconnects that would be based on these Chern insulators is small, leading to a large contact resistance between them and the standard metallic electrodes [2]. As this presents a major bottleneck for the performance of proposed devices an method of increasing the Chern number is pivotal. However, the enhancement of the Chern number is normally complicated and has been realized in only a few structures by means of doping [3,4]. This can result in structures with Chern number as high as 5 [3,4], but with a downside of structural disorder inherent to doping [5]. 
Tuesday, March 7, 2023 4:36PM  4:48PM 
K43.00007: Observation of Ferroelectricity in Antiferromagnetic Topological Heterostructures Yufei Liu Magnetic topological Insulators have caught huge attention for its potential to achieve dissipationless edge state. MnBi_{2}Te_{4}, the first intrinsic antiferromagnetic TI, has already received focus research interest recently and exhibited many novel topological behavior. Quantum Anomalous Hall effect was observed in oddlayered MnBi_{2}Te_{4 }thin flake up to 1.4 Kelvin. Evenlayered 2dimensional MnBi_{2}Te_{4}, whcih is antiferromagnetic axion insulator, is the perfect platform to investigate quantized topological magnetoelectric coupling and layerlocked berry curvature. Here, we report a unconventional ferroelectric behavior by stacking MnBi_{2}Te_{4} with InBiSe_{3}, a trivial semiconctuor sharing a similar crystal structure as MnBi_{2}Te_{4} with 6 percent lattice mismatch. MnBi_{2}Te_{4} based moire heterostructure have not been reported and similar ferroelectricity has been observed in graphenebased moire systems. Combined with its magnetoelectric coupling and ferromagnetic order, this multiferroic system shows potential for further exploring various topological phenomena. 
Tuesday, March 7, 2023 4:48PM  5:00PM 
K43.00008: Towards a robust Majorana platform based on magnetic topological insulator nanoribbons Thomas L Schmidt, Declan Burke, Malcolm R Connolly, Daniele Di Miceli, Detlev Grützmacher, Dennis Heffels, Jan Karthein, Kristof Moors, Thomas Schäpers, Michael Schleenvoigt, Peter Schüffelgen, Llorenç Serra, Justus Teller, Julian Legendre, Kaycee Underwood, Eduárd Zsurka Magnetic topological insulators (MTIs) are promising materials for realizing a topological phase with Majoranas (propagating edge modes and endlocalized bound states) when combining them with proximityinduced superconductivity. With detailed simulations, we investigate Majoranas in realistic MTI nanoribbons covered by a superconductor. We find that both endlocalized and propagating Majoranas can be realized in such a structure, and that they give rise to distinct transport signatures in normalsuperconductor (NS) or NSN junctions. We also study the impact of disorder and, while the quantum anomalous Hall edge channels of a MTI nanoribbon are quite fragile, the topological phase appears to be robust. This robustness can be confirmed by comparing the tunneling conductance at the end of a proximitized MTI nanoribbon to normal transport in a comparable MTI device without superconductor on top. Finally, we report on the latest status on our experimental platform towards MTIbased Majorana devices, based on an in situ nanofabrication process. 
Tuesday, March 7, 2023 5:00PM  5:12PM 
K43.00009: Quantum oscillation studies in ferromagnetic Mn(Bi_{1x}Sb_{x})_{4}Te_{7} Chaowei Hu, Joss P AyresSims, Jonathan M DeStefano, Elliott W Rosenberg, David E Graf, Xiaodong Xu, JiunHaw Chu The magnetic topological materials family MnBi_{2n}Te_{3n+1} provide a unique platform to study the interplay between magnetism and topology. Their great tunability with the structure (by modifying n), chemical doping (with Sb, Pb etc.), and dimensionality (bulk to thinfilm device) gives rise to capability to host rich variations of topological states and magnetic structures, paving ways to emergent physics such as quantum anomalous Hall effect and axion insulators. We report our growths and Shubnikov–de Haas oscillation studies on bulk Mn(Bi_{1x}Sb_{x})_{4}Te_{7 }(0.25<x<0.31) crystals with ferromagnetic ground state, which have been predicted to host different topological states including Axion insulator, Weyl semimetal, and topological crystalline insulator. Using Sb doping to introduce ferromagnetism to the ground state, and to finetune the carrier from electron to hole, we map out the evolution of the Fermi surface near the charge neutrality point, shedding light on the effect of magnetism on the bands structure of this system. 
Tuesday, March 7, 2023 5:12PM  5:24PM 
K43.00010: A quantitative theory of backscattering in quantum spin Hall materials Tomasz Dietl It is demonstrated quantitatively and with no adjustable parameters that the presence of paramagnetic acceptor dopants elucidates a variety of hitherto puzzling properties of twodimensional topological semiconductors, such as HgTe quantum wells, at the topological phase transition and in the regime of the quantum spin Hall effect. The concepts of charge correlation, Coulomb gap, anisotropic exchange interaction between edge electrons and holes localized on acceptors, strong coupling limit of the Kondo effect, and the influence of the Luttinger effects on the sp exchange anisotropy explain: (i) high hole mobilities compared with electron mobilities, (ii) a short topological protection length, and (iii) the temperature dependence of conductance in the edge transport regime in HgTe quantum wells [1,2]. The extension of the theory to (Hg,Mn)Te quantum wells brings two new ingredients: (i) precessional spin dephasing leading to Mninduced appreciable backscattering between the helical states despite the absence, in the case of spd exchange interaction between edge electrons and Mn spins, of the Kondo effect and exchange anisotropy; (ii) the formation of acceptor bound magnetic polarons that destroy the Kondo effect associated with acceptor holes. These phenomena explain the opposite temperature dependence of edge conductance in HgTe and (Hg,Mn)Te quantum wells. 
Tuesday, March 7, 2023 5:24PM  5:36PM 
K43.00011: Topological Edge Modes in the ManyBody Spectrum of a Ladder Quantum Paramagnet Niclas P Heinsdorf We investigate the excitations of a quantum paramagnet on a onedimensional ladder, and show that by applying an external magnetic field a topological phase transition is enforced. Because the triplon excitations of the ladder are gapped, the scattering and decay phase space is much smaller than in spin wave spectra, and the topological excitations are more robust. We demonstrate the emergence of surface modes at the end of the ladder and show their stability over large portions of the phase diagram, even when manybody interaction are included, which we treat using Density Matrix Renomarmalization Group and Time Evolution Methods. 
Tuesday, March 7, 2023 5:36PM  5:48PM 
K43.00012: Tuning TomonagaLuttinger Liquid in the Topological Edge Channel of bilayer FeSeS by Chemical Pressure Basu D Oli, Qiang Zou, Huimin Zhang, Lian Li Iron chalcogenide FeSe exhibits nematicity without longrange magnetic order, which is shown to be a quantum paramagnet dominated by antiferromagnetic (AFM) fluctuations, offering an ideal platform to probe the interplay of magnetism, nematicity, and superconductivity. In epitaxial thin film of FeSe grown on SrTiO_{3}(001) by molecular beam epitaxy, we have reported robust edge states whose density of states follows a universal scaling with both energy and temperature, characteristic of TomonagaLuttinger liquid (TLL) behavior [Nano Letters 21,6253 (2021)]. In this work, by the isovalent substitution of S into epitaxial bilayer FeSe films on SrTiO_{3}(001) substrates, chemical pressure is applied to tune the TLL behavior in the topological edge channels. Using scanning tunneling microscopy and spectroscopy, we observe similar edge states at the Fermi level for different S concentrations. The experimental Vshaped dI/dV tunneling spectra are fitted using the TLL model. We find that the TLL parameter decreases as S concentration increases, suggesting increasing electronelectron interactions. These results demonstrate that chemical pressure is an effective method to tune the interplay between topology and AFM fluctuations in Fe chalcogenides. 
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