Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M52: Magnetic Topological Materials 3: Doped Bi2Te3Focus Live
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Sponsoring Units: DMP GMAG Chair: Kenji Yasuda, Massachusetts Institute of Technology MIT |
Wednesday, March 17, 2021 11:30AM - 12:06PM Live |
M52.00001: Nonreciprocal charge transport at magnetic topological insulator interfaces Invited Speaker: Kenji Yasuda The surface state of a topological insulator has provided a simple yet effective platform for studying spin-orbit effects in quantum materials over the past decade. Especially, when the spin-momentum-locked electrons are coupled to a magnetic constituent, the electronic motion of the surface state is strongly influenced by the magnetization direction. One of the most representative examples is the quantum anomalous Hall effect, where unidirectional chiral edge mode appears at the edge of the sample due to spontaneous magnetization. In this talk, we will introduce another example of exotic transport in magnetic topological insulators: nonreciprocal charge transport, where the sample's resistance depends on the current direction [1]. Interestingly, we observe two types of nonreciprocal transport (also called unidirectional resistance) in Cr-doped (Bi,Sb)2Te3 in a different measurement configuration; in-plane and out-of-plane magnetization. Under in-plane magnetization, the interaction between the spin-momentum locked two-dimensional surface state and the in-plane magnetization gives sizable unidirectional resistance [2]. Meanwhile, in quantum anomalous Hall regime under out-of-plane magnetization, we observe a large nonreciprocal transport at the one-dimensional chiral edge state [3]. We will discuss the origin of these effects in detail. Nonreciprocal transport can be implemented in other topological materials such as quantum Hall states and magnetic Weyl semimetals to study the dissipation mechanism of the topological modes. |
Wednesday, March 17, 2021 12:06PM - 12:18PM Live |
M52.00002: Interfacing topological insulators and ferrimagnets: Bi2Te3 and Fe3O4 heterostructures grown by molecular beam epitaxy Vanda Mota Pereira, Chi-Nan Wu, Cariad-A. Knight, Arnold Choa, Liu Tjeng, Simone Altendorf Since their initial theoretical prediction, topological insulators have been extensively studied due to the multitude of features stemming from their topological surface states. Many of the recent studies have concentrated on breaking the time reversal symmetry by introducing magnetic order in the system, as this can lead to exotic phenomena such as the quantum anomalous Hall effect. A promising experimental route is to interface topological insulators and magnetic insulators in thin film heterostructures, making use of the magnetic proximity effect. Here we investigate heterostructures grown by molecular beam epitaxy consisting of Bi2Te3 and Fe3O4 layers. By growing two different types of heterostructures, Fe3O4 on Bi2Te3 and Bi2Te3 on Fe3O4, we explore differences in chemical stability, crystalline quality, electronic structure, and transport properties. We find the heterostructure Bi2Te3 on Fe3O4 to be a more viable approach, with transport signatures in agreement with a gap opening in the topological surface states. |
Wednesday, March 17, 2021 12:18PM - 12:30PM Live |
M52.00003: Magnetic Weyl semimetal emerging from non-magnetic doping Virginia Carnevali, Dimuthu Obeysekera, Andrew R Supka, Clarina Dela Cruz, Marco Fornari, Junjie Yang The investigation of new mechanism that could result in a controllable ferromagnetic (FM) component in a Weyl semimetal in the absence of external fields is of utmost importance. Here we report the enhancement of FM component induced by non-magnetic doping and the emergence of magnetic Weyl states in SrMn1-xZnxSb2. SrZnSb2 results non-magnetic, while both SrMnSb2 and SrZn0.25Mn0.75Sb2 are magnetic with strong spin-orbit coupling due to the presence of Mn. SrMnSb2shows an AFM behavior caused by the inverse spin polarization of neighbors Mn. On the contrary, the substitution of one Mn with Zn induces a total magnetization on SrZn0.25Mn0.75Sb2, turning the system on a ferromagnet. From the computed band structures and fermi surface, we deduce a nature of semimetal, small-gap insulator and Weyl semimetal for SrZnSb2, SrMnSb2 and SrZn0.25Mn0.75Sb2 respectively. Further, SrMn1-xZnxSb2 (x = 0.065 and 0.262) were also synthetized and characterized. Non-trivial de-Haas van Alphen and Shubnikov – de Haas oscillations were measured, reinforcing the theoretical finding of Weyl semimetal nature for SrMn1-xZnxSb2. |
Wednesday, March 17, 2021 12:30PM - 12:42PM Live |
M52.00004: High-throughput search of magnetic topological insulators and semi-metals using spin-orbit spillage and machine-learning methods Kamal Choudhary, Kevin Garrity Magnetic topological insulators and semi-metals have a variety of properties that make them attractive for applications including spintronics and quantum computation. However, very few such materials are known to date. In this work, we use a systematic high-throughput density functional theory based computational workflow to identify possible intrinsic Chern insulators and semi-metals among 40000 three-dimensional materials in the JARVIS-DFT database (https://jarvis.nist.gov/jarvisbdft). First, we screen materials with net magnetic moment>0.5 μB, and spin-orbit spillage > 0.5. Then we carry out Wannier charge centers, Chern number, anomalous Hall conductivity, surface bandstructure, Fermi-surface and magnetic ordering analysis to determine the exotic characteristics of the screened compounds. We also train machine learning model for predicting spillage, bandgap and magnetic moment using JARVIS-ML models to further accelerate the screening process. The computational prediction if realized experimentally will open a new paradigm in the physics of topological quantum materials. |
Wednesday, March 17, 2021 12:42PM - 12:54PM Live |
M52.00005: Charge Density Waves as a Tool to Achieve Idealized Topological Semimetals Shiming Lei, Samuel M. L. Teicher, Andreas Topp, Kehan Cai, Jingjing Lin, Fanny Rodolakis*, Jessica L McChesney, Maxim Krivenkov, Dmitry Marchenko, Andrei Varykhalov, Christian Ast, Roberto Car, Jennifer Cano, Maia Garcia Vergniory, Nai Phuan Ong, Leslie M Schoop Topological materials have been the source of many exciting new discoveries. The fast development of high throughput materials search programs has led to the identification of many topological materials in inorganic materials database. Despite the rapid progress, many topological materials that have been discovered suffer from non-ideal band structures, i.e. the topological bands are frequently convoluted with trivial ones, and band structure features of interest can appear far below the Fermi level. One strategy that has been utilized to achieve "better" topological materials has been chemical doping, which can shift the chemical potential to the points of interest. This strategy, however, does not help to remove interfering bands from the Fermi level. Here we introduce a new strategy to design nearly idealized topological semimetals, by taking advantage of a charge density wave (CDW) and non-symmorphic symmetry. We further show experimental verification of this strategy in single crystals of GdSbxTe2-x-δ by studying the electronic structure with angle-resolved photoemission spectroscopy (ARPES). |
Wednesday, March 17, 2021 12:54PM - 1:06PM Live |
M52.00006: Topological surface currents accessed through reversible hydrogenation of the bulk Haiming Deng, Lukas Zhao, Kyungwha Park, Jiaqiang Yan, Kamil Sobczak, Lia Krusin-Elbaum Hydrogen, one of the most promising clean fuel alternatives if efficiently incorporated within a solid, can also drastically modify its electronic and structural state. Here we report a new hydrogenation approach resolving an outstanding challenge in chalcogenide classes of topological materials — the control of intrinsic bulk conduction — using an aqueous HCl solution rich in H+. The technique tunes carrier densities by over 1020 cm-3, moving the Fermi level across the surface states within the bulk gap to achieve ambipolar conduction from p (hole-like) to n-type (electron-like) and back. Electrons are donated by a weak binding of H+ to Te(Se) chalcogen, a bond controllably removed by a low-temperature anneal. We demonstrate hydrogen-tunability of the canonical TI, such as Bi2Te3 and Bi2Se3, and of the recently discovered intrinsic magnetic topological materials MnX2Y4 (X=Bi, Sb and Y=Te, Se) and their superlattices1. The technique significantly expands the availability of robust materials platforms for harnessing emergent topological quantum phenomena and higher-order topological orders. |
Wednesday, March 17, 2021 1:06PM - 1:18PM Live |
M52.00007: Quasi-Periodic Bulk-Boundary Correspondence and the 1D Metal-Insulator Transition Dan Borgnia, Robert-Jan Slager, Ashvin Vishwanath The regime of strong aperiodicity is not frequently discussed in the context of topological materials. Aiming to better understand such systems, this work generalizes results from translation-invariant systems to their aperiodic counterparts. We connect bulk-boundary correspondence and the gap-labeling theorem to the dynamics of quasi-periodic eigenfunctions. Focusing on the almost-Matthieu operator (Andre-Aubry-Harper model) and its 1D Metal-Insulator Transition (MIT), we provide a novel approach to Barry Simon's "Dry Ten Martini" problem in the singularly continuous regime. Further applications of this formalism are discussed, including the generalization to other quasi-periodic models and the breakdown of bulk-boundary correspondence in quasi-crystals. |
Wednesday, March 17, 2021 1:18PM - 1:30PM Live |
M52.00008: Disentangling the magnetic proximity effect in topological insulators with Mg(Al,Fe)2O4/Bi2Se3 thin films Lauren Riddiford, Alexander Grutter, Timothy S Pillsbury, Nitin Samarth, Yuri Suzuki Magnetic topological insulators (TIs) have attracted attention as platforms for hosting the quantum anomalous Hall effect, characterized by the emergence of a quantum Hall effect in zero magnetic field and dissipation-less conduction. Magnetic TIs have been achieved through the addition of magnetic dopants to TIs or by a magnetic proximity effect induced in TIs due to an adjacent ferromagnet. Anomalous Hall effect, magnetoresistance, and polarized neutron reflectometry (PNR) measurements have been used as evidence of a magnetic proximity effect in a TI. In this talk, we have studied heterostructures of the ferromagnetic insulator Mg(Al,Fe)2O4 and TI Bi2Se3 to explore a possible magnetic proximity effect. They exhibit a nonlinear Hall effect that can be interpreted in terms of an anomalous Hall effect or multiband carriers in the TI, especially when the ferromagnetic insulator has an in-plane easy axis. Polarized neutron reflectometry provides two possible magnetic profiles – a sharp interface with a proximity effect and a highly disordered interface with no proximity effect. Further measurements of interface quality are required to prove the existence of induced magnetism in TIs. |
Wednesday, March 17, 2021 1:30PM - 1:42PM Live |
M52.00009: Effect of dilute magnetism in a topological insulator Firoza Kabir, Md Mofazzel Hosen, Xiaxin Ding, Christopher Lane, Gyanendra Dhakal, Yangyang Liu, Klauss Dimitri, Christopher Sims, Sabin Regmi, Luis Persaud, Yong Liu, Arjun Pathak, Jian-Xin Zhu, Krzysztof Gofryk, Madhab Neupane Three-dimensional topological insulators (TI) have emerged as a unique state of quantum matter and generated enormous interests in condensed matter physics. The surfaces of a three-dimensional (3D) TI are composed of a massless Dirac cone, which is characterized by the Z2 topological invariant. Here, by using a combination of first principles calculations, magneto-transport, angle-resolved photoemission spectroscopy (ARPES), and time resolved ARPES (tr-ARPES), we study the electronic properties of Gadolinium (Gd) doped Sb2Te3. Our study shows that Gd doped Sb2Te3 is a spin-orbit-induced bulk band-gap material, whose surface is characterized by a single topological surface state. Introducing dilute 4f-electron magnetism into the Sb2Te3 TI system by Gd doping creates surface magnetism in this system. Our results provide a new platform to investigate the interaction between dilute magnetism and topology in doped topological materials. |
Wednesday, March 17, 2021 1:42PM - 1:54PM Live |
M52.00010: Nanoscale synthesis and characterization of heterostructures coupling antiferromagnetic MnTe and topological insulator Bi2Te3 Alexander LaFleur, Bryan Rachmilowitz, He Zhao, Zheng Ren, Hong Li, Shrinkala Sharma, Nastaran Alamgir Tehrani, Ilija Zeljkovic Bulk MnTe experiences a transition to an antiferromagnetic phase below ~307 K. As the search for topological materials with underlying magnetic ordering continues, MnTe coupled to a topological insulator could be a promising constituent of superlattice heterostructures that feature both magnetic ordering and topological phases of matter. Here, we use molecular beam epitaxy (MBE) to grow MnTe films, and bilayers of MnTe and prototypical topological insulator Bi2Te3. We characterize their structural properties using in-situ reflection high-energy electron diffraction (RHEED) and low-temperature scanning tunneling microscopy (STM). We further study the electronic properties of these films using scanning tunneling spectroscopy, magnetization and transport measurements. Our experiments aim to advance the understanding of the interplay between magnetically ordered and topological states of matter. |
Wednesday, March 17, 2021 1:54PM - 2:06PM Live |
M52.00011: Scanning SQUID Microscopy of the Quantum Anomalous Hall Effect George Ferguson, Run Xiao, David Low, Ling-Jie Zhou, Anthony R. Richardella, Cui-Zu Chang, Nitin Samarth, Katja Nowack We report magnetic imaging of Cr-doped (Bi,Sb)2Te3 heterostructures in the quantum anomalous hall regime. We use a scanning superconducting quantum interference device (SQUID) microscope with micrometer scale spatial resolution to image the magnetic fields above current biased devices. Using these images, we reconstruct the current density, allowing us to visualize where current flows in the devices. We also use top and back gates to study how magnetization is affected by electrostatic gating. By performing these measurements as a function of current bias, gate voltage and magnetization direction, we construct a comprehensive picture of electronic transport in our devices. |
Wednesday, March 17, 2021 2:06PM - 2:18PM Live |
M52.00012: Spontaneous ferromagnetism and finite surface energy gap in the topological insulator Bi2Se3 from surface BiSe antisite defects Suhas Nahas, Biplab Sanyal, Annica M Black-Schaffer We perform ab-initio calculations on BiSe antisite defects in the surface of Bi2Se3, finding strong low-energy defect resonances with spontaneous ferromagnetism, fixed to an out-of-plane orientation due to an exceptionally large magnetic anisotropy energy. For antisite defects in the surface layer, we find semi-itinerant ferromagnetism and strong hybridization with the Dirac surface state, generating a finite energy gap. For deeper-lying defects, such hybridization is largely absent, the magnetic moments become more localized, and no energy gap is present. |
Wednesday, March 17, 2021 2:18PM - 2:30PM Live |
M52.00013: The Rabbi Limit of Current Rectification in Solids Oles Matsyshyn, Francesco Piazza, Roderich Moessner, Inti A Sodemann We investigate rectified currents in response to oscillating electric fields in materials lacking inversion and time-reversal symmetries, which in second-order perturbation theory are inversely proportional to the relaxation rate, and, therefore, naively diverge in the ideal clean limit. Employing a combination of the Keldysh technique and Floquet theory, we show that this is an artefact of perturbation theory and that there is a well-defined periodic steady-state akin to Rabbi oscillations leading to finite rectified currents in the clean limit. In this Rabbi regime, the rectified current would scale as the square root of the radiation intensity, in contrast with the linear scaling expected in the perturbative regime, allowing to detect it in experiments readily. More generally, our description provides a smooth interpolation from the ideal isolated Periodic Gibbs Ensemble describing the Rabbi oscillations in the clean limit to the perturbative regime of rapid relaxation due to strong coupling to a thermal bath. |
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