Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session S68: Magnetotransport in Topological Materials IFocus Recordings Available
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Sponsoring Units: DMP GMAG DCMP Chair: Junyi Zhang, Johns Hopkins University; Nicodemus Varnava Room: Hyatt Regency Hotel -Hyde Park B |
Thursday, March 17, 2022 8:00AM - 8:12AM |
S68.00001: Catching Axions with Density Waves Noah Bray-Ali
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Thursday, March 17, 2022 8:12AM - 8:24AM |
S68.00002: First-principles study of magnetic states and the anomalous Hall conductivity of MNb3S6 (M=Co, Fe, Mn, and Ni) Hyowon Park, Olle Heinonen, Ivar Martin Inspired by the observation of the extremely large anomalous Hall effect in the absence of applied magnetic fields or uniform magnetization in CoNb3S61,2, we performed first-principles study of this and related compounds of the MNb3S6-type with different transition metal M ions to determine their magnetic orders and the anomalous Hall conductivity (AHC)3. We find that non-coplanar antiferromagnetic ordering is favored relative to collinear or coplanar order in the case of M=Co, Fe and Ni, while ferromagnetic ordering is favored in MnNb3S6 at low temperatures. The AHC in these materials with non-coplanar spin ordering can reach about e2/h per crystalline layer, while being negligible for coplanar and collinear cases. We also find that the AHC depends sensitively on doping and reaches a maximum for intermediate values of the local spin exchange potential between 0.3 and 0.8 eV. Our AHC results are consistent with the reported Hall measurements in CoNb3S6 and motivate the magnetic phase diagram study of a triangular lattice without inversion symmetry using an atomistic model calculation4. |
Thursday, March 17, 2022 8:24AM - 8:36AM |
S68.00003: Observation of room temperature Topological Hall Effect in Van der Waal ferromagnet Fe5GeTe2 Brian W Casas, Yan Xin, Juan J Macy, Luis Balicas Here we report a novel electrical Topological Hall response in the absence of a Lorentz Force in Fe5GeTe2 that survives up to room temperature. The origin of this Hall like response is likely due to the intrinsic topological magnetic defects such as anti-merrons. Measurements of off diagonal thermal quantities reveals similarly intriguing results. All such quantities have been measured with field along the plane. However, severe differences between samples that have differing thermal histories is observed and is likely linked to the melting of magnetic order of the Fe(1) sublattice, though additional origins will be discussed. High resolution TEM was utilized to better understand the structure of quenched and slow cooled samples. We will discuss the multiple possible origins of this response. The possible origins of this behavior will be examined. |
Thursday, March 17, 2022 8:36AM - 8:48AM |
S68.00004: Magneto-electric response and Quantum Anomalous Hall Effect in Topological Antiferromagnets Chao Lei, Nezhat Pournaghavi, Anna Pertsova, Carlo M Canali, Olle Heinonen, Robert J McQueeney, Allan H MacDonald I will discuss the magneto-electric and transport properties of thin films formed from bulk normal and antiferromagnetic topological insulators. In the thick film limit, both systems have the quantized magneto-electric response properties (QTME) that are characteristic of topological insulators. In non-magnetic case, thin films with surface magnetism exhibit a quantized anomalous Hall effect when the magnetizations on the top and bottom surfaces are parallel, and a QTME response when the magnetizations have opposite orientations and the films are sufficiently thick. I will present a unified picture [1] of both effects that associates finite thickness corrections to the QTME with non-locality in the side-wall current response function, emphasizing that the side-wall response is non-zero only near the surface. In the magnetic case, the anomalous Hall effect vanishes when the product of time-reversal and inversion is a good quantum number [2,3], and the magneto-electric response then has both surface and bulk contributions that are quantized only when summed [4]. Quantum anomalous Hall effects occur in many configurations of magnetic topological insulator thin films [5], including in some collinear configurations in which the total spin magnetization vanishes [6]. |
Thursday, March 17, 2022 8:48AM - 9:00AM |
S68.00005: Mesoscopic Transport Behaviors in Miniaturized Quantum Anomalous Hall devices Gang Qiu, Peng Zhang, Peng Deng, Lixuan Tai, Sukong Chong, Christopher Eckberg, Kang-Lung Wang The quantum anomalous Hall (QAH) systems are considered an important candidate for quantum device applications because they offer lossless current-carrying capability in absence of an external magnetic field. However, a comprehensive understanding of mesoscopic transport in sub-micron size regime QAH has yet been established, which is crucial for designing and modeling miniaturized QAH devices. Here, the QAH effects are successfully demonstrated in narrow Hall-bar devices with channel width down to 600 nm. Through measurement of size-dependent breakdown current, we confirm that the chiral edge states in QAH are confined at the physical boundary of the QAH mesa with its width on the order of Fermi wavelength. The narrow channel provides an additional back-scattering path through percolative hopping between local compressible puddles. The information on domain dynamics and phase coherent length is obtained through analysis of large resistance fluctuations, which is associated with collective interference between intersecting paths along domain walls. Our work suggests the QAH edge states are mesoscopically different from quantum Hall edge states and are fundamentally more robust against device scaling. |
Thursday, March 17, 2022 9:00AM - 9:12AM |
S68.00006: Anomalous transport in a crystal with an intrinsic 90°-twist geometry—Co2MnGa Nicholas P Quirk, Guangming Chen, Nan Yao, N. Phuan Ong We find that platelets (10 x 10 x 1 μm) cut from single crystals of the magnetic Weyl semimetal Co2MnGa using a focused ion beam (FIB) have an intrinsic "twist." Measuring the resistance of the lamellae using the four corner contacts on the top face gives Rxx > Ryy, but measuring it using the contacts on the bottom gives Rxx < Ryy, by the same amount. The anomalous resistances appear to be confined to the surfaces, e.g., when the current is applied along an x-direction edge on the top face, the other x-direction edge on the top has an anomalously high voltage drop, but the x-direction edges on the bottom match the expected values for an isotropic slab of the same dimensions. We also observe a pattern of magnetic domains with a matching "twist." However, we find that the resistance anisotropy is unchanged by applied magnetic fields that are orders of magnitude higher than the coercive field of the material. |
Thursday, March 17, 2022 9:12AM - 9:24AM |
S68.00007: Disorder-driven Negative Magnetoresistance and Anomalous Hall Effect in Magnetic Semimetal EuB6 Jihun Park, Xiaohang Zhang, Shanta Saha, Kideuk Nam, Heshan Yu, Naila Al Hasan, Suchismita Sarker, Apurva Mehta, Jong-Seong Bae, Johnpierre Paglione, Richard L Greene, Seunghun Lee, Ichiro Takeuchi Europium hexaboride (EuB6), a well-known ferromagnetic semimetal, is attracting renewed interest owing to the possible emergence of topologically non-trivial phases, such as the magnetic Weyl semimetal state and a quantum anomalous Hall insulator1–4. In this work, we discuss the effect of stoichiometric disorder on spin-dependent transport characteristics in EuB6 thin films. A high-throughput combinatorial approach is employed to perform systematic investigation of the B:Eu ratio. Off-stoichiometric regions above critical points show significant enhancement in negative magnetoresistance and the anomalous Hall effect. With careful structural and electrochemical analyses, these results provide clues for the disorder-induced topological phase transition in EuB6. |
Thursday, March 17, 2022 9:24AM - 9:36AM |
S68.00008: Tuning the Exotic Quantum Phase of EuA2X2 (A = Cd, Zn, Mg, Sb; X = As, P) by using Chemical substitution. Gadeer Alqasseri, Barun Ghosh, Arun Bansil, Fazel Tafti, Bahadur Singh, Sugata Chowdhury Intrinsic magnetic topological materials (IMTMs) are promising for next-generation spintronics applications. These materials exhibit topologically protected exotic states relevant to their functionality, which can be further enhanced by tuning their topology and magnetism. Among the different classes of IMTMs, EuCd2P2 family has been observed to present unique responses in its electrical resistivity. Here, using first-principles, density-functional theory based modeling, we explore the electronic structures of EuA2X2 (A = Cd, Zn, Mg, Sb; X = As, P). Our calculations reveal that 3d and 4f orbital localization plays a key role in controlling the bandgap and the electronic states near the Fermi level in this materials family. We show that topological properties can be tuned via chemical substitutions and are not sensitive to spin-orbit coupling effects. Our study highlights the complex relationship between the topology and chemistry of this materials family. |
Thursday, March 17, 2022 9:36AM - 9:48AM |
S68.00009: Evolution of Singular Anisotropic Magnetoresistance in Magnetic Weyl Metals Younghyuk Kim, Yuxuan Xiao, Rajasekhar Medapalli, Eric E Fullerton, Matthew J Gilbert The confluence between magnetism and topology is an opportunity to explore the interaction between local and non-local orders within the same material. Amongst candidate materials that may host both orders, Holmium (Ho), shows a distinct magnetic phase change from conical ferromagnet phase to helical antiferromagnetic phase as temperature increases and is thus of great interest as a potential topological magnet. In this talk, we describe theoretical and experimental quantum transport results in epitaxial Holmium films under a variety of conditions. Calculations show that Ho, when in the helical spin antiferromagnetic phase, is a Weyl semimetal. We find that the anisotropic magnetoresistance (AMR) under the high magnetic field applied in the basal plane possesses sharp delta function like six-fold symmetric peaks in the AMR in the spin-saturated ferromagnetic phase when the field is along the a-axis. These features are superimposed on a two-fold symmetry contribution. At higher temperatures, when the magnetic phase of Holmium changes from conical spin to helical spin, there is a symmetry transition within the AMR where the 6-fold contribution dominates the response. Furthermore, we compare the conditions under which we find singular AMR in Ho to the well-known AMR of magnetic Weyl semimetal CeAlGe to show that the resultant AMR is unlikely to arise from the formation of magnetic domains. |
Thursday, March 17, 2022 9:48AM - 10:00AM |
S68.00010: Axion electrodynamics with magnetoelectric Chromia Syed Qamar Abbas Shah, Ather Mahmood, Allan H MacDonald, Christian Binek Axion electrodynamics with magnetoelectric Chromia |
Thursday, March 17, 2022 10:00AM - 10:12AM |
S68.00011: Electron-magnon coupling and quasiparticle lifetimes on the surface of a topological insulator Kristian Mæland, Hâkon I Røst, Justin W Wells, Asle Sudbo The fermionic self-energy on the surface of a topological insulator proximity coupled to ferro- and antiferromagnetic insulators is studied. An enhanced electron-magnon coupling is achieved by allowing the electrons on the surface of the topological insulator to have a different exchange coupling to the two sublattices of the antiferromagnet. Such a system is therefore seen as superior to a ferromagnetic interface for the realization of magnon-mediated superconductivity. The increased electron-magnon-coupling simultaneously increases the self-energy effects. In this paper we show how the inverse quasiparticle lifetime and energy renormalization on the surface of the topological insulator can be kept low close to the Fermi level by using a magnetic insulator with a sufficient easy-axis anisotropy. We also consider a set of material and instrumental parameters where these self-energies should be measurable in angle-resolved photoemission spectroscopy experiments, paving the way for a measurement of the interfacial exchange coupling strength. |
Thursday, March 17, 2022 10:12AM - 10:24AM |
S68.00012: Effect of the external fields in high Chern number quantum anomalous Hall insulators Yuriko C Baba, Mario Amado, Enrique Diez, Rafael Molina, Francisco Dominguez-Adame Topological magnetic insulators have been discovered as a new platform for observing Quantum Anomalous Hall states with high Chern number C. In three-dimensional structures of stacking layers of magnetically doped and undoped topological insulators, the number of chiral edge channels can be controlled by the width and number of layers. More specifically, samples of Cr-doped Bi2(Se,Te) have been recently measured by Zhao et al. [1] in transport experiments showing this feature up to C = 5. |
Thursday, March 17, 2022 10:24AM - 11:00AM |
S68.00013: Charge-density-wave phase and magnetoresitance in Ag2-δTe Invited Speaker: Marie-Louise Saboungi Considerable excitement was generated by the observation of large and linear positive magnetoresistance (MR) in non-magnetic silver chalcogenides. Renewed interest in these materials was kindled by the discovery that Ag2Te in particular is a topological insulator with gapless linear Dirac-type surface states. High-pressure x-ray diffraction studies, combined with first-principles electronic structure calculations, have identified three phase transitions as the pressure is increased: an isostructural transition identified with an electronic topological transition followed by two structural phase transitions. These recent studies were carried out on nominally stoichiometric Ag2Te. For the present work we have prepared single-phase self-doped Ag2-δTe samples with a well-characterized silver deficit (δ = 2.0.10-4) for structural and electrical transport measurements over extended ranges of pressure (0-43 GPa), temperature (2-300 K) and magnetic field (0-9 T). The temperature dependence of the resistivity exhibits anomalous behavior at 2.3 GPa, slightly above the isostructural transition, which we postulate is due to Fermi surface reconstruction associated with a charge-density-wave (CDW) phase. The anomaly is enhanced by the application of a 9T magnetic field and shifted to higher temperature, implying that the electronic Zeeman energy is sufficient to alter the gapping of the Fermi surface. A peak in the pressure dependence of the resistivity and a sudden drop in the pressure dependence of the mobility, occurring at 2.3 GPa, provide additional evidence for a CDW phase at pressures slightly above the isostructural transition. |
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