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 S52: Magnetic Topological Materials 6: HallLive
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Sponsoring Units: DMP GMAG Chair: Luis Balicas, National High Magnetic Field Laboratory |
Thursday, March 18, 2021 11:30AM - 11:42AM Live |
S52.00001: Scaling behavior of the quantum phase transition from a quantum anomalous Hall insulator to an axion insulator. Xinyu Wu, Di Xiao, Chuizhen Chen, Jian Sun, Ling Zhang, Moses H W Chan, Nitin Samarth, X. C. Xie, Xi Lin, Cui-Zu Chang We performed the scaling behavior study of quantum phase transition from quantum anomalous Hall state(QAHS) to axion insulator state in magnetic topological insulator sandwich samples. A power-law dependence of the derivative of longitudinal resistance with magnetic field on temperature was observed at the transition point. We extracted the critical exponent κ ~ 0.38±0.02, which agrees well with the recent high precision calculation result of ν ~ 2.6 using the Chalker-Coddington network model in quantum Hall system. We also measured the resistance of the axion insulator state, which is as high as 1.3 GΩ at 50 mK using a quasi-DC measurement method. Our work indicates that the QAHS to axion insulator state transition shares the same universality class with the plateau to plateau transition in quantum Hall system.[1] |
Thursday, March 18, 2021 11:42AM - 11:54AM Live |
S52.00002: Hall viscosity, anisotropy & internal angular momentum Pranav Rao, Barry Bradlyn Inspired by recent hydrodynamic experiments on graphene and chiral active fluids, we examine the viscous response of systems in two spatial dimensions, allowing for anisotropy and internal spin degrees of freedom [1]. Though there are generally six non-dissipative (Hall) viscosities, we show there are only three independent Hall contributions to the viscous force density, meaning three components are redundant. We describe a similar redundancy for dissipative viscosities, and present hydrodynamic implications of this for pipe flows and free surface waves. Turning our attention to quantum systems on the lattice, we extend the Kubo formalism for viscosity to systems with internal degrees of freedom and discrete translational symmetry, and highlight the importance of properly considering internal angular momentum through some examples. Lastly, we mention an extension of our formalism to three dimensions and describe a novel viscosity coefficient that can emerge in magnetic crystals with cubic symmetry. |
Thursday, March 18, 2021 11:54AM - 12:06PM Live |
S52.00003: Giant Anomalous Nernst Effect in Nodal Web Ferromagnets Fe3X (X = Ga, Al) Akito Sakai, Susumu Minami, Takashi Koretsune, Taishi Chen, Tomoya Higo, Yangming Wang, Takuya Nomoto, Motoaki Hirayama, Shinji Miwa, Daisuke Nishio-Hamane, Fumiyuki Ishii, Ryotaro Arita, Satoru Nakatsuji Recently, large anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) have been reported in topological magnets, where the large Berry curvature is induced by characteristic band crossings so-called Weyl points and nodal lines [1-4]. For example, Weyl ferromagnet Co2MnGa shows a large ANE ~6 μV/K at room temperature (T) [5]. This enables us to study thermoelectric applications by ANE, which has great potential such as efficient and flexible coverage of a heat source. For this purpose, substantial improvements are still necessary not only for the performance but also for the associated material costs, safety, and stability. |
Thursday, March 18, 2021 12:06PM - 12:18PM Live |
S52.00004: Giant Anomalous Hall Effect in quasi-2D Layered Antiferromagnet Co1/3NbS2 Giulia Tenasini, Edoardo Martino, Nicolas Ubrig, Nirmal Jeevi Ghimire, Helmuth Berger, Oksana Zaharko, Fengcheng Wu, John Mitchell, Ivar Martin, Laszlo Forro, Alberto Morpurgo The discovery of the anomalous Hall effect (AHE) in bulk metallic antiferromagnets (AFMs) motivates the search of the same phenomenon in two-dimensional (2D) systems, where a quantized anomalous Hall conductance can in principle be observed. Here, we present experiments on the layered AFM Co1/3NbS2, which exhibits AHE below the Néel temperature TN=29 K in bulk crystals [1]. Our transport measurements on micro-fabricated devices reveal a pronounced anisotropy in the resistivity –indicative of the 2D character of the electronic properties– and show an extremely large AHE, with an anomalous Hall conductance of the order of e2/h per layer at low temperature [2]. The observed strong anisotropy of transport and the very large anomalous Hall conductance per layer make the properties of Co1/3NbS2 compatible with the presence of partially filled topologically non-trivial 2D bands originating from the magnetic superstructure of the antiferromagnetic state. Isolating atomically thin layers of this material and controlling their charge density may therefore provide a viable route to reveal the occurrence of the quantized AHE in a 2D AFM. |
Thursday, March 18, 2021 12:18PM - 12:30PM Live |
S52.00005: Giant Anomalous Hall Effect due to Double-Degenerate Quasi Flat Bands Wei Jiang, Duarte Sousa, Jianping Wang, Tony Low We propose a novel approach to achieve giant AHE in materials with flat bands (FBs). FBs are accompanied by small electronic bandwidths, which consequently increases the momentum separation (K) within pair of Weyl points and thus the integrated Berry curvature. Starting from a simple model with a single pair of Weyl nodes, we demonstrated the increase of K and AHE by decreasing bandwidth. It is further expanded to a realistic pyrochlore lattice model with characteristic double degenerated FBs, where we discovered a giant AHE while maximizing the K with nearly vanishing band dispersion of FBs. We identify that such model system can be realized in both pyrochlore and spinel compounds based on first-principles calculations, validating our theoretical model and providing a feasible platform for experimental exploration. |
Thursday, March 18, 2021 12:30PM - 12:42PM Live |
S52.00006: Persistent currents and spin torque caused by percolated quantum spin Hall state Antonio Zegarra, Carlos Egues, Wei Chen Motivated by recent experiments, we investigate the quantum spin Hall state in 2D topological insulator/ferromagnetic metal planar junctions by means of a tight-binding model and linear response theory. We demonstrate that whether the edge state Dirac cone is submerged into the ferromagnetic subbands and the direction of the magnetization dramatically affect (i) how the edge state percolates into the ferromagnet, and (ii) the spin-momentum locking of the edge state. Laminar flows of room temperature persistent charge and spin currents near the interface are uncovered. In addition, the current-induced spin polarization at the edge of the 2D topological insulator is found to be dramatically enhanced near the impurities. The current-induced spin polarization in the ferromagnet is mainly polarized in the out-of-plane direction, rendering a current-induced spin torque that is predominantly field-like. |
Thursday, March 18, 2021 12:42PM - 12:54PM Live |
S52.00007: A new cubic Hall viscosity in three-dimensional topological semimetals Iigo Robredo, Pranav Rao, Fernando De Juan, Aitor Bergara, Juan Luis Mañes, Alberto Cortijo, Maia Garcia Vergniory, Barry Bradlyn The interplay between geometry and topology has been a guiding paradigm for recent developments in condensed matter physics. In chiral crystals, it has been shown that crystal symmetries can stabilize exotic multifold fermions, where 3,4, or 6 bands become degenerate at high-symmetry momenta. Due to the crucial role of spatial geometry in stabilizing these degeneracies, it is natural to expect that multifold fermions will also exhibit nontrivial responses to geometric deformation. In this talk, we explore this issue for the first time by examining the viscous response of a threefold degenerate "spin-1" fermion in the cubic (magnetic) space group P213 (198), with time-reversal symmetry breaking chiral magnetism. We derive the non-dissipative Hall viscosity for a spin-1 fermion following two different approaches. We find that the cubic symmetry of the unperturbed Hamiltonian allows for the appearance of a new, fundamentally three dimensional nondissipative viscous force. |
Thursday, March 18, 2021 12:54PM - 1:06PM Live |
S52.00008: Longitudinal and anomalous Hall conductivity of a general two-band model Johannes Mitscherling In the last decades, theoretical progress was made in describing transport phenomena that are based on so-called interband coherence effects, that is, going beyond independent quasiparticles. In recent years, there is increasing interest in such effects in multiband systems due to advances in experimental techniques. |
Thursday, March 18, 2021 1:06PM - 1:18PM Not Participating |
S52.00009: Quantum anomalous Hall effect in two-dimensional magnetic insulator heterojunctions Jinbo Pan, Jiabin Yu, Yanfang Zhang, Shixuan Du, Anderson Janotti, Chaoxing Liu, Qimin Yan In this talk, we propose an alternative approach to realize the quantum anomalous Hall (QAH) effect, a typical example of a magnetic topological phase, via engineering two-dimensional (2D) magnetic van der Waals heterojunctions. Instead of a single magnetic topological material, we search for the combinations of two 2D (typically trivial) magnetic insulator compounds with specific band alignment so that they can together form a type-III broken gap heterojunction with topologically non-trivial band structure. By combining the data-driven materials search, first-principles calculations, and the symmetry-based analytical models, we identify 8 type-III broken gap heterojunctions consisting of 2D ferromagnetic insulators in the MXY compound family as a set of candidates for the QAH effect. In particular, we directly calculate the topological invariant (Chern number) and chiral edge states in the MnNF/MnNCl heterojunction with ferromagnetic stacking. This work illustrates how data-driven material science can be combined with symmetry-based physical principles to guide the search for heterojunction-based quantum materials hosting the QAH effect and other exotic quantum states in general. |
Thursday, March 18, 2021 1:18PM - 1:30PM Live |
S52.00010: Unconventional Anomalous Hall Effect in a New Noncollinear Ferromagnetic Weyl Semimetal CeAlSi with Tunable Fermi Level Hung-Yu Yang, Bahadur Singh, Jonathan Gaudet, Baozhu Lu, Cheng-Yi Huang, Wei-Chi Chiu, Shin-Ming Huang, Baokai Wang, Faranak Bahrami, Bochao Xu, Jacob D Franklin, Ilya Sochnikov, David E Graf, Guangyong Xu, Yang Zhao, Christina Hoffmann, Hsin Lin, Darius Torchinsky, Collin Leslie Broholm, Arun Kumar Bansil, Fazel Tafti Ferromagnetic Weyl semimetals (FM WSMs) have drawn significant attention since they feature an interesting combination of magnetism and band topology. As a result, FM WSMs can host unique electromagnetic responses by bringing the Fermi level close to the Weyl nodes. Here, based on transport and quantum oscillation experiments aided by first-principles calculations, we show that CeAlSi is a new noncollinear FM WSM in which the Fermi level can be tuned to lie as close as 1 meV to the Weyl nodes [1]. Importantly, we observe an unconventional anomalous Hall effect (AHE) that appears (disappears) as the Fermi level lies close to (away from) the Weyl nodes. The close link between the novel AHE and the proximity of Weyl nodes to the Fermi level establishes a new transport response that is induced by the Weyl nodes in CeAlSi. We will discuss the role of topological surface states living on the ferromagnetic domain walls in generating the unusual electromagnetic response in CeAlSi. |
Thursday, March 18, 2021 1:30PM - 1:42PM Live |
S52.00011: Tuning Chern Number in Quantum Anomalous Hall Insulators Yifan Zhao, Ruoxi Zhang, Ruobing Mei, Ling-Jie Zhou, Hemian Yi, Ya-Qi Zhang, Jiabin Yu, Run Xiao, Ke Wang, Nitin Samarth, Moses H W Chan, Chaoxing Liu, Cui-Zu Chang 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 Chern number. Up to now, the zero magnetic field QAH effect has been realized only for C = 1. Here we used molecular beam epitaxy to fabricate magnetic TI multilayers and realized the well-defined QAH effect with tunable Chern number up to 5. We further demonstrated that in the same multilayer configuration, the Chern number can be tuned by varying either the Cr doping level or the thickness of the interior magnetic TI layer. One theoretical model is developed to explain our experimental observations and establish the phase diagram for the QAH effect with the tunable Chern number. Our results facilitate the applications of dissipationless chiral edge currents in energy-efficient electronic devices and open the possibility for developing multi-channel quantum computing and high-capacity chiral circuit interconnects. |
Thursday, March 18, 2021 1:42PM - 1:54PM Live |
S52.00012: Quantum anomalous Hall effect using interfacial Green's function method for an accurate mass gap Jinwoong Kim, David Vanderbilt By using a Green’s function method with DFT-based tight-binding parameters, we investigate the quantum anomalous Hall (QAH) effect at the interface between topological insulators (SnTe, SnSe) and magnetic insulators (EuS, EuSe, EuTe). QAH or axionic states, a subject of recent broad interest, are achieved by introducing an effective Zeeman field to a topological insulator whose surface Dirac cone then acquires a mass gap, resulting in exotic electromagnetic responses within the mass gap. A number of studies have demonstrated the appearance of such states by using diverse interfacial, magnetic-element-doped, and magnetic-topological systems in agreement with predictions. Although achieving a large mass gap is critical for further investigations and room temperature devices, the microscopic mechanisms determining the size of the mass gap have not been clearly addressed. In this study, we enumerate several combinations of topological crystalline insulators and magnetic insulators in a search for an optimal electronic structure, where a large mass gap is isolated inside a bulk insulating gap. The underlying mechanisms and their dependence on factors such as an external field will be discussed. |
Thursday, March 18, 2021 1:54PM - 2:06PM Live |
S52.00013: Unconventional anomalous Hall, Nernst and thermal Hall responses in Fe3GeTe2 Danilo Ratkovski, Juan Macy, Yu Che Chiu, Wenkai Zheng, Brian Casas, Eun Sang Choi, Luis Balicas Here, we evaluate the anomalous Hall, Nernst and thermal responses of the layered ferromagnet Fe3GeTe2 as a function of the temperature and field orientation. The Hall conductivity sxyA increases and saturates to large values upon cooling, buy the anomalous Nernst SxyA is observed to change its sign below T = 50 K. This is surprising given that both quantities result from the texture of the Berry curvature near the Fermi level, with the change in the sign in SxyA suggesting a possible topological transition. In support of this assertion, the anomalous thermal Hall kxyA is seen to display an anomaly around T = 50 K, albeit its ratio with respect to sxyA indicates that the Wiedemann Franz law is satisfied by this compound. Surprisingly, one observes anomalous transport quantities also for electrical currents and thermal gradients applied along the magnetic field. Furthermore, for this field orientation the anomalous quantities do not follow the magnetization. |
Thursday, March 18, 2021 2:06PM - 2:18PM Live |
S52.00014: Half-integer quantized anomalous thermal Hall effect and topological Chern number in the Kitaev material α-RuCl3 Yuichi Kasahara, Taichi Yokoi, Sixiao Ma, Shigeru Kasahara, Takasada Shibauchi, Nobuyuki Kurita, Hidekazu Tanaka, Joji Nasu, Yukitoshi Motome, Ciaran Hickey, Simon Trebst, Yuji Matsuda Kitaev quantum spin liquid (QSL) displays the fractionalization of quantum spins into Majorana fermions. In magnetic fields, the emergence of Majorana edge current leads to a half-integer quantized thermal Hall conductance, which has recently been reported for the two-dimensional honeycomb material α-RuCl3 [1]. We find that the thermal Hall plateau appears even for a magnetic field with no out-of-plane components, which is in stark contrast to the conventional electronic Hall effect that requires a perpendicular magnetic field. Field-angular variation of the topological Chern number determined by the sign of the quantized thermal Hall conductance is consistent with that predicted for the pure Kitaev QSL. These results provide evidence that the Kitaev interaction is of prime importance for realizing the non-Abelian topological order in α-RuCl3 [2]. |
Thursday, March 18, 2021 2:18PM - 2:30PM On Demand |
S52.00015: Correlation Between Chiral Fluctuations and Topological Hall Effect Above Room Temperature Tan Dao, Jiadong Zang The topological Hall effect is used exclusively as a signature of the magnetic skyrmions in a material. However, this phenomenon might have originated from other mechanisms such as thermally driven topology in chiral magnets. The relationship between topological spin structures and Hall conductivity were investigated at low temperatures; however, much remained unknown for the emergent topology at elevated temperatures. In this project, we performed the Monte Carlo simulation of a two-dimensional model of the chiral magnet and studied the correlation between Hall conductivity and the chiral fluctuations at elevated temperatures. We used both the topological charge and scalar chirality to characterize the topology of the spin-lattice. The Hall conductivity was calculated by applying the Kubo formula to the tight-binding model. Massive ensemble averages were taken to derive the thermal average of both the chirality and Hall conductivity. The results show there is a correlation between the topology and the Hall conductivity for a large number of averages. |
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