Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A59: Magnetic Weyl Semimetals: Materials DiscoveryFocus
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Sponsoring Units: DMP Room: Mile High Ballroom 3C |
Monday, March 2, 2020 8:00AM - 8:12AM |
A59.00001: Stabilizing ferromagnetism in the Weyl semimetal candidate EuCd2As2 Brinda Kuthanazhi, Na Hyun Jo, Yun Wu, Tae-Hoon Kim, Lin Zhou, Erik I Timmons, Lin-Lin Wang, Andriy Palasyuk, Kyungchan Lee, Benjamin Schrunk, Benjamin Ueland, Anton Burkov, Ruslan Prozorov, Sergey L. Bud'ko, Adam Kaminski, Paul C Canfield EuCd2As2 is predicted to host a variety of topological features in its band structure depending on its magnetic ground state. A ferromagnetic state with spins oriented along the crystallographic c axis is expected to have a single pair of Weyl points near the Fermi energy. [1] Previously published work on EuCd2As2 report it to have an antiferromagnetic ground state. [2,3] Here, we present the synthesis and characterization of single crystals of EuCd2As2 that, with varying initial stoichiometries, can be tuned from an antiferromagnetic ground state (TN ~ 9.5 K) to a ground state with a clear ferromagnetic component (TC ~ 30 K). Measurements of magnetization, magneto-optical Kerr rotation, specific heat and resistivity will be presented and discussed. |
Monday, March 2, 2020 8:12AM - 8:24AM |
A59.00002: A New Magnetic Topological Quantum Material Candidate by Design: EuSn2P2 Xin Gui, Ivo Pletikosic, Huibo Cao, Hung-Ju Tien, Xitong Xu, Ruidan Zhong, Guangqiang Wang, Tay-Rong Chang, Shuang Jia, Tonica Valla, Weiwei Xie, Robert J. Cava Magnetism, when combined with an unconventional electronic band structure, can give rise to forefront electronic properties such as the quantum anomalous Hall effect, axion electrodynamics and Majorana fermions. Here we report the characterization of high-quality crystals of EuSn2P2, a new quantum material specifically designed to engender unconventional electronic states plus magnetism. EuSn2P2 has a layered, Bi2Te3-type structure. Ferromagnetic interactions dominate the Curie-Weiss susceptibility, but a transition to antiferromagnetic ordering occurs near 30 K. Neutron diffraction reveals that this is due to two-dimensional ferromagnetic spin alignment within individual Eu layers and antiferromagnetic alignment between layers - this magnetic state surrounds the Sn-P layers at low temperatures. The bulk electrical resistivity is sensitive to the magnetism. Electronic structure calculations reveal that EuSn2P2 might be a strong topological insulator, which can be a new magnetic topological quantum material (MTQM) candidate. The calculations show that surface states should be present, and they are indeed observed by ARPES measurements. |
Monday, March 2, 2020 8:24AM - 8:36AM |
A59.00003: Angle resolved photoemission spectrocopy study on EuCd2As2 Na Hyun Jo, Yun Wu, Lin-Lin Wang, Kyungchan Lee, Brinda Kuthanazhi, Benjamin Schrunk, Sergey L. Bud'ko, Paul C Canfield, Adam Kaminski Recently, DFT calculations on EuCd2As2 in a FM ordered state with Eu moments alligned along the c-axis revealed a single pair of Weyl points. However, previous experimental studies on EuCd2As2 show that the magnetism is an A-type AFM with TN ~ 9.5 K. Here, by discovering and taking advantage of EuCd2As2 chemical tunability, we report successful growths of single crystals of EuCd2As2 with two different magnetic ground states: FM-EuCd2As2 and AFM-EuCd2As2. Angle resolved photoemission spectrocopy shows an anomalous suppression of scattering in ferromagnetic Weyl semimetal candidate EuCd2As2. Unexpectedly, scattering rate is suppressed only in one of the bands over limited energy range that is controlled by presence of another, completely filled band. Such selectivity is highly unusual and can have potential impact on spin transport. |
Monday, March 2, 2020 8:36AM - 8:48AM |
A59.00004: Ultrafast carrier dynamics in the ferromagnetic semimetal EuCd2As2 Kenneth O'Neal, LaMoyne Tyler Mix, Min-Cheol Lee, Brinda Kuthanazhi, Na Hyun Jo, Sergey L. Bud'ko, Paul C Canfield, Rohit P Prasankumar, Dzmitry Yarotski We report the near-infrared pump-probe ultrafast response of the ferromagnetic candidate for Weyl semimetal EuCd2As2. A non-pulsewidth limited rise dynamic is followed by two distinct decay processes – a sub-picosecond decay attributed to electron-phonon relaxation and a several picosecond long process which stems from demagnetization of the in-plane magnetic moment. Exponential fits of the decay processes down to 6 K reveal sensitivity to the long range magnetic ordering. These findings indicate strong coupling between charge and spin degrees of freedom, a step towards magnetic field or light control of topological Weyl states. |
Monday, March 2, 2020 8:48AM - 9:00AM |
A59.00005: Single crystal growth of Weyl semimetal candidates Sudha Krishnan, Tiglet Besara Following the discovery of topological Weyl semimetals in non-magnetic materials, ferromagnetic Weyl semimetals have now been discovered with materials such as YbMnBi2, Co2MnGa, and Co3Sn2S2. With the different structures of the already discovered ferromagnetic Weyl semimetals, this suggests that more ternary intermetallic compounds can be found displaying the connection between ferromagnetism and topology. Here, we report a systematic growth study and search for other ternary intermetallics displaying magnetism and topology. We utilize metallic self-flux methods to grow single crystals of compounds isostructural to YbMnBi2 and Co2MnGa. |
Monday, March 2, 2020 9:00AM - 9:12AM |
A59.00006: Giant room temperature anomalous Hall effect and magnetically tuned topology in the ferromagnetic Weyl semimetal Co2MnAl Peigang Li, Jahyun Koo, Wei Ning, Jinguo Li, Leixin Miao, Lujin Min, Yanglin Zhu, Yu Wang, Nasim Alem, Chao-Xing Liu, Zhiqiang Mao, binghai yan Weyl semimetals have been extensively studied due to their exotic properties such as topological surface states and anomalous transport phenomena. Their band structure topology is usually predetermined by material parameters and can hardly be manipulated once the material is formed. By calculations and experiments, we reveal a tunable, giant anomalous Hall effect in the ferromagnet Co2MnAl. The transition between Weyl points and nodal rings occurs when rotating the magnetization axis. We propose a material recipe to generate the giant anomalous Hall effect by gaping nodal rings without requiring the existence of Weyl points. In addition, experiments show the strong anomalous Hall effect even at room temperature with a large Hall angle 21%. Our work reveals an ideal intrinsically magnetic platform to explore the interplay between magnetic dynamics and topological physics for the development of a new generation of spintronic devices. |
Monday, March 2, 2020 9:12AM - 9:24AM |
A59.00007: Magnetotransport properties and fermiology in a layered rare-earth intermetallics Takashi Kurumaji, Min Gu Kang, Shiang Fang, David E Graf, Linda Ye, Yang Zhao, Mun K. Chan, Takehito Suzuki, Riccardo Comin, Jeffrey Lynn, Joseph G Checkelsky While a growing number of nonmagnetic topological phases are being proposed and discovered, the search for materials that combine nontrivial electronic topology with strong correlations has advanced at a slower pace [1]. Most studies have focused on doping magnetic elements into nonmagnetic topological materials [2] in which random disorder and low magnetic transition temperature are inevitable. Realization of magnetic topological state in stoichiometric compounds has the potential to enable an approach to the intrinsic properties of the system, and provide a chance to design candidates for higher temperature quantized anomalous Hall effect and axion electrodynamics [3]. We will discuss our recent attempts to design, synthesize, and characterize magnetic topological semimetals with a focus on layered materials with non-trivial magnetic order arising from rare-earth ions. We will describe transport, spectroscopic, and neutron scattering measurements as well as ab-initio calculations to develop an understanding of their underlying electronic/magnetic structure. |
Monday, March 2, 2020 9:24AM - 10:00AM |
A59.00008: Layered Rare-Earth Intergrowth Compounds: A Platform for Correlated and Topological Properties Invited Speaker: Julia Chan The single crystalline growth of lanthanide based intermetallics has garnered much interest in the last two decades, primarily due to their unusual magnetic and electrical properties. To study the intrinsic and physical properties of highly correlated quantum systems, the growth of single crystalline intermetallics is critical. With our ongoing efforts to grow single crystals of rare earth materials, the opportunity exists to discover new compositions or robust structure types enabling subsequent substitution for correlation studies for compounds with competing magnetic behavior. In this talk, examples of the strategic selection of rare earth layered intergrowth compounds will be presented. In particular, we will present a new class of Ce-based intermetallics for fundamental correlation between the subunits and structural motifs common in highly correlated materials. To advance the field, discovering new families of highly correlated material is vital to the advancement of correlated and topological materials. |
Monday, March 2, 2020 10:00AM - 10:12AM |
A59.00009: Synthesis and characterization of a new topological semimetal enabled by a charge density wave Shiming Lei, Jingjing Lin, Andreas Topp, Christian R Ast, N. Phuan Ong, Leslie Schoop Formation of charge density waves (CDWs) is a known mechanism to induce an energy gap or partial energy gap at the Fermi surface. Therefore, introducing a CDW can be a way to modify the band structure. In this work, we discuss the potential of a CDW being the key to realize materials with clean topological semimetal band structures, by gapping out unwanted trivial states at the Fermi level. We will introduce a class of materials that exhibits CDWs and discuss the effect of the structural distortion on the electronic structure. Finally, we will present electrical transport and angle resolved photoemission spectroscopy (ARPES) data on one magnetic topological semimetal candidate, which was identified through this method. |
Monday, March 2, 2020 10:12AM - 10:24AM |
A59.00010: Quantum Anomalous Hall Effect in Intrinsic Magnetic Topological Insulator MnBi2Te4 Yujun Deng, Yijun Yu, Meng Zhu Shi, Zhongxun Guo, Zihan Xu, Jing Wang, Xianhui Chen, Yuanbo Zhang In a magnetic topological insulator, nontrivial band topology conspires with magnetic order to produce exotic states of matter that are best exemplified by quantum anomalous Hall (QAH) insulators and axion insulators. Here, we probe quantum transport in MnBi2Te4 thin flake—a topological insulator with intrinsic magnetic order. In this layered van der Waals crystal, the ferromagnetic layers couple anti-parallel to each other, so bulk MnBi2Te4 is an antiferromagnet. Atomically thin MnBi2Te4, however, becomes ferromagnetic when the sample has odd number of septuple layers. Signatures of QAH effect are observed in few-layer MnBi2Te4 samples. Our results establish MnBi2Te4 as an ideal arena for further exploring various topological phenomena. |
Monday, March 2, 2020 10:24AM - 10:36AM |
A59.00011: Routes to quantum anomalous Hall effect from superlattice-like magnetic topological insulators Qihang Liu, Hongyi Sun, Pengfei Liu, Ni Ni, Chaowei Hu, Kyle Gordon, Daniel Dessau Recently, MnBi2Te4 thin films were observed to manifest the quantum anomalous Hall effect (QAHE) at a temperature of 4.5 K under magnetic field. By realizing a bulk MnBi4Te7 with alternating [MnBi2Te4] and [Bi2Te3] layers, we suggest that MnBi4Te7 is a Z2 antiferromagnetic topological insulator with a small out-of-plane saturation field of ~ 0.2 Tesla [1]. Using model Hamiltonian analysis and first-principle calculations, we then establish a topological phase diagram and map on it with different two-dimensional configurations, which is taken from the recently-grown magnetic topological insulators MnBi4Te7 and MnBi6Te10. These configurations manifest various topological phases, including quantum spin Hall effect with and without time-reversal symmetry, as well as QAHE. We then provide design principles to trigger QAHE by tuning experimentally accessible knobs, such as slab thickness and magnetization [2]. Our work reveals that superlattice-like magnetic topological insulators with tunable exchange interaction serve as an ideal platform to realize the long-sought QAHE in pristine compounds. |
Monday, March 2, 2020 10:36AM - 10:48AM |
A59.00012: A new method for tuning topological insulators with large carrier densities (~1020/cc) to CNP and beyond Haiming Deng, Lukas Zhao, Kyungwha Park, Lia Krusin-Elbaum We report a new powerful method for tuning chemical potential in topological materials, capable of changing carrier densities by more than 1020/cc. It involves the incorporation of H+ into the device structure. The process is reversible by low-temperature annealing (< 100oC), and a finetuning of the chemical potential is easily achieved. We demonstrate this method to be effective in a variety of 3D bulk topological insulators (TIs) (e.g. Bi2Te3 and Sb2Te3), as well as in intrinsic magnetic TIs, such as Mn(Bi, Sb)2Te4. For example, Bi2Te3 which is initially p-type with hole carrier densities of >1019/cc is converted to n-type with n >1019/cc. This electron doping method should apply to other materials as well. Using a proper annealing protocol, the chemical potential is finetuned across the charge-neutral point (CNP), where the Hall resistance exhibits the ambipolar behavior and Rxx has a maximum at the type conversion. This method does not create additional defects that reduce carrier mobility, nor induce any significant top-bottom surface asymmetries typical of electrostatic gating. The mechanism responsible for the H+ doping will be discussed. |
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