Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J59: Magnetic Weyl semimetals |
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Sponsoring Units: DMP Room: Mile High Ballroom 3C |
Tuesday, March 3, 2020 2:30PM - 2:42PM |
J59.00001: Single-crystal neutron diffraction study of Dirac material EuMnSb2 John Wilde, Benjamin Ueland, Simon Xavier Riberolles, Dominic H Ryan, Andreas Kreyssig, David Vaknin, Thomas Heitmann, Yong Liu, Sergey L. Bud'ko, Robert McQueeney EuMnSb2 is a potential Dirac semimetal with Dirac fermions located in the Sb layers. These Dirac fermions have the potential to be controlled by a magnetic field if the magnetic order of the Eu or Mn sublattices couples to them. A large anisotropic magnetoresistance suggests that this coupling is strong in EuMnSb2. Here we present results from single-crystal neutron diffraction experiments on EuMnSb2. We find that the Eu and Mn magnetic sublattices order antiferromagnetically (AF) at 24(1) K and 324(1) K, respectively, with an additional magnetic phase transition at 9(1) K related to the coupling between both magnetic sublattices. Neutron diffraction shows that each AF phase is characterized by a magnetic unit cell equal to the chemical unit cell in contrast to Mossbauer measurements which suggest incommensurability. Our analysis indicates that the ordered Eu moments chiefly point perpendicular to the Sb planes and that the Mn moments align within the planes. |
Tuesday, March 3, 2020 2:42PM - 2:54PM |
J59.00002: Visualization of Antiferromagnetic Domain Walls in the Topological Semimetal EuMnBi2 Wenbo Ge, Paul Sass, Weida Wu, Dimuthu Obeysekera, Junjie Yang The layered compound EuMnBi2 is one such system where magnetic order coexists with Dirac fermions. The interplay between the uniaxial A-type antiferromagnetic (AFM) Eu magnetic order and quasi-2D Dirac fermions was recently observed to give rise to a multilayer quantum Hall state at high magnetic field above the spin-flop transition (> 5 T)1. In this talk we will present magnetic imaging of AFM domain walls (DW) and their evolution across the spin-flop transition in the topological semimetal EuMnBi2 using our cryogenic magnetic force microscope2. Our results reveal that curvilinear AFM DWs naturally form when the sample is cooled below TN (~ 22 K). Magnetic field dependence of the DW signal suggests enhanced susceptibility inside the DWs in the A-type AFM state. The DWs disappear above the spin-flop transition, and re-nucleate at different locations after reentry into the A-type AFM state. The direct visualization of AFM DWs in topological semimetals will help in the understanding of topological phenomena in correlated magnetic systems. |
Tuesday, March 3, 2020 2:54PM - 3:06PM |
J59.00003: Signature of spin-fermion coupling in the magnetic excitations of Dirac semimetal YbMnBi2. Aashish Sapkota, Igor Zaliznyak, John Tranquada, Laura Classen, Cedomir Petrovic, Aifeng Wang, Matthew Stone, Andrei T Savici, Vasile O Garlea In Dirac semimetals, coupling of Dirac charge carriers with magnetism can lead to novel phenomena with potential for technological applications [1, 2]. From this perspective, 112 ternary pnictogens (A,R)MnX2 (A=Ca,Sr;R=Yb,Eu;X=Bi,Sb) represent an interesting family of Dirac materials where both the magnetism and Dirac electrons coexist, thereby providing an opportunity to study the spin-fermion coupling between Mn spins and Dirac electrons in Bi layer [2]. Previous studies of spin excitations in (Sr,Ca)MnBi2 using inelastic neutron scattering (INS) have found no indication of such coupling because anomalous broadening found in itinerant magnets was absent [2]. However, our recent INS measurements of spin waves on YbMnBi2 found a small and q-independent broadening in the spin waves, which is consistent with a significant spin-Dirac fermion coupling. Theoretical calculations show that the effect of coupling on the spin excitations is suppressed by a vanishing carrier density of states at the Dirac point. [1] A. Sapkota et al, arxiv 1908.08114 (2019), [2] M. C. Rahn et al, Phys. Rev. B 95, 134405 (2017). |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J59.00004: A Charge Density Wave Transition in NaxMnBiy magnetic semimetal Despina Louca, Aaron Wegner The I-Mn-V antiferromagnet, NaMnBi, was recently reported to develop a very large magnetoresistance (MR) up to 10,000 % at 2 K under a magnetic field of 9 T. A strong positive MR is present in crystals showing a semiconductor-to-metal transition (SMT) while, in the absence of an SMT, a modest (20 %) MR is achieved. Here, we show that upon cooling below the magnetic transition, a spatial modulation appears due to charge and defect ordering in a checkerboard pattern, with two kinds of displacement vectors q1=(2/3, 0, 1) and q2=(2/3, 1/3, 1/2). This couples to a superlattice transition (Ts) that lowers the symmetry from centrosymmetric P4/nmm to non-centrosymmetric P-4m2. In crystals with a large MR a close to room temperature Ts is observed with both q1 and q2 present. In crystals with low MR however, Ts is lower and only q1 is observed. Ts breaks spatial inversion symmetry and marks the onset of a charge density wave instability. |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J59.00005: Exotic phenomena in topological Weyl line magnets Ilya Belopolski, Guoqing Chang, Tyler Cochran, Jiaxin Yin, Songtian Sonia Zhang, Zijia Cheng, Xian Yang, Nana Shumiya, Daniel Multer, Maksim Litskevich, Zahid Hasan Topological electronic phases in intrinsic magnets are of current interest. One common magnetic topological object is the Weyl line, a two-fold band degeneracy along a closed curve in bulk momentum space. Weyl lines arise naturally in the absence of time-reversal symmetry (absent in magnets) and in the presence of mirror symmetry (common in many space groups). I will present the observation of Weyl lines by ARPES in the room temperature magnet Co2MnGa [1]. On the surface of the magnet, I observe drumhead surface states, pinpointing the bulk-boundary correspondence and suggesting a Berry phase topological invariant associated with the Weyl line. Next, the intrinsic Berry curvature contribution to the anomalous Hall response, determined from quantum transport, agrees with a prediction of the Weyl line Berry curvature based on ARPES and first-principles calculations [1]. I will comment on composite topological structures arising from pinning and linking of different Weyl lines. Lastly, I will discuss future directions in Co2MnGa and other topological Weyl line magnets. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J59.00006: Band splitting and signatures of Fermi-arcs in the magnetic Weyl candidate CeBi Yu Liu, Christian Matt, Harris Pirie, Nathan Drucker, Robert-Jan Slager, Na Hyun Jo, Brinda Kuthanazhi, Sergey L. Bud'ko, Paul C Canfield, Jennifer E. Hoffman Weyl semimetals are characterized by topologically protected Weyl nodes in the bulk and Fermi-arcs states with momentum-locked spin or chirality on the surface. Among various Weyl semimetals, magnetic Weyl semimetals are particularly attractive because they have fewer, more widely separated Weyl nodes. Additionally, their Weyl nodes can potentially be manipulated by external magnetic field or changes in the local moment ordering. Here we present a scanning tunneling microscopy and quasiparticle interference (QPI) study of the magnetic Weyl semimetal candidate CeBi. In particular, we resolve splitting of the Bi p and Ce d bands of CeBi with 9T external magnetic field, which is the necessary precondition for Weyl nodes. We also observe signatures of Fermi-arcs in the QPI pattern. The QPI-derived band structure of CeBi in different magnetic phases is also studied. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J59.00007: Magnetic semimetals and quantized anomalous Hall effect in EuB6 Si-Min Nie Exploration of the novel relationship between magnetic order and topological semimetals has received enormous interest in a wide range of both fundamental and applied research. Here we predict that \soft" ferromagnetic (FM) material EuB6 can achieve multiple topological semimetal phases by simply tuning the direction of the magnetic moment. Explicitly, EuB6 is a topological nodal-line semimetal when the moment is aligned along the [001] direction, and it evolves into a Weyl semimetal with three pairs of Weyl nodes by rotating the moment to the [111] direction. Interestingly, we identify a novel semimetal phase featuring the coexistence of a nodal line and Weyl nodes with the moment in the [110] direction. Topological surface states and anomalous Hall conductivity, which is sensitive to the magnetic order, have been computed and are expected to be experimentally observable. Large-Chern-number quantum anomalous Hall effect can be realized in its [111]-oriented quantum-well structure. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J59.00008: Real-space imaging of surface-magnetism in magnetic Weyl semimetal candidate CeBi Christian Matt, Yu Liu, Nathan C Drucker, Harris Pirie, Robert-Jan Slager, Na Hyun Jo, Brinda Kuthanazhi, Sergey L. Bud'ko, Paul C Canfield, Jennifer E. Hoffman Magnetic Weyl semimetals host chiral Weyl nodes in their bulk which are formed by broken time-reversal symmetry, caused, for example, by intrinsic ferromagnetic order. While the bulk-surface correspondence dictates the formation of Fermi arcs connecting Weyl nodes with opposite chirality, their momentum-space contour and their connectivity (i.e. which Weyl points they are connecting to), depends on surface properties. Here we use spin-polarized scanning tunneling microscopy to map the magnetic structure on the surface of CeBi at various bulk-magnetic phases. We find unexpected surface-magnetic order with strongly bias-dependent magnetic contrast and phase. We discuss our observation in terms of Fermi arc connectivity and magnetic interactions in CeBi. |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J59.00009: Effect of chemical substitution on the anomalous Hall effect of chiral-lattice antiferromagnet CoNb3S6 Dina Michel, Nishchal Thapa Magar, Nicholas Bishop, Lekhanath Poudel, Jeffrey Lynn, John Mitchell, Nirmal Ghimire An ordinary Hall effect in a conductor arises due to the Lorentz force acting on the charge carriers. In ferromagnets, an additional contribution to the Hall effect, the anomalous Hall effect (AHE), appears proportional to the magnetization. It is also known that the AHE can arise in non-collinear and non-coplanar antiferromagnets due to scalar spin chirality. However, recently a large AHE was observed in a collinear antiferromagnet CoNb3S6 [1]. More recent theories have indicated that the AHE in CoNb3S6 may be inherently related to the chirality associated with the crystal structure [2,3]. The presence of a small ferromagnetic component plays an important role in revealing the AHE in CoNb3S6 which was limited to a narrow temperature range just below the Néel temperature. Here we will show that partial substitution of Co by Fe widens the temperature range and that the AHE in Co1-xFexNb3S6 saturates at lower temperatures. We will discuss the importance of this observation in the context of recent theories of the AHE in collinear antiferromagnets. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J59.00010: In-plane antiferromagnetic moments in axion topological insulator candidate EuIn2As2 Yang Zhang, Ke Deng, Xiao Zhang, Meng Wang, Cai Liu, Eike F. Schwier, Shiv Kumar, Chaoyu Chen, Bing Shen Topological insulator with antiferromagnetic order can serve as an ideal platform for the realization of axion electrodynamics. In this paper, we report a systematic study of the axion topological insulator candidate EuIn2As2. A linear energy dispersion across the Fermi level confirms the existence of the proposed hole-type Fermi pocket. Spin-flop transitions occur with magnetic fields applied within the ab-plane while are absent for fields parallel to the c-axis. Anisotropic magnetic phase diagrams are observed and the orientation of the ground magnetic moment is found to be within the ab-plane. The magnetoresistivity for EuIn2As2 behaves non-monotonic as a function of field strength. It exhibits angular dependent evolving due to field-driven and temperature-driven magnetic states. These results indicate that the magnetic states of EuIn2As2 strongly affect the transport properties as well as the topological nature. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J59.00011: Determination of the magnetic order in the EuIn2As2, an axion insulator candidate Simon Riberolles, Brinda Kuthanazhi, Na Hyun Jo, Adam Kaminski, Paul C Canfield, Thomas Heitmann, Dominic H Ryan, Benjamin Ueland, Robert McQueeney EuIn2As2 is a Zintl compound recently reported as a possible first example of axion insulator with antiferromagnetic (AF) long-range order and characteristic colossal negative magnetoresistance. It thus represents a unique platform to investigate the physics of axion insulators and, in particular, to focus on the interplay between the AF order and the topologically protected features of the electronic band structure. Here, we present results from single-crystal magnetic diffraction experiments. Surprisingly, our results reveal the successive stabilization of two different sets of magnetic Bragg reflections indexed with the propagation vectors k1=(0,0,0.31) and k2=(0,0,0) below TN1=17 and TN2=15 K, respectively, despite the chemical unit cell containing only a single Eu site. The two magnetic structures are believed to coexist at T≤TN2, and were individually determined from refinement analyses. We will describe both magnetic structures and discuss the influence of our results upon the current understanding of the material’s topological features. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J59.00012: Anomalous Hall effect with colossal magnetic saturation and coercivity in (111)-oriented pyrochlore Eu2Ir2O7 thin films Xiaoran Liu, Wenbo Ge, Mikhail Kareev, Fangdi Wen, Eun Sang Choi, Weida Wu, Jak Chakhalian The pyrochlore iridates are a class of candidate materials for the topological Weyl semimetal, which can host hidden topological and magnetic phases in the thin film form. To investigate the intrinsic properties relevant to the Ir sublattices, we fabricated high-quality epitaxial (111)-oriented pyrochlore Eu2Ir2O7 thin films on YSZ substrates, in which the Eu3+ ions are magnetically silent. Temperature-dependent transport reveals an onset of the metal-insulator transition at TC around 110 K, similar to the bulk value. However, magneto-transport measurements reveal that below TC in the insulating phase with the non-colinear all-in-all-out antiferromagnetic order, a distinct anomalous Hall effect (AHE) emerges combined with a butterfly-shape hysteretic magnetoresistance. The AHE gets strongly pronounced at lower temperatures, with colossal enhancement of both the saturation field HS and the coercivity HC. These findings are signatures of the emergent interacting topological phases in thin films of [111]-oriented pyrochlore iridates. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J59.00013: Domain Wall-Dependent Electrical Transport in a Magnetic Weyl Semimetal Nicholas Quirk, Guangming Chen, Nan Yao, N. Phuan Ong Magnetic force microscopy reveals that microscale lamellae fabricated by focused ion beam microscopy of Co2MnGa—a ferromagnetic Weyl semimetal—contain just a few magnetic domains which form a pattern that is different on the top surface than on the bottom. Transport measurements of electrical devices made from these lamellae with contacts on both faces show large resistance anisotropies that depend on in-plane and out-of-plane magnetic fields in different ways. These data taken together describe a very non-homogeneous current distribution which may result from the interaction of the Weyl fermions in this material with the magnetic domain walls. |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J59.00014: Imaging the Electronic Structures in Antiferromagnetic Dirac Semimetal GdSbTe Balaji Venkatesan, Chih-Chuan Su, Ramesh Babu, Raman Sankar, Fangchang Chou, Guang-Yu Guo, Tien-Ming Chuang Dirac semimetals (DSMs) are characterized by topology- or symmetry- protected band crossings near the Fermi level that can form Dirac points, nodal lines, nodal surface or nodal chains in the electronic band structures. These unique band crossings can give rise to interesting properties such as large anisotropic magnetoresistance, chiral anomaly and flat optical conductivity. Moreover, magnetic DSMs can break the time reversal symmetry, where the interplay of symmetry, relativistic effects and the magnetic order can lead to novel quantum phases and offer the tunability of these quantum states with magnetic field. GdSbTe is a non-symmorphic semimetal with an antiferromagnetic phase below T=13K. By using spectroscopic imaging - scanning tunneling microscopy and first principle calculation, we investigated the surface and bulk electronic structures in GdSbTe across the magnetic transition and under magnetic field. Our quasiparticle scattering interference imaging and calculation showed the linear dispersion from Dirac bands over a wide energy range and are robust against different magnetic orders, suggesting the rigidity of the non-symmorphic crystalline symmetry protection in GdSbTe. |
Tuesday, March 3, 2020 5:18PM - 5:30PM |
J59.00015: Interplay between topology and magnetic excitations in topological nodal semimetal CeAlGe Thanh Nguyen, Mingda Li, Fei Han, Nina Andrejevic, Ricardo Pablo Pedro, Matthew Stone, Songxue Chi, Jaime Fernandez-Baca, Masaaki Matsuda, David Tennant Weyl semimetals (WSMs) comprise of a novel condensed matter phase that carries emergent quasiparticles of topologically nontrivial chiral Weyl fermions and represent a key recent advance in the realm of strongly spin-orbit-coupled materials. Magnetic WSMs, in which the nontrivial Weyl fermions break time-reversal symmetry, enable the possibility of further discovering unexplored spin-electronic phenomena enabling feasible spintronic applications. In this talk, I will present results obtained from neutron scattering experiments performed on predicted type-II WSM, CeAlGe. Our measurements of low-energy spin waves as well as crystal-field excitations using time-of-flight and triple-axis inelastic neutron scattering highlight phenomena intertwining its nontrivial topology with its unusually intricate magnetic structure. Through the investigation of these excitations, this study elucidates the substantial coupling between the exotic magnetic configuration and the topological semimetallic electronic structure within this material, which may see a valuable role in future practical applications. |
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