Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session A43: Magnetic Weyl SemimetalsFocus
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Sponsoring Units: DMP Chair: Ryotaro Arita, Univ of Tokyo Room: Room 317 |
Monday, March 6, 2023 8:00AM - 8:36AM |
A43.00001: Magnetic Weyl nodal ring states and Landau quantization in square-net magnets Invited Speaker: Shiming Lei Magnetic topological semimetals (TSMs) have been the source of many new discoveries in recent years. Among the magnetic TSMs, the Weyl nodal line semimetals are probably the least experimentally studied due to the limited materials that are avaliable. To date, the only magnetic compound that has been identified to host Weyl nodal ring states with spectroscopic evidence is the Heusler alloy Co2MnGa, which has led to fruitful research results, enabling the discovery of the drumhead surface bands, giant anomalous Nernst effect, and more recently the linked-loop quantum state. In spite of these achievements, Landau quantization of the Weyl nodal line states and their associated magnetotransport properties remain largely unexplored. In this talk, I will introduce a new magnetic Weyl nodal line semimetal with a layered square-net structure motif, featuring the Weyl nodal ring states in the close vicinity of the Fermi level. I will go through the design strategy, and the detailed experimental verification including angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. In this talk, the field tunable topological phase transition and the exotic magneto-transport properties in the Landau quantized regime will also be discussed. |
Monday, March 6, 2023 8:36AM - 8:48AM |
A43.00002: Neutron scattering and magnetic excitations in Weyl semimetal NdAlSi Chris J Lygouras, Jonathan Gaudet, Hung-Yu Yang, Xiaohan Yao, Andrey Podlesnyak, Predrag Nikolic, Jose A Rodriguez, Fazel Tafti, Collin L Broholm Realizing time-reversal symmetry breaking by magnetic order in Weyl semimetals has been a long-standing goal, one that can potentially lead to unique physical phenomena. NdAlSi is an interesting example of a non-centrosymmetric material where Weyl electrons may mediate the magnetic exchange interactions that result in a helical magnetic structure. To gain further insight into the relevant exchange interactions in NdAlSi, we used inelastic neutron scattering to probe the magnetic excitations in the paramagnetic, incommensurate, and commensurate states of NdAlSi. We highlight the importance of the crystal electric field; our approach to modeling the data; and how we might gain more insight into possible Weyl-mediated exchange interactions. |
Monday, March 6, 2023 8:48AM - 9:00AM |
A43.00003: GdAlSi: A potential candidate for antiferromagnetic Weyl semimetal Jadupati Nag We report the theoretical prediction of a new Weyl semimetal (WSM), which is also supported by our experimental findings. WSMs are novel quantum phase, which are known to host emerging Weyl fermions as a result of either broken time-reversal symmetry or inversion symmetry. We predict such a unique WSM feature in a non-centrosymmetric magnetic crystal GdAlSi. GdAlSi crystallizes in a perfect body-centered tetragonal structure with space group I41md (#109). The magnetization data indicates antiferromagnetic ordering with the ordering temperature (TN) 32 K. Our ab-initio simulation confirms emerging topological properties in this system. Moreover, our experimental results further support the novel WSM phase in GdAlSi. Hence, our study on this quantum composite material provides a new direction for exploring the emerging topological states for future research and applications in topotronics. |
Monday, March 6, 2023 9:00AM - 9:12AM |
A43.00004: Evidence of robust edge mode in magnetic Weyl semimetal Co3Sn2S2 PRANAB K NAG, Noam Morali, Rajib Batabyal, Jahyun Koo, Hengxin Tan, Raquel Queiroz, Maxim Breitkreiz, Piet Brouwer, Enke Liu, Claudia Felser, Binghai Yan, Nurit Abraham, Haim Beidenkopf Ferromagnetic Weyl semimetal Co3Sn2S2 belongs to the family of magnetic topological materials where time-reversal symmetry is spontaneously broken. We use scanning tunneling microscopy and spectroscopy to study its topological nature by measuring robust boundary responses. Upon studying several clean step edges we have identified the existence of an edge mode within a formerly unidentified energy window. With the support of ab initio calculation, we are able to relate this edge mode to a new set of Weyl nodes within a corresponding energy interval. Furthermore, we have observed the systematic evolution of the edge mode from a state bound to a single vacancy through a growing cluster of vacancies. This observation demonstrates the deep relation between defect states and Fermi arc states at extended crystallographic boundaries, both governed by the topological semimetallic nature of the bulk. |
Monday, March 6, 2023 9:12AM - 9:24AM |
A43.00005: Theory of spin/charge transport and magnetic orderings in stacked kagome-lattice Weyl semimetal Akihiro Ozawa, Kentaro Nomura Weyl semimetals are zerogap semiconductors with relativistic linear dispersion near the Fermi level. In particular, magnetic Weyl semimetals, in which time-reversal symmetry is broken by magnetism, exhibit an anomalous Hall effect. Co3Sn2S2 (CSS) is a stacked kagome lattice system that is a candidate material for magnetic Weyl semimetals. CSS shows an out-of-plane ferromagnetic ordering and a giant intrinsic anomalous Hall effect originated from the Berry curvature generated by the Weyl points. On the other hand, it has been reported experimentally that carrier-doped systems show changes in magnetic structure such as suppression of out-of-plane magnetic moments. However, theoretical investigations on the magnetic orderings in the case of different electron numbers are still needed. Furthermore, in systems with strong spin-orbit coupling such as CSS, transport phenomena such as anomalous/spin Hall effects are expected to correlate with magnetic orderings. |
Monday, March 6, 2023 9:24AM - 9:36AM |
A43.00006: Thickness dependent quantum oscillations in ferromagnetic Weyl metal SrRuO3 Wei-Li Lee, Wei-Cheng Lee, Steffen Wiedmann, Akhilesh K Singh, Uddipta Kar, Yu-Te Hsu, Steffen Wiedmann, Maarten Berben, Chih-Yu Lin, Bipul Das, Cheng-Tung Cheng, Chia-Hung Hsu, Song Yang, Chun-Yen Lin In a Weyl semimetal, a unique Weyl-orbit quantum oscillations can appear resulting from a nonlocal cyclotron orbit that connects the top and bottom Fermi-arc surface states via the bulk Weyl nodes. In this talk, such a Weyl-orbit effect (WOE) will be demonstrated in untwinned thin films of ferromagnetic Weyl metal SrRuO3 (SRO). From thickness dependent measurements of the quantum oscillations with field strength up to 35 T, we uncovered an unusual small Fermi pocket with a light effective mass and an oscillation frequency of about 30 T, which behaves as a 2D-like Fermi-pocket and shows a maximum oscillation amplitude for film thickness in a range of 10 – 20 nm. In addition, unusual thickness dependent phase shift and also concave downward curvature in the Landau fan diagram were observed, which agrees well with the simulated curves based on WOE theory with nonadiabatic correction terms. Discussions between the simulations and experimental data will be provided to address the issue regarding the optimum film thickness for achieving dominant WOE contribution in Weyl metal SRO. |
Monday, March 6, 2023 9:36AM - 9:48AM |
A43.00007: Observation of spiral spin order and A-phase in Weyl semimetal SmAlSi Xiaohan Yao, Jonathan Gaudet, Predrag Nikolic, David E Graf, Fazel Tafti The role of Weyl electrons in magnetism has drawn great attention recently. Previous work on NdAlSi [1] has shown a minor spiral modulation of spins, which could be due to nesting between the Weyl nodes; however, a fully spiral order has not been observed. We present evidence of a spiral magnetic order in SmAlSi induced by Weyl-mediated RKKY interactions [2]. We highlight the role of structural symmetries, nesting between the Weyl nodes, and magneto-crysgtalline anisotropy in the spiral magnetic state of SmAlSi. Furthermore, we map the phase diagram, observe a large topological hall effect (THE) within the A-phase, and show that its angle dependence is consistent with the Weyl-mediated DM interactions. Specifically, we show that the angle dependence of THE is consistent with a considerable helical component of the spiral order in SmAlSi as expected from Weyl-mediated RKKY interactions [3]. |
Monday, March 6, 2023 9:48AM - 10:00AM |
A43.00008: Maximizing anomalous Hall effect by tuning the Fermi level in simple Weyl semimetal EuCd2Sb2 Masaki Uchida, Mizuki Ohno, Susumu Minami, Yusuke Nakazawa, Shin Sato, Markus Kriener, Ryotaro Arita, Masashi Kawasaki Magnetic Weyl semimetal EuCd2Sb2 provides an ideal platform for demonstrating and utilizing intrinsic anomalous Hall effect originating in the Weyl nodes. This is because EuCd2Sb2 in the forced ferromagnetic state hosts a simple Weyl-node-related band structure near the Fermi energy. Here we report growth of EuCd2Sb2 single-crystalline films by molecular beam epitaxy and observation of large anomalous Hall effect dependent on the carrier density. By approaching the Fermi level to Weyl node energy positions with controlling growth conditions and applying electrostatic gating, the anomalous Hall angle is largely enhanced as compared to previously reported EuCd2Sb2 single-crystalline bulk one. As also confirmed by first-principles calculations of intrinsic anomalous Hall conductivity, the observed anomalous Hall effect shows a sharp peak as a function of the carrier density, revealing clear energy dependence of the Weyl-node-based anomalous Hall effect. Our present work paves the way for further exploring the potential of Weyl-node-based exotic magnetotransport using film techniques. |
Monday, March 6, 2023 10:00AM - 10:12AM |
A43.00009: Structure-driven topological phase transition in a quasi-one-dimensional ferromagnetic Weyl semi-metal Chin Shen Ong, Ola K Forslund, Nicolas Gauthier, Hiroshi Uchiyama, Daniel Mazzone, Masafumi Horio, Nami Matsubara, Elisabetta Nocerino, Deepak J Mukkattukavil, Konstantinos Papadopoulos, Kazuya Kamazawa, Kazuhiko Ikeuchi, Romain Sibille, Antonio M dos Santos, Hidenori Takagi, Masahiko Isobe, Jun Sugiyama, Johan Chang, Yasmine Sassa, Olle Eriksson, Martin Mansson The quasi-one-dimensional compound K2Cr8O16 is famous for exhibiting a structure-driven ferromagnetic-metal-to-ferromagnetic-insulator transition (FM-MIT), a transition not explained by the Hubbard model. Since the doped metallic phase of K2Cr8O16 belongs to the special class of magnetic Weyl semimetals, this FM-MIT is also a topological phase transition. In this work, we show that the unique topological phase transition from a trivial insulator to a topological magnetic Weyl semimetals is structure driven, and can be achieved using temperature as the tuning parameter. Through inelastic X-ray and neutron scattering experiments combined with first-principles theoretical calculations, we demonstrate the absence of divergences in the electronic and magnetic responses across the transition, disproving the previous proposition that this transition is driven by a Peierls instability. We further establish that the exchange interaction is mainly superexchange in nature, which persists across the transition, as opposed to double-exchange type as previously proposed. This work represents the first systematic study of a topological phase transition for a structure driven ferromagnetic Weyl semimetal, paving way for novel topological spintronics devices. |
Monday, March 6, 2023 10:12AM - 10:24AM |
A43.00010: Carrier density dependence of the anomalous Hall conductivity of EuCd2As2 Yue Shi, Zhaoyu Liu, Qianni Jiang, Elliott W Rosenberg, Jonathan M DeStefano, Cheng-Chien Chen, Jiun-Haw Chu Ideal magnetic Weyl semimetals with a single pair of Weyl points close to the Fermi level are highly desirable for realizing the intrinsic topological effects associated with Weyl fermions. The antiferromagnetic layered EuCd2As2 was predicted as an ideal Weyl semimetal in the spin polarized ferrimagnetic (FM) state. However, the direct experimental demonstration of the ideal FM Weyl state in EuCd2As2 is still missing. Here, we successfully synthesized highly insulating crystals EuCd2As2 and were able to control the carrier density by several orders of magnitude. We observed colossal negative magnetoresistance in low carrier density EuCd2As2 when the external magnetic field polarized the spins. However, the anomalous Hall conductivity (AHC) decreases by several orders of magnitude as the carrier density decreases. This behavior is inconsistent with the AHC resulted from the Berry curvature generated by the Weyl points, which is independent of carrier density. Our electrical transport result suggests that more studies are required to elucidate the nature of the topological phase of FM EuCd2As2. |
Monday, March 6, 2023 10:24AM - 10:36AM |
A43.00011: Discovery of a topological quantum link Ilya Belopolski, Guoqing Chang, Tyler A Cochran, Zi-Jia Cheng, Xian Yang, Cole Hugelmeyer, Kaustuv Manna, Jia-Xin Yin, Guangming Cheng, Maksim Litskevich, Nana Shumiya, Songtian Sonia Zhang, Chandra Shekhar, Niels B Schröter, Alla Chikina, Craig Polley, Balasubramanian Thiagarajan, Mats Leandersson, Johan Adell, Shin-Ming Huang, Nan Yao, Vladimir N Strocov, Claudia Felser, Zahid M Hasan Quantum phases can be classified by topological invariants, which take on discrete values capturing global information about the quantum state. Over the past decades, these invariants have come to form our foundation for understanding superfluids, magnets, the quantum Hall effect, topological insulators and Weyl semimetals. We introduce a remarkable linking number (knot theory) invariant associated with loops of electronic band crossings in the mirror-symmetric ferromagnet Co2MnGa [1-4]. By ARPES, we observe three intertwined degeneracy loops in the bulk Brillouin zone three-torus, T3, and find that each loop links each other loop twice. We explicitly draw the link diagram of this linked loop quantum state and conclude, in analogy with knot theory, a linking number of (2,2,2). On the sample surface, we further predict and observe Seifert boundary states protected by the bulk linked loops, suggestive of a Seifert bulk-boundary correspondence. Our quantum loop link motivates the application of knot theory to the exploration of quantum matter. |
Monday, March 6, 2023 10:36AM - 10:48AM Author not Attending |
A43.00012: Magnetic field-controlled phonon softening in topological semimetal Co2MnGa Thanh Nguyen, Nathan C Drucker, Nina Andrejevic, Mingda Li, Ahmet Alatas, Haowei Xu One of the several promising applications of topological semimetals (TSMs) is magnetic-field enhanced thermoelectric performance. Time-reversal-symmetry (TRS)-broken TSMs further enable enhanced transport properties even at zero field and give rise to a range of intriguing phenomena such as chiral magnetic effects as well as large anomalous Hall and Nernst effects. Magnetic topological semimetal Co2MnGa has received considerable attention due to exotic magnetic nodal-line, Weyl points and linked-loop phases and giant anomalous Nernst effect, which is attributed to a large net Berry curvature near the Fermi level. The phonon and magnon properties of this material, including the possible interactions between them, remain elusive. In this talk, we present inelastic scattering measurements performed on Co2MnGa performed at room temperature under a magnetic field to map the field-dependent phonon dispersions. We observe an anomalous strong tuning of an optical phonon to external magnetic field, when the magnetic field is located in the plane of the phonon wavevector. The tuning does not appear when the field is applied perpendicular to the plane of the phonon wavevector. The observed phenomena may originate from magnon-phonon interactions in the material at the crossing point which may lead to further discussions on topics such as non-Hermitian physics. |
Monday, March 6, 2023 10:48AM - 11:00AM |
A43.00013: Raman scattering study of magnetism in the kagome materials Fe3Sn2 and Co3Sn2S2 Rudolf U Hackl, Leander Peis, Ge He, Daniel Jost, Lilian Prodan, Vladimir Tsurkan, Christoph Meingast, Ramona Stumberger, Andreas Baum, Changjiang Yi, Youguo Shi, Nico Unglert, Malhar Kute, Brian Moritz, Thomas Devereaux Fe3Sn2 and Co3Sn2S2 have triangularly coordinated layers of 3d transition metal ions sitting on a kagome network. Both compounds have Dirac and Weyl nodes in the band structure. Fe3Sn2 is ferromagnetic below 670 K, and the spins start to reorient from perpendicular to parallel to the kagome layer below 150 K. This reorientation was first observed by Mössbauer spectroscopy but has in general little influence on other observables such as thermal expansion or magnetization. In our Raman study we find anomalies in both line width and energy of the lowest A1g phonon where the Sn atoms vibrate perpendicular to the Fe plane. We interpret the anomaly in terms of an enhanced spin-phonon coupling below approximately 150 K. Co3Sn2S2 starts ordering antiferromagnetically below 175 K and turns into a ferromagnet in the low-temperature limit. The spins are first in plane, and the order is nearly frustrated. Below 100 K the spins point along the c-axis and are parallel. The A1g phonon couples strongly to a continuum as highlighted by the asymmetric Fano-type line shape. The asymmetry is maximal close to the magnetic transition. We argue that small changes of the lattice have an effect on the magnetism. |
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