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 Y52: Magnetic Topological Materials 9: TheoryLive
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Sponsoring Units: DMP GMAG Chair: Nirmal Ghimire, George Mason University |
Friday, March 19, 2021 11:30AM - 11:42AM Live |
Y52.00001: Prediction of Anti-Kramers Antiferromagnetism (AKAF) in doped FeSb2 Igor Mazin, Libor Smejkal, Michelle Dawn Johannes, Rafael J Gonzalez-Hernandez, Klaus Koepernik A new phenomenon, overlooked for decades, here called AKAF, was recently discussed by several groups. It represent a fully compensated antiferromagnet with no net magnetization, which nonetheless manifests time-reversal symmetry breaking typical of ferromagnets: exchange band splitting, anomalous Hall effect (AHE), and magneto-optical Kerr effect (MOKE). In this talk we concentrate on a well known semiconductor, FeSb2, and show several highly unexpected features: (1) DFT ground state is not paramagnetic, and not, as suggested by some, ferro-, but antiferromagnetic (AF); (2) two different AF exist, and the most stable one corresponds to the so-called AFMe order, observed experimentally in CrSb2 (which is NOT AKAF); (3) the energetically favorable AKAF state is metallic, and the nonmagnetic one semiconducting, which is highly unusual; (4) the AFMe order is preferential to the AKAF one, since it has a pseudogap at EF. and the paramagnetic one is realized in nature because of the gap opening; (5) doping with Cr or Co reduces the advantage of (pseudo-)gapping, and in a narrow range of doping the AKAF becomes the ground state. We calculate AHE and MOKE, which appear nonzero for two magnetization directions, including the DFT easy axis. |
Friday, March 19, 2021 11:42AM - 11:54AM Live |
Y52.00002: Pinch-point singularities in spectroscopy guarantee topological criticality Han Yan Electron and magnon bands with topological features have been a major interest in condensed matter physics in the past few decades. In this work we discuss how to identify topologically critical gap-closing points from energy and momentum-resolved spectroscopy without any additional knowledge of the system. More explicitly, observing an n-fold pinch point on two bands with a gapless point sufficiently guarantees that the gapless point is topologically critical, with local Berry curvature n*Pi encoded at the pinch point. These results offer deepr insight and generalization on the theoretical study of two-fold pinch points on magnon bands, which focused on their "dynamical spin ice" nature [1,2]. |
Friday, March 19, 2021 11:54AM - 12:06PM Not Participating |
Y52.00003: Unlocking of time reversal, space-time inversion and rotation invariants in magnetic materials Jian Yang, Chen Fang Time reversal (T) and space inversion are symmetries of our universe in the low-energy limit. Fundamental theorems relate their corresponding quantum numbers to the spin for elementary particles: T2 = (PT)2 = −1 for half-odd-integer spins and T2 = (PT)2 = +1 for integer spins. Here we show that for elementary excitations in magnetic materials, this “locking” between quantum numbers is lifted: T2 and (PT)2 take all four combinations of +1 and −1 regardless of the value of the spin, where T now represents the composite symmetry of time reversal and lattice translation. Unlocked quantum numbers lead to new forms of minimal coupling between these excitations and external fields, enabling novel physical phenomena such as the“cross-Lamor precession”, indirectly observable in a proposed light-absorption experiment. We list the magnetic space groups with certain high-symmetry momenta where such excitations may be found. |
Friday, March 19, 2021 12:06PM - 12:18PM Live |
Y52.00004: Non-Axionic Magnetic Higher-Order Topological Insulators and High-Throughput Magnetic Topological Materials Discovery from Magnetic Topological Quantum Chemistry Benjamin Wieder, Yuanfeng Xu, Luis Elcoro, Zhida Song, Maia Garcia Vergniory, Barry Bradlyn, Nicolas Regnault, Yulin Chen, Claudia Felser, Andrei B Bernevig In [Bradlyn*, Elcoro*, Cano*, Vergniory*, Wang*, et al., Nature (2017)], we introduced Topological Quantum Chemistry (TQC) - a real-space predictive theory of band topology in nonmagnetic crystals. TQC subsequently fueled the successful identification of tens of thousands of topological materials, and the discovery of novel topological insulating (TI) phases. This past year, we have completed the 70-year-old problem of group theory in magnetic crystals, facilitating the formulation of a complete position-space theory of band topology in magnetic solids - Magnetic Topological Quantum Chemistry (MTQC) [Elcoro*, Wieder*, et al., arXiv:2010.00598 (2020)]. Through MTQC, we have derived the complete symmetry-based indicators of spinful magnetic band topology, discovered novel exceptions to symmetry-enhanced fermion doubling theorems, and uncovered the existence of magnetic TIs with response theories beyond axion electrodynamics. We have additionally employed MTQC to perform the first high-throughput search for magnetic topological materials [Xu, et al., Nature (2020)], discovering over 100 magnetic TIs and semimetals with experimentally established magnetic structures. |
Friday, March 19, 2021 12:18PM - 12:30PM Live |
Y52.00005: Flavors of noise in a magnetic Weyl semimetal Shu Zhang, Yaroslav Tserkovnyak Weyl semimetals (WSMs) are a relatively new class of topological quantum materials that exhibit exotic transport phenomena such as the chiral anomaly, the chiral magnetic effect and the anomalous Hall effect etc. Many magnetic-WSM materials are discovered recently, opening more possibilities for spintronic applications. To model the transport in a magnetic WSM, at least three degrees of freedom---charge, valley and spin---need to be considered. In this work, we investigate the magnetic noise induced by the fluctuations of the charge current, the valley (chiral) current and the spin density, respectively. In particular, the valley current behaves like spins in generating a magnetic field by the demagnetization kernel. In a simple treatment in the diffusive regime, we show that the three contributions have very different frequency dependence, which could be used to extract the valley related properties in a quantum-impurity relaxometry measurement. Our approach could be generalized to discuss WSMs with broke time-reversal symmetry and may inspire further study of transport in topological semimetals in general by non-invasive means. |
Friday, March 19, 2021 12:30PM - 12:42PM Live |
Y52.00006: Analog of Lieb-Schultz-Mattis argument for low dimensional antiferromagnets in the semiclassical sigma model framework Akihiro Tanaka, Yoshihiko Nonomura We revisit the Lieb-Schultz-Mattis argument as generalized by Oshikawa, and discuss how the program can be carried through in the sigma model + topological term effective action framework for antiferromagnets. We emphasize the role played by large gauge transformations, which effect the ground state Schroedinger wave functional in subtle but nevertheless crucial ways. The same language will be employed to see how entanglement properties are governed by the topological terms. We will also discuss how the above relates to the AKLT wavefunction. |
Friday, March 19, 2021 12:42PM - 12:54PM Live |
Y52.00007: Strong anisotropy and gyrotropy of electromagnetic wave propagation in magnetic Weyl semimetals Alexey Belyanin, QianFan Chen, Ivan Oladyshkin, Maria Erukhimova, Mikhail Tokman We report theoretical studies of the optical properties and electromagnetic modes of time-reversal-symmetry breaking Weyl semimetals. We present rigorous quantum-mechanical derivation of bulk and surface conductivity tensors including both intraband and interband optical transitions and taking into account all possible combinations of bulk-to-bulk, bulk-to-surface, and surface-to-surface transitions. We show how the information about the electronic structure of Weyl semimetals, such as position and separation of Weyl nodes, Fermi energy, surface states etc. can be unambiguously extracted from their optical response, namely from the reflection, transmission, and polarization change of incident radiation. We predict the optical Hall effect and anomalous dispersion for surface polaritons excited by a nanotip. We show that magnetic Weyl semimetals represent a new class of gyrotropic materials with unique electrodynamics due to the combination of strongly anisotropic and gyrotropic bulk conductivity, surface conductivity, and surface dipole layer. |
Friday, March 19, 2021 12:54PM - 1:06PM Live |
Y52.00008: Topological magnetism in correlated insulators and Weyl semimetals Predrag Nikolic Spin-orbit interaction is the driving force in many topological materials. Its effect on the spins of localized electrons is seen through the short-range Dzyaloshinskii-Moriya (DM) and chiral interactions. Additional itinerant electrons that interact with local moments can further mediate RRKY interactions (Heisenberg, Kitaev and DM) as well as multi-spin chiral interactions among the moments. When induced by Weyl electrons, these spin interactions can be oscillatory on both short and intermediate length scales, and feature an algebraic envelope dependence on the spatial separation between the spins. After outlining the theory behind these conclusions, I will discuss the observable implications: magnetic orders with skyrmions and hedgehogs, spin-momentum locking of magnetic excitations, transport properties, and the prospects for 3D quantum spin liquids with topological order. I will also propose a neutron scattering method for characterizing the spectrum of Weyl electrons from the damping of spin waves. |
Friday, March 19, 2021 1:06PM - 1:18PM Live |
Y52.00009: Topological Hopf insulating phases of magnetic materials Yuxin Wang, Pallab Goswami Hopf insulators are examples of three-dimensional, time-reversal symmetry breaking states, whose topological properties cannot be addressed by the Altland-Zirnbauer scheme of symmetry classification. Our current understanding of these exotic insulators is based on the third homotopy classification of two-band, Bloch Hamiltonians. Can any realistic magnetic material support Hopf insulator phase? To answer this question affirmatively, we have developed third homotopy classification scheme for n-band, Bloch Hamiltonians, belonging to the unitary class, with n≥2. We also describe the nature of surface states and topological phase transitions for Hopf insulators. Our results lay the foundations for the theoretical and experimental search for new types of topological states in spin-orbit coupled, magnetic materials. |
Friday, March 19, 2021 1:18PM - 1:30PM Live |
Y52.00010: Controlling a quantum point junction on the surface of an antiferromagnetic topological insulator Nicodemos Varnava, Justin Wilson, Jed Pixley, David Vanderbilt The surface and edges of topological materials can host physics, such as unidirectional charge or spin transport, that is unavailable in isolated one- and two-dimensional systems. However, to fully control the mixing and interference of edge-state wave functions, one needs robust and tunable junctions. We propose to achieve this control using an antiferromagnetic topological insulator that supports two distinct types of gapless unidirectional channels on its surface, one from antiferromagnetic domain walls and the other from single-height steps. The distinct geometric nature of these edge modes allows them to intersect robustly to form quantum point junctions, and their presence at the surface makes them subject to control by magnetic and electrostatic tips like those used in scanning probe microscopes. Remarkably, we show that the scattering at these junction can be described by a two-level quantum system and that the junctions are fully tunable. These facts render them a promising candidate for a universally programmable single-qubit gate. Prospects for realizing such junctions are encouraged by recent material candidate proposals, potentially leading to exciting applications in quantum computing and sensing. |
Friday, March 19, 2021 1:30PM - 1:42PM Live |
Y52.00011: New topological invariants and real-space constructions of topological states protected by magnetic space group symmetries Bingrui Peng, Yi Jiang, Chen Fang Using real-space recipes, we construct all gapped topological states protected by magnetic space groups (MSGs) for free fermion systems with spin-orbit coupling, and enumerate all topological invariants for MSG symmetries and their corresponding surface states, many of which are discovered for the first time. Among these real-space topological states, a vast number of them are beyond the scope of layer constructions. We also derive explicit formulas of symmetry-based indicators (SIs) for all MSGs and make quantitative mappings between SIs and topological invariants. A byproduct of our work is that we also find all Weyl semimetal states that can be determined by SIs in MSGs. Their existence are characterized by either quantized Berry phase of a certain loop or the difference between Chern numbers of two high-symmetry planes in Brillouin zone. We hope our work will facilitate the discovery of more magnetic topological materials in the future. |
Friday, March 19, 2021 1:42PM - 1:54PM Live |
Y52.00012: From symmetry-based indicators to topological states in magnetic space groups Bingrui Peng, Yi Jiang, Chen Fang Symmetry-based indicators(SI) is a powerful tool for diagnosing topological materials. The recent development of SI theory in non-magnetic space groups and its application in the high-throughput calculations, which discovered a large amount of new topological materials, has drawn much attention in the community. However, the SI theory for magnetic space groups(MSGs) has only been partly tackled, due to their vast number (1651 MSGs in total) and complicated structure. Here we develop the complete theory of SI in all MSGs, which includes the explicit formulas of SIs, their physical meanings and quantitative mappings to topological invariants, and their correspondence to real-space constructions of magnetic topological states. With the help of the real-space recipe, we enumerate all the gapped topological crystalline states protected by MSG symmetries. Generic Weyl states that can be diagnosed using SIs are also identified as a byproduct. Our results can be readily applied to high-throughput searching for new magnetic topological materials. |
Friday, March 19, 2021 1:54PM - 2:06PM Live |
Y52.00013: Towards a Theory of Surface Orbital Magnetization Daniel Seleznev, David Vanderbilt The bulk values of macroscopic quantities like electric polarization and orbital magnetization for crystalline systems of independent electrons may be explicitly computed using k-space integrals of quantities involving Berry connections and curvatures of Bloch functions. In recent years, attention has focused on expressing the same quantities locally via one-particle density matrices. Such decompositions may be exploited to define surface dependent analogues of bulk quantities; an example is the anomalous Hall conductivity (AHC) on the surface of a bulk. In this talk, we explore the possibility of defining a surface orbital magnetization on the boundary of a three-dimensional bulk system. We address this question via a combination of formal arguments and test calculations on a layered realization of the Haldane model. While we focus here on topologically trivial materials, we also speculate on the connections to topological ones, e.g., the relation between orbital magnetization and AHC on the surface of a material with a non-zero Chern-Simons axion coupling. |
Friday, March 19, 2021 2:06PM - 2:18PM Live |
Y52.00014: Inversion-Asymmetric Chiral Hinge States Due to the Inversion-Z4 Topology Yutaro Tanaka, Ryo Takahashi, Tiantian Zhang, Shuichi Murakami We study positions of chiral hinge states in higher-order topological insulators with inversion symmetry, and we find inversion-asymmetric hinge states, which are different from the conventional inversion-symmetric hinge states in the higher-order topological insulators. First, we find all possible configurations of the hinge states in the higher-order topological insulators in all type-I magnetic space groups with inversion symmetry. In particular, in a layered antiferromagnet, we find a difference in the hinge states between the cases with an even and odd number of layers. In the case of an even number of layers, which does not preserve inversion symmetry, positions of hinge states are not inversion symmetric. Nonetheless, these inversion-asymmetric hinge states result from the bulk topology. We show that their inversion-asymmetric configurations are uniquely determined from the symmetries and the topological invariant. In addition, we discuss the emergence of inversion-asymmetric hinge states in the axion insulator EuIn2As2. |
Friday, March 19, 2021 2:18PM - 2:30PM On Demand |
Y52.00015: Topological Invariant for Bosonic Bogoliubov-de Gennes Systems with Disorder Yutaka Akagi The highly successful studies on the topological phases for fermions have been extended to bosonic systems (see, e.g., a review [1]). As a difference from fermionic systems, bosonic Bogoliubov-de Gennes systems possess a unique mathematical property---non-Hermiticity. In addition, since topological invariants that characterize the topological phases are usually defined in terms of Bloch wave functions in crystal momentum space, the determination of the invariants for disordered systems is still very challenging. |
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