Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T69: Magnons in Magnetic Topological MaterialsFocus Recordings Available

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Sponsoring Units: DMP GMAG DCMP Chair: Chunli Huang, University of Texas Austin and Los Alamos National Lab Room: Hyatt Regency Hotel Jackson Park A 
Thursday, March 17, 2022 11:30AM  11:42AM 
T69.00001: Search for topological magnons using band representations of magnetic space groups Mohammed Karaki, Xu Yang, Mohamed Nawwar, Archibald Williams, Joshua E Goldberger, Vicky DoanNguyen, Yuanming Lu In contrast to their electronic counterparts, a systematic framework to understand magnon band topology is still lacking. By combining the recently developed formalism of magnetic topological quantum chemistry with a general framework that maps an arbitrary linear spin wave system into a freefermion Hamiltonian with the same band topology, we achieve a classification of possible magnon band structures that arise from localized magnetic moments in any crystalline symmetry setting. This allows us to perform an efficient search for candidate materials hosting topological magnons based on the pool of existing magnetic materials. 
Thursday, March 17, 2022 11:42AM  11:54AM 
T69.00002: Nontrivial Topological Properties of a Onedimensional Magnonic Crystal Kwangyul Hu, Denis R Candido, Michael E Flatté A magnonic crystal provides a promising realization of a bosonic topological insulator with length scales much shorter than photonic ones. We derive the nontrivial topological phase diagram of a onedimensional magnonic crystal that is a magnonic analogue of the SuSchriefferHeeger model. We studied a YIG based semiinfinite magnonic crystal polarized with an outofplane magnetic field. We obtained dispersion relations of the magnonic crystal using the linearized LandauLifshitzGilbert equation. We used an analytic calculation to obtain Zak phases and confirmed a change to the nontrivial phase in certain magnon bands. Finally, we employed a micromagnetic modeling program to simulate the finite structure and excitation of topologically nontrivial and robust magnonic edge states, which may have applications to quantum information science. 
Thursday, March 17, 2022 11:54AM  12:06PM 
T69.00003: MagnetoRaman study of magnon and spinorbit excitations in CoTiO_{3} Thuc T Mai, Kevin F Garrity, Daniel Shaw, Yufei Li, Jeffrey R Simpson, Rolando Valdes Aguilar, Kate A Ross, Angela R Hight Walker With a J_{eff}=1/2 ground state of Co^{2+} ions in a stacked honeycomb lattice, CoTiO_{3} has recently been in the spotlight due to its promise of exotic magnetic behavior. Specifically, a Dirac crossing was measured in its magnon spectrum by two different groups via inelastic neutron scattering. This spectrum displays a linear crossing at the Dirac point, as well as higher energy spinorbit excitons, coming from the J=5/2 multiplets. Thus, CoTiO_{3} is a promising material to explore the properties of topological magnons. 
Thursday, March 17, 2022 12:06PM  12:18PM 
T69.00004: Terahertz spectroscopy evidence of an electromagnon in honeycomb Dirac magnet CoTiO_{3} Yufei Li, Chase L Lyon, Daniel M Heligman, Evan Jasper, Daniel Shaw, Kate A Ross, Rolando Valdes Aguilar In CoTiO_{3}, the Co^{2+} ions form a honeycomb lattice in the ab plane. Scattering measurements indicate inplane ferromagnetic interactions and interplane antiferromagnetic interactions. The measured magnon dispersion relation shows nodal lines around the K points, just like the electrons in graphene. This makes this material a potential platform for the existence of topological magnons. 
Thursday, March 17, 2022 12:18PM  12:30PM 
T69.00005: Magnons in the antiferromagnetic insulator CoTiO_{3} Jeongheon Choe, David Lujan, Martin A Rodriguezvega, Gaihua Ye, Jiaming He, T. Nathan Nunley, Aritz Leonardo, M. Arruabarrena, A. Ayuela, Jianshi Zhou, Gregory A Fiete, Rui He, Xiaoqin (Elaine) Li Dirac materials have been extensively studied in fermionic particle system like graphene and topological insulators. Recently, bosonic Dirac magnons have been reported in the antiferromagnetic insulator CoTiO_{3}, where the magnetic lattice of the Co^{2+} ions is a stacked honeycomb lattice analogous to graphene lattice. Here we report Raman spectra of multiple magnon modes below the transition temperature, which is consistent with the recent inelastic neutron scattering experiments [1, 2]. We will also discuss magneticfield and temperature dependence of magnons in this antiferromagnetic insulator. 
Thursday, March 17, 2022 12:30PM  12:42PM 
T69.00006: SpinSpace Groups and Magnon Band Topology Alberto Corticelli, Roderich Moessner, Paul McClarty Band topology is both constrained and enriched by the presence of symmetry. The importance of antiunitary symmetries such as time reversal was recognized early on leading to the classification of topological band structures based on the tenfold way. Since then, lattice point group and nonsymmorphic symmetries have been seen to lead to a vast range of possible topologically nontrivial band structures many of which are realized in materials. 
Thursday, March 17, 2022 12:42PM  12:54PM 
T69.00007: Magnon Topology in Chiral Crystals: Multifold Crossings and Nodal Planes Niclas P Heinsdorf, Darshan G Joshi, Andreas P Schnyder, Tobias Weber, Marc A Wilde, Michal Stekiel, Christian Pfleiderer We investigate the topology of magnon excitations in magnets with chiral space groups. The presence of (magnetic) screw rotations leads to symmetry enforced Weyl points and nodal planes in the magnon band structure. In addition, there are multifold crossings pinned at highsymmetry points.We systematically analyze the band topology of these crossings and calculate their topological charges. In particular, we find that the magnon nodal planes carry a quantized topological charge similar to magnon Weyl points. Analogous to the protected spin currents on the surface of topological insulators, the topologically nontrivial magnon crossings result in protected surface modes of heat quanta. We propose several candidate materials and calculate their magnon band structures, topological invariants,and topologically protected surface modes. Further, we investigate the excitations of a quantum paramagnet on a onedimensional ladder, and show that by applying an external magnetic field a topological phase transition is enforced. Because the triplon excitations of the ladder are gapped, fewer scattering channels are available and the topological exciations are potentially more robust than their spinwave counterparts. 
Thursday, March 17, 2022 12:54PM  1:06PM 
T69.00008: JackiwRebbi magnons Pablo I Martinez, Tami PeregBarnea, Bill Coish We theoretically study the elementary excitations of a magnetic topological insulator. These excitations combine aspects of JackiwRebbi bound states and magnon modes. Specifically, we consider the delocalized excitations of a quantum ferromagnet in contact with a 2D Dirac electron system. The local magnetization gaps the electron system, leading to bound states at magnetic domain walls (JackiwRebbi modes). The delocalized (magnon) excitations of this system thus inherit a charge from the delocalized JackiwRebbi bound states. We show that these composite excitations (JackiwRebbi magnons) can be probed experimentally through the characteristic chiral current distribution and through unique characteristics of the longitudinal and transverse spin susceptibilities. 
Thursday, March 17, 2022 1:06PM  1:18PM 
T69.00009: Nonlinear magnon spin Nernst effect in antiferromagnets and straintunable pure spin current Hiroki Kondo, Yutaka Akagi The topological properties of magnons, as exemplified by the magnon spin Nernst effect (SNE) in antiferromagnets [1], offer great potential for spin current generation in magnetic materials. However, many of them are studied in the linear response regime and their realization is often limited to the magnets which have, such as, nonnegligible DzyaloshinskiiMoriya (DM) interaction. On the other hand, nonlinear topological responses of magnons are expected to be realized in a wide range of materials, as can be imagined from the nonlinear Hall effect in electronic systems [2]. 
Thursday, March 17, 2022 1:18PM  1:30PM 
T69.00010: Intrinsic topological magnons in arrays of magnetic dipoles Paula Mellado We study a simple magnetic system made out of periodically modulated magnetic dipoles with an easy axis. Upon adjusting the modulation amplitude alone, chains and twodimensional stacked chains exhibit a rich magnon spectrum where frequency gaps and magnon speeds are easily manipulable. The blend of anisotropy due to dipolar interactions between magnets and geometrical modulation induces a magnetic phase, with fractional Zak number in infinite chains and end states in open onedimensional systems. In two dimensions, it gives rise to topological lattices with protected chiral edge modes. Tuning the amplitude in twodimensional lattices causes a band touching, which triggers the exchange of the Chern numbers of volume bands and switches the sense of motion of the topological modes at the edges of stripes. 
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