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 B44: Topological MagnonsFocus
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Sponsoring Units: DMP Chair: Kelly Luo, Cornell University; Rolando Aguilar, Ohio State University Room: Room 316 |
Monday, March 6, 2023 11:30AM - 12:06PM |
B44.00001: Gaps in Topological Magnon Spectra: Intrinsic vs. Extrinsic Effects Invited Speaker: Andy Christianson Revealing and understanding the presence of a gap at a putative magnon crossing point is an essential part of characterizing the magnetic topological properties of a material. An important experimental approach in this regard is an inelastic neutron scattering study of a single crystal to probe the momentum and energy dependence of the spin excitation spectrum. Here we discuss this approach for studies of topological magnons and other cases where the scattering intensity rapidly disperses in the vicinity of a singularity or band crossing in the spin excitation spectrum. In particular, we show the apparent difference in energies between modes at the Dirac wave-vector, i.e. gap size, is extremely sensitive to experimental conditions including sample mosaic, resolution, and data processing. Accounting for instrumental resolution effects is required to accurately compare measurements to analytic models. As a test case, we examine CrCl3 which orders antiferromagnetically at 14 K with ferromagnetic alignment of spins within honeycomb lattice planes which are in turn stacked antiferromagnetically along the c-axis. Our inelastic neutron scattering measurements provide a comprehensive map of the excitation spectrum near the Dirac point and establish CrCl3 as an ideal quasi-two-dimensional (2D) Heisenberg ferromagnet with a gapless Dirac magnon. However, we also find that the size of a topological magnon gap to be considerably overestimated if “typical” Q-integration ranges are used. This result provides an explanation of the apparent discrepancies between first-principles calculations and spectroscopic measurements of Dirac magnon gap sizes of recently studied materials and further provides guidelines for accurate measurements of topological magnon gaps. |
Monday, March 6, 2023 12:06PM - 12:42PM |
B44.00002: Interacting Topological Magnons in Ferromagnets and Antiferromagnets Invited Speaker: Alexander Mook Topological magnets support magnetic excitations with a topologically nontrivial spectrum. As a result, they exhibit chiral edge states akin to those known from the quantum Hall effect. These edge states are envisioned to facilitate backscattering-free information channels for magnetic signals [1]. Since spin excitations do not carry charge, they do not suffer from Joule heating and facilitate ultra-low energy computation. However, in contrast to electrons, there is no conservation law for spin excitations. This gives rise to particle number-nonconserving many-body interactions the influence of which on quasiparticle topology is an open issue of fundamental interest in the field of topological quantum materials. Herein, I concentrate on magnons - the elementary spin excitations of ferromagnets and antiferromagnets - and discuss several aspects of many-body effects caused by particle-number nonconservation. These include (i) quantum damping due to spontaneous quasiparticle decay [2], (ii) interaction-stabilized topological gaps in the single-particle spectrum [3], and (iii) a topological hybridization of states belonging to different particle number sectors [4]. These effects highlight the fundamental difference between electronic and magnonic topology. |
Monday, March 6, 2023 12:42PM - 12:54PM |
B44.00003: Polarization dependent magneto-Raman study of phonons and magnons in CoTiO3 Thuc T Mai, Maria F Munoz, Tehseen Adel, Yufei Li, Kevin F Garrity, Daniel Shaw, Timothy N DeLazzer, Jeffrey R Simpson, Kate A Ross, Rolando Valdes Aguilar, Angela R Hight Walker We performed a study of the symmetry of the Raman active phonons and magnons in exotic quantum magnet CoTiO3. Using polarization dependent Raman, we mapped out the symmetry of the various quasiparticles in an isotropic and anisotropic cut of the sample, the ab-plane and ac-plane respectively. The study was carried out in its magnetically ordered phase below 38 K. In addition, we measured the polarization dependent of the magnons as a function of applied magnetic field. The results are analyzed using the Raman tensors of the magnetic point group in the ordered phase. |
Monday, March 6, 2023 12:54PM - 1:06PM |
B44.00004: Ring Exchange Effects on Magnons in Topological Dirac Magnet CoTiO3 Rolando Valdes Aguilar, Yufei Li, Thuc T Mai, Evan Jasper, Mohammed Karaki, Kevin F Garrity, Chase L Lyon, Daniel Shaw, Timothy N DeLazzer, Adam Biacchi, Rebecca L Dally, Daniel M Heligman, Jared Gdanski, Tehseen Adel, Alex D Giovannone, Maria F Munoz, Jeffrey R Simpson, Kate A Ross, Nandini Trivedi, Yuan-Ming Lu, Angela R Hight Walker The study of the interplay between topology and magnetism has begun to show a plethora of candidate materials where magnons can be classified as topological (arxiv:2206.06248). One recent example of a potential topological magnetic material is CoTiO3. We use a combination of time domain THz (TDTS) and magneto-Raman spectroscopies to measure the low energy magnetic spectrum in CoTiO3 and detect the two lowest energy magnons, one of which is the gapped Goldstone mode which responds to THz magnetic field perpendicular to the honeycomb plane, and the other is an inversion even optical magnon which is active for THz magnetic field parallel to the honeycomb plane. We build a theoretical model that can explain the opening of the gap of the pseudo Goldstone magnon and the magnetic field dependence of the two magnons with the apperance of a small ring exchange interaction between 6 spins in a hexagonal plaquette. This study shows the power of combining TDTS and Raman spectroscopies with theoretical modeling to understand the nature of the magnetic excitations in quantum magnets. |
Monday, March 6, 2023 1:06PM - 1:18PM |
B44.00005: Origin of dielectric response and magneto-electric properties of CoTiO3 Rathnayaka Mudiyanselage Dilan K Rathanayaka, Despina A Louca, John A Schneeloch Magnetic materials with kagome and honeycomb lattices harbor topological properties. One example is CoTiO3 that consists of layers of Co2+ ions on a honeycomb lattice and exhibits Dirac magnons, magnetoelectric coupling, and spin-orbit excitons. The system orders into an A-type antiferromagnetic structure below TN = 38 K, with the spins aligned ferromagnetically within the plane and antiferromagnetically out of plane. Moreover, CoTiO3 has a large dielectric response at high temperature and low frequencies. The analysis of the local structure from neutron diffraction data indicate that the Ti atoms are displaced in the TiO6 octahedra that coincide with the dielectric response at high temperature. The Ti displacements are enhanced with increasing temperature, indicating that the Ti distortions lead to dipolar interactions leading to the dielectric behavior. From inelastic neutron scattering data, low energy (<15 meV) magnon peaks below TN and higher-energy “spin-orbit excitons” (SEOs) near 29 and 58 meV are clearly observed. The SEOs represent crystal field excitations and remain present up to at least 300 K, in contrast to the spin waves, though the SEOs soften across TN. Our results could provide insight into the relation between the local structure and magnetic or electric properties. |
Monday, March 6, 2023 1:18PM - 1:30PM |
B44.00006: Gapless Dirac magnons in CrCl3 Despina A Louca, John A Schneeloch Bosonic Dirac materials are testbeds for dissipationless spin-based electronics. In the quasi two-dimensional honeycomb lattice of CrX3 (X = Cl, Br, I), Dirac magnons have been predicted at the crossing of acoustical and optical spin waves, analogous to Dirac fermions in graphene. Here we show that, distinct from CrBr3 and CrI3, gapless Dirac magnons are present in bulk CrCl3, with inelastic neutron scattering intensity at low temperatures approaching zero at the Dirac K point. Upon warming, magnon-magnon interactions induce strong renormalization and decreased lifetimes, with a ~25% softening of the upper magnon branch intensity from 5 to 50 K, though magnon features persist well above TN. Moreover, on cooling below ~50 K, an anomalous increase in the a-axis lattice constant and a hardening of a ~26 meV phonon feature are observed, indicating magnetoelastic and spin-phonon coupling arising from an increase in the in-plane spin correlations that begins tens of Kelvin above TN. |
Monday, March 6, 2023 1:30PM - 1:42PM |
B44.00007: Competing Magnetic Energy Scales in the Topological Ferrimagnet TbMn6Sn6 Benjamin G Ueland, Simon X Riberolles, Tyler J Slade, Douglas L Abernathy, Garrett E Granroth, Bing Li, Yongbin Lee, Paul C Canfield, Liqin Ke, Robert J McQueeney TbMn6Sn6 is a metallic ferrimagnet displaying signatures of topological electrons and topological magnons arising from ferromagnetism and spin-orbit coupling within the Mn kagome layers. We present inelastic neutron scattering data showing strong ferromagnetic (FM) interactions within a Mn layer and reveal a magnetic bandwidth of ∼ 230 meV. The low-energy magnetic excitations are characterized by strong FM Mn-Mn and antiferromagnetic (AFM) Mn-Tb nearest-neighbor interlayer exchange, as well as weaker and competing longer range FM and AFM Mn-Mn interlayer exchange similar to those driving helical AFM order in YMn6Sn6. When these measurements are combined with density-functional theory calculations, we find that competing Mn-Mn interlayer magnetic interactions occur in all RMn6Sn6 compounds with R = Y, Gd-Lu. This results in magnetic instabilities and tunability when Mn-R interactions are weak. In the case of TbMn6Sn6, the strong AFM Mn-Tb coupling ensures a highly stable three-dimensional ferrimagnet. |
Monday, March 6, 2023 1:42PM - 1:54PM |
B44.00008: Detecting Thermal Hall in an Insulating Néel Antiferromagnet Mohamed E Nawwar, Jiamin Wen, Joseph P Heremans Topological magnons have attracted the interest of many scientists thanks to their great potential in developing the field of spintronics. Two experimental measurements that have proved to be reliable in the search for topological magnon candidates are magnon thermal Hall and Inelastic Neutron Scattering (INS). Magnon thermal Hall, however, is preferred over INS in exploring a wide range of materials given its lower cost and availability. To date, most of the topological magnon candidates are ferromagnets such as Lu2V2O7 and Cu (1,3 bdc). In both materials, a magnon thermal Hall effect was observed and was attributed to Dzyaloshinskii Moriya (DM) interaction1,2. In our talk, we report the first thermal Hall data detected on an insulating antiferromagnet in MnPS3 and MnPSe3 below their Néel temperatures and discuss their potential as topological magnon materials. |
Monday, March 6, 2023 1:54PM - 2:06PM |
B44.00009: Magnons and paramagnons in (111)-oriented thin films of pyrochlore iridates Michael Terilli, Jak Chakhalian, Yue CaO, Xun Jia, Wanzheng Hu, Huyongqing Chen, Gregory A Fiete, Pontus Laurell, Mikhail Kareev, Tsung-Chi Wu, Xiaoran Liu, John W Freeland, Jianshi Zhou Rare-earth pyrochlore iridates R2Ir2O7 (e.g., R = Y, rare-earth elements) are an exciting class of quantum materials which exhibit emergent electronic and magnetic phenomena. These materials exhibit all-in-all-out magnetic ground states stabilized by the competition between the Heisenberg antiferromagnetic correlation and Dzyaloshinskii–Moriya interaction. The nature of the magnetism is encoded not only in the magnetic ground state, but also in the magnon quasiparticles, as well as in their evolution across the Neel transition. In this talk, I will present our recent work in fabricating the high quality Y2Ir2O7 thin films and mapping the magnetic excitations with resonant inelastic x-ray scattering (RIXS). The low-temperature magnon dispersion is consistent with an all-in-all-out magnetic ground state. Towards the Neel transition, the magnon gap does not soften with temperature, which we propose as arising from the spin-orbital nature of the isospin. Moreover, a paramagnon excitation persists above Tc = 140K, indicating the presence of small antiferromagnetic domains above Tc. |
Monday, March 6, 2023 2:06PM - 2:18PM |
B44.00010: Jackiw-Rebbi Magnons Pablo I Martinez We study a system of two-dimensional Dirac electrons (as is realized on the surface of a 3D topological insulator) coupled to an array of localized spins. The spins are coupled ferromagnetically to each other, forming an ordered ground state with low-energy spin-wave excitations (magnons). The Dirac electrons couple to the spins through a spin-dependent effective Zeeman field. The out-of-plane effective Zeeman field therefore serves as a Dirac mass that gaps the electronic spectrum. Once a spin is flipped, it creates a surrounding domain in which the sign of the Dirac mass is opposite to that of the rest of the sample. Therefore, an electronic bound state appears on the domain wall, as predicted by Jackiw and Rebbi. However, in a quantum magnet, a localized spin flip does not produce an eigenstate. Instead, the eigenstates correspond to delocalized spin waves (magnons). As in the case of the single flipped spin, the delocalized magnon also binds an in-gap electronic state. We name this excitation a `Jackiw-Rebbi-Magnon' (JRM) and study its signature in the spin susceptibility. We show that when coupled to a reservoir, the JRMs couple to magnons producing magnon-JRM polaritons. This results in a quantum phase transition (QPT) when the lower polariton energy is below the fully polarized ferromagnetic ground state. |
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