Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V44: Topological MagnonsInvited

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Sponsoring Units: DCMP Chair: Nandini Trivedi, Ohio State University Room: BCEC 210C 
Thursday, March 7, 2019 2:30PM  3:06PM 
V44.00001: Topological magnon bands in ultrathin film pyrochlore iridates and iron jarosites Invited Speaker: Gregory Fiete We describe recent theoretical efforts of our group aimed at exploring twodimensional magnetic systems described by local moment models that possess topological band structures of the lowenergy spin fluctuations. We study two systems: (1) Ultrathin film pyrochlore iridates grown along the [111] direction and (2) the iron jarosites. For the thin film pyrochlore iridates, we consider a bilayer and trilayer and find that in the trilayer case the ground state is the allin–allout (AIAO) state, whereas the bilayer has a deformed AIAO state. For range of parameters the lowest magnon band in the trilayer and bilayer case has a nonzero Chern number. We calculate the magnon Hall response for both geometries, finding a striking sign change as a function of temperature. For the iron jarosites, we study magnetic and topological properties of antiferromagnetic kagome spin systems in the presence of both in and outofplane DzyaloshinskiiMoriya interactions. The inplane interactions stabilize a canted noncollinear “umbrella” magnetic configuration with finite scalar spin chirality. We derive expressions for the canting angle and use the resulting order as a starting point for a spinwave analysis. We find topological magnon bands, characterized by nonzero Chern numbers. We calculate the magnon thermal Hall conductivity and propose the iron jarosites as a promising candidate system for observing the magnon thermal Hall effect in a noncollinear spin configuration. We also show that the thermal conductivity can be tuned by varying an applied magnetic field or the inplane DzyaloshinskiiMoriya strength. In contrast to previous studies of topological magnon bands, the effect is found to be large even in the limit of small canting. 
Thursday, March 7, 2019 3:06PM  3:42PM 
V44.00002: Topological spin excitations in a threedimensional antiferromagnet Invited Speaker: Yuan Li The recent discovery of topological semimetals, which possess distinct electronband crossing with nontrivial topological characteristics in the bulk, has stimulated intense research interest. By extending the notion of symmetryprotected band crossing into one of the simplest magnetic groups, namely by including the symmetry of timereversal followed by spaceinversion, we predict the existence of topological magnonband crossing in threedimensional antiferromagnets [1]. The crossing may take the forms of Dirac points and nodal lines, in the presence and absence, respectively, of the conservation of the total spin along the ordered moments. In a concrete example of a Heisenberg spin model for a "spinweb" compound, Cu_{3}TeO_{6}, we theoretically demonstrate the presence of Dirac magnons over a wide parameter range using linear spinwave approximation [1]. Inelastic neutron scattering experiments have been carried out to detect the bulk magnonband crossing in a singlecrystal sample [2]. The highly interconnected nature of the spin1/2 lattice suppresses quantum fluctuations and facilitates our experimental observation, leading to remarkably clean experimental data and very good agreement with the linear spinwave calculations. The predicted topological magnon Dirac points are confirmed. Further studies will be discussed, including determination of noncollinear spin canting in the ground state with neutron diffraction, and search for topological magnon surface states with inelastic neutron scattering. 
Thursday, March 7, 2019 3:42PM  4:18PM 
V44.00003: Topology of magnons: classification and application to honeycomb Kitaev magnets Invited Speaker: YuanMing Lu Compared to their electronic counterparts in topological band theory, much less in known about the topology of spin wave excitations in a general magnetic order. Here we provide a generic theory framework to classify and compute the topology of magnons, by mapping an arbitrary linear spin wave into a local freefermion Hamiltonian with exactly the same spectrum, symmetry implementation and band topology. This allows us to achieve a full classification and calculation on any topological properties of magnon bands. We apply this fermionization approach to honeycomb Kitaev magnet αRuCl_{3}, and show the existence of topologically protected magnon band crossings, and fieldinduced magnon Chern bands under small magnetic fields. 
Thursday, March 7, 2019 4:18PM  4:54PM 
V44.00004: Discovery of coexisting Dirac and triply degenerate magnons in a threedimensional antiferromagnet Invited Speaker: Jinsheng Wen Topological magnons are emergent quantum spin excitations featured by magnon bands crossing linearly at the points dubbed nodes, analogous to fermions in topological electronic systems. In this talk, we show direct spectroscopic evidence for the coexistence of symmetryprotected Dirac and triply degenerate nodes, the latter involving threecomponent magnons beyond the Dirac–Weyl framework, by measuring spin excitations of a threedimensional antiferromagnet Cu3TeO6 with inelastic neutron scattering. Our theoretical calculations show that the observed topological magnon band structure can be well described by the linearspinwave theory based on a Hamiltonian dominated by the nearestneighbor exchange interaction J1. As such, we showcase Cu3TeO6 as an example system where Dirac and triply degenerate magnonic nodal excitations coexist, demonstrate an exotic topological state of matter, and provide a fresh ground to explore the topological properties in 
Thursday, March 7, 2019 4:54PM  5:30PM 
V44.00005: The surprising usefulness of magnons at intermediate and high energies: from frustration to topology Invited Speaker: Roderich Moessner Magnons are among the oldest known elementary excitations, yet very recent developments have yielded plenty of suprises. This talk will cover three aspects of magnons in frustrated magnets  (i) their topological properties, (ii) their survival at intermediate energies, as well as (iii) their unexpected utility in an S=1 spin liquid state. 
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