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 R52: Magnetic Topological Materials 5: MagnonsLive

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Sponsoring Units: DMP GMAG Chair: Pavlo Sukhachov, Yale University 
Thursday, March 18, 2021 8:00AM  8:12AM Live 
R52.00001: BoseEinstein condensate of Dirac magnons Pavlo Sukhachov, Saikat Banerjee, Alexander Balatsky A Diraclike quasiparticle spectrum is generated for localized magnetic moments on a 2D honeycomb lattice. The bosonic nature of these excitations and pumping allow for an accumulation of Dirac bosons and open up the possibility of the BoseEinstein condensation at the Dirac point. 
Thursday, March 18, 2021 8:12AM  8:24AM Live 
R52.00002: ParaHermitian perturbation theory and nodal lines in magnon systems Geremia Massarelli, Ilia Khait, Arun Paramekanti ParaHermitian Hamiltonians are commonplace in many areas of physics, arising from Bogoliubov Hamiltonians of bosons. These are achieved, for example, in magnon and triplon systems as well as in interacting condensates. We present a systematic quasidegenerate perturbation theory scheme for paraHermitian Hamiltonians, which faithfully reproduces the spectrum of any set of neighbouring energy eigenstates order by order, regardless of possible degeneracies. As an application, we show how the constraint of magnetic inversion symmetry (i.e. PT symmetry) generically gives rise to nodal lines in magnon band structures, much like in spinless electron systems. We verify our predictions by studying a spin model for CoTiO3, in which topological magnonic nodal lines have recently been discovered. 
Thursday, March 18, 2021 8:24AM  8:36AM Live 
R52.00003: Lightinduced topological magnons in twodimensional van der Waals magnets Emil Vinas Boström, Martin Claassen, James McIver, Gregor Jotzu, Angel Rubio, Michael Sentef Driving a twodimensional Mott insulator with circularly polarized light breaks timereversal and inversion symmetry, which induces an opticallytunable synthetic scalar spin chirality interaction in the effective lowenergy spin Hamiltonian. We show that this mechanism can stabilize topological magnon excitations in honeycomb ferromagnets and in optical lattices. We find that the irradiated quantum magnet is described by a Haldane model for magnons and hosts topologically protected edge modes. The evolution of the magnon spectrum is studied in the Floquet regime and via time propagation of the magnon Hamiltonian for a slowly varying pulse envelope. Compared to a magnetooptical interaction based on the AharanovCasher effect, the dimensionless lightmatter coupling parameter at fixed electric field strength is enhanced by five orders of magnitude and the topological band gap by ten orders of magnitude. This increase of the coupling parameter allows to induce a topological gap of the order of 2 meV with realistic laser pulses, bringing an experimental realization of lightinduced topological magnon edge states within reach. 
Thursday, March 18, 2021 8:36AM  8:48AM Live 
R52.00004: Thermal and magnetoelastic properties of αRuCl_{3} in the fieldinduced low temperature states Rico Schoenemann, Shusaku Imajo, Franziska Weickert, Jiaqiang Yan, David George Mandrus, Yasumasa Takano, Eric Brosha, Priscila Rosa, Stephen E Nagler, Koichi Kindo, Marcelo Jaime Here we discuss the implications that new magnetocaloric, thermal expansion and magnetostriction data of αRuCl_{3} single crystals have on its temperaturefield phase diagram and uncover the magneticfield dependence of an apparent energy gap Δ(H), extracted from thermal expansion measurements, that evolves when the low temperature antiferromagnetic order is suppressed. We show that, depending on how the thermal expansion data is modeled, Δ(H) can follow a cubic field dependence and remain finite at zero field, consistent with the pure Kitaev model. Our quasiisothermal magnetocaloric effect data provides, below 1 K, unambiguous evidence for dissipative phenomena at the critical field, smoking gun for a first order phase transition. On the other hand, our results show little support for a phase transition from a quantum spin liquid to a polarized paramagnetic state above the critical field. The results are discussed in the context of possible quantum critical and QSL behavior in αRuCl_{3}. 
Thursday, March 18, 2021 8:48AM  9:00AM Live 
R52.00005: Topological thermal Hall effect of strongly interacting bosons in a disordered quantum magnet SrCu_{2}(BO_{3})_{2} Shota Suetsugu, Taichi Yokoi, Ibuki Tanaka, Yuichi Kasahara, Shigeru Kasahara, Zhong Chengchao, Keisuke Totsuka, Hiroshi Kageyama, Yuji Matsuda Topological phases of bosons, where topological edge states and dissipationless transport of bosons are expected, have attracted considerable interest. A suitable candidate is a disordered quantum magnet SrCu_{2}(BO_{3})_{2} [1] where the ground state is given by tiling quantum mechanical singlets on orthogonally arranged dimers of S = 1/2 Cu moments. The low energy excitation is triply degenerate S = 1 bosonic quasiparticles called triplon. It has been shown that the DzyaloshinskiiMoriya interactions yield the triplon bands with nontrivial Chern numbers. Although topological modes associated with such a state are predicted to be detectable by the thermal Hall effect [2], no discernible thermal Hall conductivity κ_{xy} has been reported so far [3]. 
Thursday, March 18, 2021 9:00AM  9:12AM Live 
R52.00006: Topological Weyl magnons and thermal Hall effect in layered honeycomb ferromagnets Shuyi Li, Andriy Nevidomskyy We study the topological properties and magnon Hall effect of a 3D ferromagnet in the ABC stacked honeycomb lattice, relevant to the recent inelastic neutron scattering study of CrI3 [1]. We find that the magnon band structure and Chern numbers of the magnon branches are significantly affected by the interlayer coupling Jc, which moreover has a qualitatively different effect in the ABC stacking compared to the AA stacking case. Tuned by the ratio of the interlayer coupling Jc and the thirdneighbour Heisenberg interaction J3, several gapless Weyl points appear, which separate the nonequivalent Chern insulating phases. We further show that the topological character of magnon bands results in nonzero thermal Hall conductivity, whose sign and magnitude depend on Jc and the intralayer couplings. Since the interlayer coupling strength Jc can be easily tuned by applying pressure to the quasi2D material such as CrI3, this provides a potential route to tuning the magnon thermal Hall effect in an experiment. 
Thursday, March 18, 2021 9:12AM  9:24AM Live 
R52.00007: Topological properties of magnons in FerromagnetSuperconductor heterostructure Bishal Parajuli, ChihChun Chien, ShiZeng Lin Studies of nontrivial band topology of magnon are emerging since they are considered to circumvent the shortcomings of traditional electronic based devices since magnons propagate without Joule heating. Here, we present topological properties of magnons in a ferromagnetsuperconductor heterostructure in the presence of a magnetic field. Magnons can propagate in the ferromagnetic thin film while magnetic fields penetrate through the typeII superconducting thin film forming a vortex lattice. We show that the ferromagnetic film influenced by the vortex lattice acts as a magnonic crystal and results in the opening of bandgaps in the spin wave spectrum for a periodic vortex lattice. Our observation supports the closing of band gaps and existence of edge states for finite number of vortices. Furthermore, we calculate the Berry curvature and Chern number for the associated magnon bands to characterize the topological properties of the magnons. 
Thursday, March 18, 2021 9:24AM  9:36AM Live 
R52.00008: Chiral Hinge Magnons in SecondOrder Topological Magnon Insulators Alexander Mook, Sebastian Diaz, Jelena Klinovaja, Daniel Loss When interacting spins in condensed matter order ferromagnetically, their ground state wave function is topologically trivial. Nonetheless, in two dimensions, ferromagnets can support spin excitations with nontrivial topology, an exotic state known as topological magnon insulator (TMI). We theoretically unveil and numerically confirm a novel ferromagnetic state in three dimensions dubbed secondorder TMI, whose hallmarks are excitations at its hinges, where facets intersect. Since ferromagnetism naturally comes with broken timereversal symmetry, the hinge magnons are chiral, rendering backscattering impossible. Hence, they trace out threedimensional paths about the sample unimpeded by defects and are topologically protected by the spectral gap. They are remarkably robust against disorder and highly tunable by atomiclevel engineering of the sample termination. Our findings empower magnonics, the harnessing of spin waves as information carries, with the tools of higherorder topology, a promising route to combine lowenergy information transfer free of Joule heating with threedimensional vertical integration. 
Thursday, March 18, 2021 9:36AM  9:48AM Live 
R52.00009: Dirac surface states in topological crystalline magnon insulators Hiroki Kondo, Yutaka Akagi The physics of topologically nontrivial magnon states provides a new perspective on transport phenomena in magnetic materials. However, few topological magnon materials in threedimensions are considered. This is partly because it is difficult to find out symmetries that would protect the surface state topologically due to Bose statistics. 
Thursday, March 18, 2021 9:48AM  10:00AM Live 
R52.00010: Spin fluctuations in quantized transport of magnetic topological insulators Yuhang Li, Ran Cheng In magnetic topological insulators, quantized electronic transport is interwined with spontaneous magnetic ordering, as magnetization controls band gaps, hence band topology, through the exchange interaction. We show that considering the exchange gaps at the meanfield level is inadequate to predict phase transitions between electronic states of distinct topology. Thermal spin fluctuations disturbing the magnetization can act as frozen disorders that strongly scatter electrons, reducing the onset temperature of quantized transport appreciably even in the absence of structural impurities. This effect, which has hitherto been overlooked, provides an alternative explanation of recent experiments on intrinsic magnetic topological insulators. 
Thursday, March 18, 2021 10:00AM  10:12AM Live 
R52.00011: πflux Dirac Triplon and Thermal Hall Effect in ShastrySutherland model Hao Sun, Bo Yang SrCu2(BO3)2 provides an excellent platform for studying ShastrySutherland model and topological triplon excitation. In this letter, we introduce a effective 2dimer tightbinding model to reveal significant physical observable when pseudospin (two sublattices) couples the tilted external magnetic field. The calculated band diagram holds a 2fold degenerate Dirac triplon excitation at the Brillouin zone center (k = 0) by inplane component part of the magnetic field. We identify this elementary excitation as having nontrivial topological properties with πBerry flux. Using the effective Hamiltonian built based on tightbinding model, we reveals the role of inplane magnetic field, which gives rise to Rashba pseudospinorbit coupling term. Further, an outofplane component of magnetic gaps the excitation spectrum resulting in a nonzero Berry curvature hot spot around the Brillouin zone center. Using a comprehensive 2dimer theoretical model, we also show the pronounced experimental signature for exotic nontrivial transportthe thermal Hall effect based on the gapped Dirac triplon bands. 
Thursday, March 18, 2021 10:12AM  10:24AM Live 
R52.00012: Quantum Halllike transitions in the thermal Hall behavior of multilayer topological magnon insulators Stephen Hofer, Trinanjan Datta, Sumanta Tewari, Dipanjan Mazumdar Twodimensional magnetic insulators can be promising hosts for topological magnons. In this study, we show that ABCstacked honeycomb lattice multilayers with alternating DzyaloshinskiiMoriya interaction (DMI) reveal a rich topological magnon phase diagram. Based on our bandstructure and Berry curvature calculations, we demonstrate jumps in the thermal Hall behavior that corroborate with topological phase transitions triggered by adjusting the DMI and interlayer coupling. We connect the phase diagram of generic multilayers to a bilayer and a trilayer system. We find an evenodd effect amongst the multilayers where the even layers show no jump in thermal Hall conductivity, but the odd layers do. We also observe the presence of topological proximity effect in our trilayer. Multilayer characterized by ferro and antiferromangetic interlayer coupling are considered in addition to rhombohedral and monoclinic stacking arrangements. Our results offer new schemes to manipulate Chern numbers and their measurable effects in topological magnonic systems. 
Thursday, March 18, 2021 10:24AM  10:36AM Live 
R52.00013: Multifunctional Antiperovskites driven by Strong Magnetostructural Coupling Harish Kumar Singh, Ilias Samathrakis, Nuno Fortunato, Jan Zemen, Chen Shen, Oliver Gutfleisch, Hongbin Zhang Based on density functional theory calculations, we elucidated the origin of multifunctional properties for cubic antiperovskites 
Thursday, March 18, 2021 10:36AM  10:48AM Live 
R52.00014: Magnetoelastic behaviors in holmium antimonide topological semimetal: magnetic torque studies Narayan Poudel, M. Mofazzel Hosen, Zahirul Islam, Dariusz Kaczorowski, Madhab Neupane, Krzysztof Gofryk HoSb crystallizes in cubic rocksalt crystals structure, and orders antiferromagnetically below 5.7 K. Our recent magnetotransport and ARPES studies of HoSb indicate that the presence of bulk band gaps at the Γ and Xsymmetry points of the Brillouin zone, but these gaps do not exhibit band inversion, therefore HoSb does not host a Dirac semimetal state. Furthermore, highfield magnetotransport measurements indicate that HoSb can be characterized as a correlated nearly complete electronholecompensated semimetal. In the course of our systematic studies on the electronic and magnetoelectric properties of this material, here we show our recent torque measurements on oriented HoSb single crystals. We show that the magnetic torque exhibits large anisotropy in the antiferromagnetic phase. The angle dependence indicates the presence of a complex magnetic structure below the Neel temperature in this cubic system. We will discuss the implications of these studies in the context of magnetic phasetransition and topological state in this material. 
Thursday, March 18, 2021 10:48AM  11:00AM Live 
R52.00015: Skyrmion lattice creep at ultralow current densities Yongkang Luo, ShiZeng Lin, Maxime Leroux, David Michael Fobes, Eric D Bauer, Joe Thompson, Marc Janoschek, Boris A Maiorov Magnetic skyrmions are wellsuited for encoding information because they are nanosized, topologically stable, and only require ultralow critical current densities jc to depin from the underlying atomic lattice. Above jc skyrmions exhibit wellcontrolled motion, making them prime candidates for racetrack memories. In thin films thermallyactivated creep motion of isolated skyrmions was observed below jc as predicted by theory. Uncontrolled skyrmion motion is detrimental for racetrack memories and is not fully understood. Notably, the creep of skyrmion lattices in bulk materials remains to be explored. Here we show using resonant ultrasound spectroscopy  a probe highly sensitive to the coupling between skyrmion and atomic lattices  that in the prototypical skyrmion lattice material MnSi depinning occurs at jc that is only 4 percent of jc. Our experiments are in excellent agreement with AndersonKim theory for creep and allow us to reveal a new dynamic regime at ultralow current densities characterized by thermallyactivated skyrmionlatticecreep with important consequences for applications. 
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