09:45  10:30

Max Hirschberger
(The University of Tokyo)
Spinchiral fluctuation processes in magnetic insulators and metals

10:30  10:35

group photo (to be published on the workshop's web page)

10:35  10:50

coffee break & discussions

10:50  11:35

Alexander Mook
(University of Basel)
Interacting Topological Magnons
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 backscatteringfree information channels for magnetic signals [1]. Since spin excitations do not carry charge, they do not suffer from Joule heating and allow for ultralow energy computation. However, in contrast to electrons, there is no conservation law for spin excitations. This gives rise to particle numbernonconserving manybody interactions the influence of which on quasiparticle topology is an open issue of fundamental interest in the field of topological quantum materials.
Herein, I discuss several aspects of quantum manybody effects caused by particlenumber nonconservation. These include
(i) the quantum damping due to spontaneous quasiparticle decay [2],
(ii) interactionstabilized topological gaps in the singleparticle spectrum [3], and
(iii) a topological hybridization of singleparticle and fewparticle states [4].
[1] Alexander Mook, Sebastián A. Díaz, Jelena Klinovaja, and Daniel Loss, "Chiral Hinge Magnons in SecondOrder Topological Magnon Insulators," Phys. Rev. B 104, 024406 (2021)
[2] Alexander Mook, Jelena Klinovaja, and Daniel Loss, "Quantum damping of skyrmion crystal eigenmodes due to spontaneous quasiparticle decay," Phys. Rev. Research 2, 033491 (2020)
[3] Alexander Mook, Kirill Plekhanov, Jelena Klinovaja, and Daniel Loss, "InteractionStabilized Topological Magnon Insulator in Ferromagnets," Phys. Rev. X 11, 021061 (2021)
[4] Alexander Mook, Rhea Hoyer, Jelena Klinovaja, Daniel Loss, "Topological Hybrids of Magnons and Magnon Bound Pairs," arXiv:2203.12374

11:35  12:20

Matthias Vojta
(Technische Universität Dresden)
Strain tuning of frustrated magnets: Spin liquids and Landau levels
Strain applied to a condensedmatter system can be used to engineer its excitation spectrum via artificial gauge fields, or it may tune the system through transitions between different phases. In the talk I will describe different settings where inhomogeneous strain is applied to localmoment magnets. For a classical spin liquid, the application of strain lifts the extensive degeneracies, and a sequence of novel phases may emerge, which I will illustrate for the Heisenberg model on the kagome lattice. In cases where the magnetic ground state is stable against small deformations, strain will mainly change the excitation spectrum, and I will discuss different types of emergent pseudoLandau levels and their properties.

12:20  14:00

lunch

14:00  14:20

Miska Elliot
(Clarendon Laboratory)
Orderbydisorder and nodal quasiparticles in the XY and Isinglike honeycomb quantum magnets CoTiO3 and FeTiO3 (virtual)
Band topology in electronic systems is known to have profound consequences on
various observable properties in semimetals and certain insulators. Insights from
this eld have reached into many areas of physics and here we describe certain uni
versal signatures of band topology in Dirac magnon materials. Here we report high
resolution inelastic neutron scattering measurements of the magnetic dynamics in
XYlike CoTiO3 and Isinglike FeTiO3 with stacked honeycomb layers. In both sys
tems we observe a winding of the inelastic neutron scattering intensity in the vicinity
of the nodal points. Furthermore, we show that ne details of the magnon band struc
ture reveal bond dependent exchange couplings and an orderbydisorder mechanism
in the XYlike CoTiO3, and we contrast the results with the Isinglike FeTiO3.

14:20  14:40

Gyungchoon Go
(Korea Advanced Institute of Science and Technology (KAIST))
Thermal Hall and spin Nernst effects of magnonpolarons in twodimensional magnets
We theoretically investigate topological transports of magnonphonon hybrid excitations in twodimensional magnetic systems. The magnetoelastic interaction opens band gaps and allows the interband transitions between different excitation states. As a result, the topological transports of the magnonpolarons and their spin angular momenta can occur. When the timereversal symmetry is broken, the magnonpolarons are topologically nontrivial, possessing finite Berry curvature.
However, the magnonpolarons show finite spin Berry curvature with vanishing Berry curvature in the antiferromagnet preserving the timereversal symmetry. We find that the intrinsic spin Nernst conductivity can be large in the clean antiferromagnets. Our results show that a simple magnet on a square lattice supports topologically nontrivial magnonpolarons their spin angular momenta, generalizing topological excitations in conventional magnetic systems.

14:40  15:00

Pontus Laurell
(University of Tennessee)
Dirac Magnons, Nodal Lines, and Nodal Plane in Elemental Gadolinium (virtual)
We investigate the magnetic excitations of elemental gadolinium (Gd), a hexagonal closepacked ferromagnet with nearperfect exchange isotropy and $S=7/2$ moments. Using inelastic neutron scattering we show that Gd is a Dirac magnon material with nodal lines at K and nodal planes at half integer ℓ. We find an anisotropic intensity winding around the Kpoint Dirac magnon cone, which is interpreted to indicate Berry phase physics. Using linear spin wave theory calculations, we show the nodal lines, which are protected by inversion and effective timereversal symmetry, have nontrivial Berry phases and topological surface modes. We also discuss the protection of the nodal plane by a combination of a screwaxis symmetry and the effective timereversal symmetry, and introduce a $\mathbb{Z}_2$ topological invariant characterizing its presence and effect on the scattering intensity. Our results suggest that the entire class of rare earth hcp ferromagnets present a simple model system for topological magnetism.
References:
A. Scheie, Pontus Laurell, P. A. McClarty, G. E. Granroth, M. B. Stone, R. Moessner, and S. E. Nagler, Phys. Rev. Lett. 128, 097201 (2022).
A. Scheie, Pontus Laurell, P. A. McClarty, G. E. Granroth, M. B. Stone, R. Moessner, and S. E. Nagler, Phys. Rev. B 105, 104402 (2022).

15:00  15:20

Peter Czajka
(Princeton University)
Thermal Hall effect from topological bosonic excitations in RuCl$_3$ (virtual)
RuCl$_3$ is thought to exhibit a strong Kitaevtype exchange interaction, but whether this interaction produces a spin liquid state with chiral Majorana edge modes is still unclear. We will discuss our new temperature thermal Hall conductivity measurements where we observe a strongly temperaturedependent signal that is inconsistent with the Majoranabased picture reported by other groups. Using the quantitative framework developed for magnonderived Hall effects, we show that this temperaturedependence can be fit to a bosonic model and that this fit produces values that are consistent with known parameters for RuCl3. Specifically, this fitting model corresponds to a magnon Chern insulatorlike state in the system’s spinpolarized phase that was previously predicted.

15:20  16:00

coffee break & discussions

16:00  16:20

Seunghwan Do
(Oak Ridge National Laboratory)
Gaps in Topological Magnon Spectra: Intrinsic vs. Extrinsic Effects (virtual)

16:20  17:05

Alexey Kovalev
(University of NebraskaLincoln)
Towards control of spin currents in magnetic insulators (virtual)
An ability to control spin currents is important for probing many spin related phenomena in the field of spintronics and for designing logic and memory devices with low dissipation. Spinorbit torque is an important example in which spin current flows across magnetic interface and helps to control magnetization dynamics. In this talk, I will first discuss transport, Halllike responses of magnons in antiferromagnetic insulators, ranging from collinear antiferromagnets [1,2] to breathing pyrochlore noncollinear antiferromagnets [3,4]. The theory also applies to noncollinear antiferromagnets, such as kagome, where we predict both the spin Nernst response [3] and the generation of nonequilibrium spin polarization [4] by temperature gradients, the latter effect constitutes the magnonic analogue of the Edelstein effect of electrons. I will further discuss the spin superfluid transport associated with collective modes in magnetic insulators [5,6]. We observe that in two dimensional systems at finite temperatures spin superfluidity is affected by the presence of topological defects. We further propose to use the Hall response of topological defects, such as merons and antimerons, to spin currents in 2D magnetic insulator with inplane anisotropy for identification of the BerezinskiiKosterlitzThouless (BKT) transition in a transistorlike geometry. Our numerical results relying on a combination of Monte Carlo and spin dynamics simulations show transition from spin superfluidity to conventional spin transport, accompanied by the universal jump of the spin stiffness and exponential growth of the transverse vorticity current. We propose a superfluid spin transistor in which the spin and vorticity currents are modulated by tuning the inplane magnet across BKT transition, e.g., by changing the exchange interaction, magnetic anisotropy, or temperature [7].
[1] V. Zyuzin, A.A. Kovalev, Phys. Rev. Lett. 117, 217203 (2016).
[2] B. Li, A.A. Kovalev. Phys. Rev. Lett. 125, 257201 (2020)
[3] B. Li, S. Sandhoefner, A.A. Kovalev, Phys. Rev. Research 2, 013079 (2020)
[4] B. Li, A. Mook, A. Raeliarijaona, A.A. Kovalev, Phys. Rev. B 101, 024427 (2020)
[5] G. G. Baez Flores, A.A. Kovalev, M. van Schilfgaarde, K. D. Belashchenko, Phys. Rev. B 101, 224405 (2020)
[6] B. Li, A.A. Kovalev, Phys. Rev. B 103, 060406 (2021)
[7] E. Schwartz, B. Li, A.A. Kovalev, arXiv:2112.14241v1

17:05  18:00

discussions

18:00  19:30

dinner
