Session K01: Condensed Matter I

Topological Thermal Hall Effect of Magnons in Magnetic Skyrmion Lattice

Topological transports of fermions are governed by the Chern numbers of the energy bands lying below the Fermi energy. For bosons, e.g. phonons and magnons in a crystal, topological transport is dominated by the Chern number of the lowest energy band when the band gap is comparable to the thermal energy. Here, we demonstrate the presence of topological transport by bosonic magnons in a lattice of magnetic skyrmions – topological defects formed by a vortex-like texture of spins. We find a distinct thermal Hall signal in the magnetic skyrmion phase of an insulating polar magnet GaV₄Se₈, identified as the topological thermal Hall effect of magnons governed by the Chern number of the lowest energy band of the magnons in a triangular lattice of magnetic skyrmions. Our findings lay a foundation for studying topological phenomena of other bosonic excitations through thermal Hall probe.   

Chiral spin texture in the anomalous Hall antiferromagnet CoNb3S6

CoNb₃S₆ (CNS) is an intercalated transition metal dichalcogenide exhibiting a large anomalous Hall effect (AHE) that cannot be explained by the collinear magnetic order previously observed by neutron diffraction. Thus, complex chiral, non-collinear, or non-coplanar spin orders have all been proposed as explanations for the observed large AHE in CNS. Here we carry out resonant elastic x-ray scattering (REXS) at the Co L₃ edge to obtain a more detailed description of the magnetic ordering in CNS. We determine the average in-plane spin orientation of the previously known q = (0.5 0 0) collinear order using full linear polarization analysis (FLPA). By comparing the results at two symmetry-related wavevectors, we find that the data is consistent with single-q, multi-domain order, rather than multi-q order. We also find satellite peaks around the collinear wavevector, indicating a long-wavelength incommensurate uniaxial modulation of the collinear order. The modulation wavevector exhibits a sample dependence which is correlated with the magnetic transition temperature, indicating that the modulation depends on Co stoichiometry. The modulation in the under-intercalated samples is consistent with a spin density wave, while that in the nearly stoichiometric sample is consistent with a helical structure, as indicated by the circular dichroism of the satellite peaks. These results suggest that a long-range chiral spin texture provides the necessary symmetry-breaking mechanism for the large anomalous Hall effect in CNS.   

Avoidance of magnetic frustration: charge and magnetic ordering in the triangular lattice Hubbard model

Geometrically frustrated magnetic materials and models have attracted significant

attention in the condensed matter community because they are apt to exhibit interesting

collective phases of matter. Geometric frustration arises in magnetic systems when all

nearest neighbor antiferromagnetic interactions cannot be minimized simultaneously.

A similar phenomenon is observed in itinerant systems, metals, because the hopping,

kinetic energy, of electrons between nearest neighbor ions does not promote a minimization

of the repulsive electron-electron interactions. We study the paradigm

extended Hubbard model of interacting electrons on the geometrically frustrated

triangular lattice. We report results from mean field calculations for a model with

an average electron filling of n = 2/3, where electrons could avoid geometric frustration

if they all were to occupy sites that define a honeycomb substructure of the parent

triangular lattice. At T = 0 K, we find that a partially ordered charge and magnetic phase

forms on a honeycomb substructure at a critical on-site interaction U_c/t = 5.00. We report

the effects of finite nearest neighbor Coulomb repulsion, V/t,  and finite temperature

on the properties of our model.

      

Dynamically tunable magnon-magnon coupling in synthetic antiferromagnets

The richness in both the dispersion and energy of antiferromagnetic magnons has spurred the magnetism community to consider antiferromagnets for future spintronic/magnonic applications. However, the excitation and control of antiferromagnetic magnons remains challenging, especially when compared to ferromagnetic counterparts. A middle ground is found with synthetic antiferromagnet metamaterials, where acoustic and optical magnons exist at GHz frequencies. In these materials, the magnon energy spectrum can be tuned by static symmetry-breaking external fields or dipolar interactions hybridizing optical and acoustic magnon branches. Here, we theoretically predict and experimentally discover an alternative pathway to strong and tunable magnon-magnon interactions. We develop a phenomenological model for the fieldlike and dampinglike torques generated by spin pumping in noncollinear magnetic multilayers separated by normal-metal spacers. We show that an asymmetry in the fieldlike torques acting on different magnetic layers can lift the spectral degeneracies of acoustic and optical magnon branches and yield symmetry-breaking induced magnon-magnon interactions. Our work extends the phenomenology of spin pumping to noncollinear magnetization configurations and significantly expands ways of engineering magnon-magnon interactions within antiferromagnets and quantum hybrid magnonic materials.   

Magnetic phase competition in the highly frustrated iridate K2IrCl6

Frustrated magnetic systems have attracted significant attention recently as the possibility of realizing novel magnetic states such as spin liquid state [1]. K₂IrCl₆ is an exciting compound to study the ground-state selection and explore the magnetic phase diagram in a model antiferromagnet with face-centered-cubic frustration. In this talk, I will discuss our elastic and inelastic neutron scattering results which indicate the coexistence of a minority type-I phase with k=(100) and a previously revealed type-III phase with k=(1 ½ 0). This coexistence is also confirmed by the resonant elastic X-ray scattering measurement. Given that the type-III magnetic order is the dominant phase, the type-I magnetic order could be stabilized through local deviations from cubic symmetry, quantum order-by-disorder effect, or both [2,3]. This study will provide a sophisticated empirical basis for the understanding of ground-state selections by various mechanisms and for the possible materialization of quantum spin liquid in frustrated magnets.

[1] L. Balents, Nature 464, 199 (2010)

[2] A. Aczel et al, Phys. Rev. B 99, 134417 (2019)

[3] R. Schick,  arXiv:2206.12102 [cond-mat.str-el] (2022)   

Exceptional points in magnetic systems

While several works have recently addressed the emergence of exceptional points (EPs), i.e., spectral singularities of non-Hermitian Hamiltonians, in the dynamics of coupled magnetic systems, their experimental signatures remain relatively unexplored. In the vicinity of an EP, magnon spectral features can be drastically modified, which, in turn, can affect the the properties routinely probed in spintronics setups, such as, e.g., spin conductivity and spin diffusion length. However, the systems discussed so far display EPs only at isolated momenta, which are not likely to influence system’s properties, such as, e.g., transport coefficients, that depend on integrals over a large number of momenta. 

In the first part of my talk I will show, by focusing on the driven magnetization dynamics of a van der Waals ferromagnetic bilayer, that EPs can appear over extended portions of the first Brillouin zone as well. Furthermore, I will discuss how the effective non-Hermitian magnon Hamiltonian, whose eigenvalues are purely real or come in complex-conjugate pairs, respects an unusual wavevector-dependent pseudo-Hermiticity. In the second part of my talk, I will show that, in the presence of gain, EPs are smoking guns of dynamical phase transitions of the non-linear spin dynamics.