Magnetic vortex liquid and anomalous thermal Hall conductivity in frustrated magnets with bond-dependent interactions

TL;DR

This paper predicts a classical vortex liquid state in frustrated Kitaev magnets that can produce large thermal Hall effects without magnetic order, due to nearly flat Chern bands in the excitation spectrum.

Contribution

It introduces a novel classical vortex liquid state in frustrated bond-dependent magnets and links it to large thermal Hall conductivity without magnetic order.

Findings

Vortex liquid state exists in frustrated Kitaev materials under magnetic fields.

The spin excitation spectrum features nearly flat Chern bands.

Large thermal Hall conductivity arises from the vortex liquid state.

Abstract

Recently, the observation of large thermal Hall conductivities in correlated insulators with no apparent broken symmetry have generated immense interest and debates on the underlying ground states. Here, considering frustrated magnets with bond-dependent interactions, which are realized in the so-called Kitaev materials, we theoretically demonstrate that a large thermal Hall conductivity can originate from a classical ground state without any magnetic order. We discover a novel liquid state of magnetic vortices, which are inhomogeneous spin textures embedded in the background of polarized spins, under out-of-plane magnetic fields. In the classical regime, different configurations of vortices form a degenerate manifold. We study the static and dynamical properties of the magnetic vortex liquid state at zero and finite temperatures. In particular, we show that the spin excitation spectrum resembles a continuum of nearly flat Chern bands, which ultimately leads to a large thermal Hall conductivity. Possible connections to experiments are discussed.