Unveiling Non-Kitaev Interactions and Field-Angle Dependence in Topological Magnon Transport of $\alpha$-RuCl$_3$

TL;DR

This paper investigates how non-Kitaev interactions and magnetic field angles influence topological magnon transport in $ ext{RuCl}_3$, revealing their effects on thermal Hall conductivity and aiding in understanding its magnetic Hamiltonian.

Contribution

It introduces a theoretical analysis of non-Kitaev interactions' impact on magnon transport in $ ext{RuCl}_3$, connecting field-angle dependence with experimental observations.

Findings

Transverse thermal conductivities depend on magnetic field orientation.

Sign and magnitude of conductivities are influenced by exchange parameters.

The model helps identify the effective Hamiltonian for $ ext{RuCl}_3$.

Abstract

Honeycomb lattice Kitaev magnets exhibit exotic magnetic properties governed by the Kitaev interaction. This study delves into $\alpha$-RuCl$_3$, a prototypical example described by effective Hamiltonians encompassing bond-dependent Kitaev interactions alongside additional terms such as the Heisenberg interaction and symmetric off-diagonal exchange interactions. These non-Kitaev terms significantly influence $\alpha$-RuCl$_3$'s low-temperature magnetism, impacting both magnetic order and excitations. We employ spin-wave theory to elucidate the topological nature of magnetic excitations within the polarized state of $\alpha$-RuCl$_3$ under an external magnetic field. Our focus lies on transverse magnon conductivities, specially the thermal Hall conductivity and spin Nernst coefficient. The calculations unveil a pronounced dependence of the magnitude and sign structure of the low-temperature transverse thermal conductivities on both the applied magnetic field's orientation and the exchange parameters within the nearest neighbor Heisenberg-Kitaev-Gamma-Gamma$'$ $(JK\Gamma\Gamma')$ model, which govern the nature and strength of spin interactions. This theoretical framework facilitates critical comparisons with experimental observations, ultimately aiding the identification of an effective Hamiltonian for Kitaev magnets exemplified by $\alpha$-RuCl$_3$.