Thermal conductivity of local moment models with strong spin-orbit coupling

TL;DR

This paper investigates how magnetic and lattice contributions affect thermal conductivity in 2D honeycomb lattice models with strong spin-orbit coupling, focusing on different regimes and relaxation mechanisms.

Contribution

It introduces a methodology to analyze thermal transport in complex 2D magnon-phonon systems with anisotropic magnetic phases and strong spin-orbit effects.

Findings

Thermal conductivity can be highly anisotropic in magnetic energy dominated regimes.

In phonon energy dominated regimes, thermal conductivity is nearly isotropic.

The methodology applies to systems lacking analytical solutions for magnon bands.

Abstract

We study the magnetic and lattice contributions to the thermal conductivity of electrically insulating strongly spin-orbit coupled magnetically ordered phases on a two-dimensional honeycomb lattice using the Kitaev-Heisenberg model. Depending on model parameters, such as the relative strength of the spin-orbit induced anisotropic coupling, a number of magnetically ordered phases are possible. In this work, we study two distinct regimes of thermal transport depending on whether the characteristic energy of the phonons or the magnons dominates, and focus on two different relaxation mechanisms, boundary scattering and magnon-phonon scattering. For spatially anisotropic magnetic phases, the thermal conductivity tensor can be highly anisotropic when the magnetic energy scale dominates, since the magnetic degrees of freedom dominate the thermal transport for temperatures well below the magnetic transition temperature. In the opposite limit in which the phonon energy scale dominates, the thermal conductivity will be nearly isotropic, reflecting the isotropic (at low temperatures) phonon dispersion assumed for the honeycomb lattice. We further discuss the extent to which thermal transport properties are influenced by strong spin-orbit induced anisotropic coupling in the local moment regime of insulating magnetic phases. The developed methodology can be applied to any 2D magnon-phonon system, and more importantly to systems where an analytical Bogoliubov transformation cannot be found and magnon bands are not necessarily isotropic.