Generic magnetic field dependence of thermal conductivity in magnetic insulators via hybridization of acoustic phonons and spin-flip excitations

Abstract

Magnetic insulators provide excellent playgrounds to realize a range of exciting spin models, some of which predict exotic spin ground states, and thermal transport properties have been taking center stage in probing the spin excitations. Despite the fact that acoustic phonons make the major contribution to heat conduction in a crystalline system, their interplay with magnetic excitations is often viewed as peripheral to the physics of interest, for instance as an inconvenient source of scattering or decoherence. Here, we present a comprehensive study on the longitudinal magneto-thermal transport in a paramagnetic effective spin-1/2 magnetic insulator CsYbSe$_2$. We introduce a minimal model requiring only Zeeman splitting and magnetoelastic coupling, and use it to argue that hybridized excitations -- formed from acoustic phonons and localized spin-flip-excitations across the Zeeman gap of the crystal electric field ground doublet -- are responsible for a striking non-monotonic field dependence of longitudinal thermal conductivity. Beyond highlighting a starring role for phonons, our results raise the prospect of universal magneto-thermal transport phenomena in magnetic insulators that originate from simple features shared across many systems.Comment: 8 pages, 4 figure