Thermal Hall effect in a phonon-glass Ba$_3$CuSb$_2$O$_9$

TL;DR

This study reports a thermal Hall effect in Ba$_3$CuSb$_2$O$_9$, indicating strong phonon scattering by orphan spins, revealing insights into heat transport in quantum spin liquids.

Contribution

It demonstrates the presence of a phonon Hall effect in a quantum spin liquid candidate, linking it to phonon scattering by orphan spins, a novel observation in this material.

Findings

Thermal Hall signal observed below 40 K.

Power-law temperature dependence of thermal conductivity.

Indication of phonon Hall effect induced by orphan spins.

Abstract

A distinct thermal Hall signal is observed in a quantum spin liquid candidate Ba$_3$CuSb$_2$O$_9$. The transverse thermal conduction shows a power-law temperature dependence below 40 K where a spin gap opens. We suggest that, through the very low longitudinal thermal conductivity and the thermal Hall signals, a phonon Hall effect is induced by strong phonon scatterings by orphan Cu$^{2+}$ spins formed in random domains of Cu$^{2+}$-Sb$^{5+}$ dumbbells.