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

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.