Enhanced thermal Hall conductivity below 1 Kelvin in the pyrochlore magnet Yb$_2$Ti$_2$O$_7$

TL;DR

This study reports a record thermal Hall effect in the frustrated pyrochlore Yb$_2$Ti$_2$O$_7$ at sub-Kelvin temperatures, revealing complex magnon and phonon interactions and their impact on thermal transport.

Contribution

First measurement of non-zero thermal Hall conductivity in Yb$_2$Ti$_2$O$_7$ below 0.29 K, highlighting magnon contributions and ruling out phonon skew scattering.

Findings

Gigantic thermal Hall effect observed below 3 K.

Sign change in $ppa_{xy}$ at lowest temperatures.

Suppression of $ppa_{xy}$ with magnetic field.

Abstract

In this letter, we report the gigantic thermal Hall effect $\kappa_{xy}$ in the low-field correlated-paramagnetic state of the frustrated pyrochlore Yb$_2$Ti$_2$O$_7$. We observed a record magnitude for the thermal Hall angle in an insulator, $\left|\kappa_{xy}\right|/\kappa_{xx}\sim 2\,\%$. The signal onsets at $T\sim 3\,$K and is severely weakened around the transition to canted ferromagnetic order at $T_\text{CFM}=0.275\,$K. Besides the large $\kappa_{xy}>0$ of the fluctuating regime, a sign change towards negative $\kappa_{xy}$ occurs at the lowest temperatures and in moderate field, where sharp magnon excitations appear in the inelastic neutron scattering spectra. We analyze the magnon-Hall signal and its suppression with field semi-quantitatively. A contribution of phonon skew scattering to $\kappa_{xy}$ is ruled out by a comparison of $\kappa_{xx}$ for Tb-, Yb-, and Y-based rare earth pyrochlore titanates. These results represent the first report of non-vanishing $\kappa_{xy}$ measured in a dilution refrigerator ($T < 0.29\,$K).