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

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, $\mid {\kappa }_{xy}\mid \mathrm{/}{\kappa }_{xx}\sim 2\mathrm{\%}$. 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).