Role of magnetic ions in the thermal Hall effect of the paramagnetic insulator TmVO$_{4}$

TL;DR

This study investigates the role of magnetic Tm$^{3+}$ ions in the thermal Hall effect of TmVO$_{4}$ and finds that they are not essential, highlighting the significance of impurities and phonons in generating the effect.

Contribution

The paper demonstrates that Tm$^{3+}$ ions are not necessary for the thermal Hall effect in TmVO$_{4}$, emphasizing the role of impurities and phonons over magnetic ions.

Findings

Negative thermal Hall conductivity observed in TmVO$_{4}$

Negligible $_{xy}$ in nonmagnetic YVO$_{4}$

Sign change of $_{xy}$ with impurity concentration

Abstract

In a growing number of materials, phonons have been found to generate a thermal Hall effect, but the underlying mechanism remains unclear. Inspired by previous studies that revealed the importance of Tb$^{3+}$ ions in generating the thermal Hall effect in a family of pyrochlores, we investigated the role of Tm$^{3+}$ ions in TmVO$_{4}$, a paramagnetic insulator with a different crystal structure. We observe a negative thermal Hall conductivity in TmVO$_{4}$ with a magnitude such that the Hall angle, $|\kappa_{xy}$/$\kappa_{xx}|$, is approximately 1 x 10$^{-3}$ at $H$ = 15 T and $T$ = 20 K, typical for a phonon-generated thermal Hall effect. In contrast to the negligible $\kappa_{xy}$ found in the nonmagnetic pyrochlore analog (where the Tb$^{3+}$ ions are replaced with Y$^{3+}$), we observe a negative $\kappa_{xy}$ in YVO$_{4}$ with a Hall angle of magnitude comparable to that of TmVO$_{4}$. This shows that the Tm$^{3+}$ ions are not essential for the thermal Hall effect in this family of materials. Interestingly, at an intermediate Y concentration of $x$ = 0.3 in Tm$_{1-x}$Y$_{x}$VO$_{4}$, $\kappa_{xy}$ was found to have a positive sign, pointing to the importance of impurities in the thermal Hall effect of phonons.