Thermal Hall conductivity in the frustrated pyrochlore magnet Tb2Ti2O7

TL;DR

This study demonstrates that thermal Hall conductivity effectively probes spin excitations in the frustrated quantum magnet Tb2Ti2O7, revealing unique characteristics distinct from conventional magnons and mapping their behavior under magnetic fields.

Contribution

It introduces thermal Hall conductivity as a novel tool to investigate spin excitations in quantum spin ice pyrochlore magnets, highlighting their unique properties and field dependence.

Findings

Large thermal Hall response despite transparency

Spin excitations differ from magnons with specific characteristics

Excitations are suppressed by magnetic field, mapped in a phase diagram

Abstract

In a ferromagnet, the spin excitations are the well-studied magnons. In frustrated quantum magnets, long-range magnetic order fails to develop despite a large exchange coupling between the spins. In contrast to the magnons in conventional magnets, their spin excitations are poorly understood. Are they itinerant or localized? Here we show that the thermal Hall conductivity $\kappa_{xy}$ provides a powerful probe of spin excitations in the "quantum spin ice" pyrochlore Tb$_2$Ti$_2$O$_7$. The thermal Hall response is large even though the material is transparent. The Hall response arises from spin excitations with specific characteristics that distinguish them from magnons. At low temperature ($T<$ 1 K), the thermal conductivity imitates that of a dirty metal. Using the Hall angle, we construct a phase diagram showing how the excitations are suppressed by a magnetic field.