Drastic enhancement of magnon thermal conductivity in the Bose-Einstein condensed state of TlCuCl$_3$

TL;DR

This study investigates how magnetic fields influence thermal conductivity in TlCuCl₃, revealing a significant increase in magnon-mediated heat transport in the Bose-Einstein condensed state, with implications for quantum magnetic materials.

Contribution

It demonstrates the drastic enhancement of magnon thermal conductivity in the Bose-Einstein condensed state of TlCuCl₃, a novel observation in quantum magnetism.

Findings

Thermal conductivity peak at 4 K suppressed by magnetic fields up to 7 T.

Emergence of a new sharp peak above 7 T attributed to magnons.

Magnon mean free path extends significantly in the Bose-Einstein condensed state.

Abstract

We have measured the thermal conductivity of a TlCuCl$_3$ single crystal in magnetic fields up to 14 T. It has been found that the temperature dependence of the thermal conductivity exhibits a sharp peak at 4 K in zero field, which is suppressed by the application of magnetic fields up to 7 T. The peak is concluded to be attributable to the enhancement of the thermal conductivity due to phonons because of the formation of a spin-gap state. In high magnetic fields above 7 T, on the other hand, another sharp peak appears around 4 K and this is enhanced with increasing magnetic field. This peak is regarded as being attributable to the enhancement of the thermal conductivity due to magnons and/or phonons because of the drastic extension of the mean free path of magnons and/or phonons in the Bose-Einstein condensed state.