Thermal conductivity of IPA-CuCl_3: Evidences of ballistic magnon transport and limited applicability of the Bose-Einstein condensation model

TL;DR

This study investigates heat transport in IPA-CuCl_3 at ultra-low temperatures and high magnetic fields, revealing ballistic magnon transport and questioning the applicability of the Bose-Einstein condensation model.

Contribution

It provides experimental evidence of ballistic magnon heat transport and highlights limitations of the BEC model in describing IPA-CuCl_3.

Findings

Ballistic phonon transport below 1 K in zero field.

Magnon contribution to heat transport at H = H_{c1}.

Limited applicability of the BEC model in this material.

Abstract

The heat transport of the spin-gapped material (CH_3)_2CHNH_3CuCl_3 (IPA-CuCl_3), a candidate quantum magnet with Bose-Einstein condensation (BEC), is studied at ultra-low temperatures and in high magnetic fields. Due to the presence of the spin gap, the zero-field thermal conductivity (\kappa) is purely phononic and shows a ballistic behavior at T < 1 K. When the gap is closed by magnetic field at H = H_{c1}, where a long-range antiferromanetic (AF) order of Cu^{2+} moments is developed, the magnons contribute significantly to heat transport and exhibit a ballistic T^3 behavior at T < 600 mK. In addition, the low-T \kappa(H) isotherms show sharp peaks at H_{c1}, which indicates a gap re-opening in the AF state (H > H_{c1}) and demonstrates limited applicability of the BEC model to IPA-CuCl_3.