Thermal conductivity in the Bose-Einstein condensed state of TlCuCl ₃

We have measured the thermal conductivity of a TlCuCl₃ single crystal in magnetic fields up to 14 T. It has been found that the temperature-dependence of the thermal conductivity along the [2 01] direction exhibits a sharp peak at 4 K in zero field, which is attributed to enhancement of the thermal conductivity due to phonons or magnons on account of the formation of a spin-gap state. In high magnetic fields above 7 T, on the other hand, it has been found that another sharp peak appears around 4 K. The temperature, where the thermal conductivity rapidly increases with decreasing temperature, is in good agreement with the Bose-Einstein condensation temperature. Accordingly, it is concluded that the peak in high magnetic fields above 7 T is due to large contribution to the thermal conductivity of magnons whose mean free path becomes drastically long owing to the suppression of the magnon-magnon scattering rate in the Bose-Einstein condensed state.