Thermal conductivity in the Bose–Einstein condensed state of TlCuCl3

Abstract 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 0 1] 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.