Low-temperature heat transport in the low-dimensional quantum magnet NiCl2-4SC(NH2)2.

We report a study of the low-temperature thermal conductivity of NiCl2-4SC(NH2)_{2}, which is a spin-1 chain system exhibiting the magnon Bose-Einstein condensation (BEC) in a magnetic field. It is found that the low-T thermal conductivity along the spin-chain direction shows strong anomalies at the lower and upper critical fields of the magnon BEC state. In this state, magnons act mainly as phonon scatterers at relatively high temperature, but change their role to heat carriers upon temperature approaching zero. The result demonstrates a large thermal conductivity in the magnon BEC state and points to a direct analog between the magnon BEC and the conventional one.