Evidence for ballistic thermal conduction in the one-dimensional spin system Sr2CuO3

We have measured the thermal conductivity along the b axis, k(b), parallel to the one-dimensional (1D) spin chain of Sr2Cu1-xPdxO3 single crystals including nonmagnetic impurities of Pd2+. in order to investigate possible ballistic thermal conduction. It has been found that the mean free path of spinons estimated from the thermal conductivity due to spinons, which is obtained by subtracting the thermal conductivity due to phonons from K-b using the Debye model, is comparable with the average length of finite spin chains between spin defects estimated from the magnetic susceptibility measurements. This proves that the thermal conduction due to spinons at low temperatures in the 1D spin system Sr2CuO3 is ballistic.