Evidence for Ballistic Thermal Conduction in the One-Dimensional S=1⁄2 Heisenberg Antiferromagnetic Spin System Sr₂CuO₃

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We have measured the thermal conductivity of the one-dimensional (1D) S=1⁄2 Heisenberg antiferromagnetic spin system of Sr₂Cu_1−xPd_xO₃ single crystals including nonmagnetic impurities of Pd²⁺. It has been found that the mean free path of spinons along the 1D spin chain at low temperatures is very close to 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 Sr₂CuO₃ is ballistic as theoretically expected [Zotos et al.: Phys. Rev. Lett. 55 (1997) 11029].