Thermal Conductivity in the Triangular-Lattice Antiferromagnet Ba₃CoSb₂O₉

We have measured the thermal conductivity in the ab-plane, κ_ab, and along the c-axis, κ_c, of single crystals of the S = 1/2 triangular-lattice antiferromagnet Ba₃CoSb₂O₉ in zero field and magnetic fields. In zero field, it has been found that both κ_ab and κ_c show a similar broad peak around 40 K, suggesting that the thermal conductivity due to phonons is dominant in this compound and that the mean free path of phonons is suppressed so much by strong magnetic-fluctuations due to the magnetic frustration. It has also been found that both κ_ab and κ_c show a kink at the antiferromagnetic transition temperature T_N. In magnetic fields parallel to the ab-plane up to 14 T, magnetic-field dependences of both κ_ab and κ_c at temperatures below and above T_N have been found to be understood taking into account the phonon-magnon scattering in the so-called 120° structure phase and the up-up-down (UUD) phase, while the minima of κ_ab and κ_c observed in the middle of the UUD phase have not been understood. This suggests the occurrence of some change of the magnetic state in the middle of the UUD phase.