Anisotropic behavior of thermal conductivity in the bose-einstein condensed state of the bond-alternating spin-chain system Pb₂V ₃O₉

We have systematically measured the thermal conductivity along the [101], [101̄] and b* directions, κ[101̄], κ[101̄] and κb*, respectively, of the S = 1/2 bond-alternating spin-chain system Pb₂V₃O₉ in magnetic fields parallel and perpendicular to the heat current. It has been found that κ[101] along the spin chains is markedly enhanced in the BEC state, which has been concluded to be due to the enhancement of the thermal conductivity due to triplons. Furthermore, it has been found that the enhancement of κ[101̄] in the BEC state is more marked in magnetic fields parallel to the [101] direction than perpendicular to the [101] direction. The BEC state may be weakened or broken in magnetic fields perpendicular to the [101] direction on account of the breaking of the rotational symmetry.