We have measured the thermal conductivity along the [101] direction, κ [101] , along the [101] direction, κ [101] , and along the b-axis, κ b , of the quasi one-dimensional S=1/2 spin system Sr2V3O9 in magnetic fields up to 14 T, in order to find the thermal conductivity due to spinons and to clarify whether the spin-chains run along the [101] or [101] direction. It has been found that both κ [101] , κ [101] and κ b show one peak around 10 K in zero field and that the magnitude of κ [101] is larger than those of κ [101] and κ b . By the application of magnetic field along the heat current, the peak of κ [101] is markedly suppressed, while the peaks of κ [101] and κ b little change. These results indicate that there is a large contribution of spinons to κ [101] and suggest that the spin-chains run along the [101] direction.
In order to clarify the origin of the enhancement of the thermal conductivity in the Bose-Einstein Condensed (BEC) state of field-induced triplons, we have measured the thermal conductivity along the [101] direction parallel to spin-chains, || [101] , and perpendicular to spin-chains, ⊥[101] , of the S = 1/2 bond-alternating spin-chain system Pb 2 V 3 O 9 in magnetic fields up to 14 T. With increasing field at 3 K, it has been found that both ||[101] and ⊥[101] are suppressed in the gapped normal state in low fields. In the BEC state of field-induced triplons in high fields, on the other hand, ||[101] is enhanced with increasing field, while ⊥[101] is suppressed. That is, the thermal conductivity along the direction, where the magnetic interaction is strong, is markedly enhanced in the BEC state. Accordingly, our results suggest that the enhancement of ||[101] in the BEC state is caused by the enhancement of the thermal conductivity due to triplons on the basis of the two-fluid model, as in the case of the superfluid state of liquid 4 He.
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