INPLANE THERMAL-CONDUCTIVITY AND LORENZ NUMBER IN YBA2CU3O7-Y

We make a tentative proposal to extract the electronic contribution to the in-plane thermal conductivity kappa(el)(T) of YBa2Cu3O7-y in the normal state. We have measured kappa(ab)(T) for single crystals with v arious oxygen contents (6.06 less than or equal to 7-y less than or eq ual to 6.93). The systematic study of the oxygen-content dependence of kappa(ab) for the insulating phase enables us to estimate the phononi c contribution kappa(ph)(T) for the metallic phase to some extent, Bas ed on the estimated kappa(ph), we examined kappa(el) from the view poi nt of whether the Wiedemann-Franz law holds or not. The estimated kapp a(el) appears to show only a weak T dependence and no significant chan ge at a characteristic temperature T below which the electrical condu ctivity sigma(ab)(T) is enhanced possibly due to the opening of a spin gap. The difference between kappa(el) and sigma(ab) suggests the viol ation of the Wiedemann-Franz law below T. We discussed the ambiguity of our analyses and other possible interpretations of the present resu lt as well as the picture of the charge transport speculated from the above suggestion.