In-plane and out-of-plane thermal conductivity of a large single crystal of YBa2Cu3O7-x

The in-plane and out-of-plane thermal conductivity, kappa(ab) and kappa(c), of a large single crystal of twinned YBa2Cu3O7-x has been studied as a function of temperature for several annealing conditions. The present result strongly supports ascribing an observed peak in kappa(ab) below T-c to an electron contribution. The superconducting enhancement of kappa(ab) becomes rather small and the position of the peak value gradually shifts to higher temperatures with an increase of annealing time. This behavior is not well explained by the Kadanoff-Martin expression on the assumption of Matthiessen's rule but probably implies that the screening effect due to charge carrier doping yields a suppression of strong electron-electron interaction in the normal state.