THERMAL-CONDUCTIVITY IN THE AB PLANE OF UNTWINNED YBA2CU3O7-DELTA

We present measurements of thermal conductivity (kappa) in the ab plane of untwinned YBa2Cu3O7-delta (delta almost equal to 0.10) for T less-than-or-equal-to 200 K. Kappa has been measured with heat flow along both the a and b axes and for two oxygen configurations of the same crystal. The conductivity anisotropy has its maximum value at the conductivity peak (T almost-equal-to 40 K), where we find kappa(a)/kappa(b) almost equal to 1.15 before and after an oxygen anneal. Very little ab anisotropy in the magnitude of kappa is observed in the normal state, though near T(c) the temperature dependences are noticeably different for the two directions: d-kappa/dT>0 along the b axis and d-kappa/dT<0 along the a axis. Near T(c) (T less-than-or-equal-to 110 K) kappa is enhanced slightly for both transport directions, features we attribute to the effect of superconducting fluctuations.