Charge-transfer and barrier-layer effects in YBa2Cu3O7 superlattices

Superconducting superlattices composed of YBa2Cu3O7 and various ''barrier-layer'' materials have been studied extensively experimentally and several theoretical attempts to correlate the resistivity R as a function of temperature T with the superlattice structure have appeared in the literature. Theoretical interest in such structures comes about primarily because of the insight they may provide into dimensionality effects, interlayer coupling, and interlayer charge redistribution in the high-Tc superconductors. In this paper, R(T) data up to T=300 K are analyzed by least-squares fitting for 17 superlattices consisting of YBa2Cu3O7 and barrier layers of PrBa2Cu3O7, Pr0.7Y0.3Ba2Cu3O7, and Pr0.5Ca0.5Ba2Cu3O7. The fitting model incorporates results from Kosterlitz-Thouless (vortex-antivortex unbinding) and Aslamazov-Larkin (fluctuation-enhanced conductivity) theories in the resistive transition region and charge-transfer effects, variable-range hopping, etc., in the normal state. The results show that ''interface'' and barrier-layer conductivity must be taken into account in order to explain the unusual normal-state R(T) behavior in some superlattices. These effects may also contribute in the transition region where vortex-antivortex unbinding and fluctuation effects play the major roles