THICKNESS DEPENDENCE OF THE SUPERCONDUCTING TRANSITION-TEMPERATURE OF YBCO

We have performed detailed structural and superconducting studies of YBCO single layers, YBCO/PrBCO/YBCO tri-layers, and YBCO/PrBCO superlattices. The results for single layers indicate that a nominally one unit cell thick YBCO film sandwiched between PrBCO layers is not superconducting. However, a superlattice with nominally one unit cell thick YBCO layers has a measurable superconducting temperature onset (T(c)o). These apparently contradictory results are reconciled by experiments on YBCO/PrBCO/YBCO tri-layers which, due to coupling across the PrBCO insulating layer, exhibit superconductivity for PrBCO thicknesses below 200 angstrom. Structural studies show that the length scale of the superconductive coupling is much larger than the length scale of the step disorder and interdiffusion present in the film.