THICKNESS DEPENDENCE OF THE SUPERCONDUCTING TRANSITION-TEMPERATURE OFYBCO

We have performed detailed structural and superconducting studies of Y BCO single layers, YBCO/PrBCO/YBCO tri-layers, and YBCO/PrBCO superlat tices. The results for single layers indicate that a nominally one uni t cell thick YBCO film sandwiched between PrBCO layers is not supercon ducting. However, a superlattice with nominally one unit cell thick YB CO layers has a measurable superconducting temperature onset (T(c)o). These apparently contradictory results are reconciled by experiments o n YBCO/PrBCO/YBCO tri-layers which, due to coupling across the PrBCO i nsulating layer, exhibit superconductivity for PrBCO thicknesses below 200 angstrom. Structural studies show that the length scale of the su perconductive coupling is much larger than the length scale of the ste p disorder and interdiffusion present in the film.