Renormalization of the Mean-Field Superconducting Penetration Depth in Epitaxial YBa sub 2 Cu sub 3 O sub 7 films.

The a-b plane penetration depth lambda (T) in YBCO films grown expitaxially on SrTiO sub 3 was determined from kinetic inductance measurements. The temperature dependence follows weak-coupling mean-field BCS theory to within ~0.5K of T sub c, resolution- limited either by phase fluctuations in the case of the thinnest films or by homogeneity. The Kosterlitz- Thouless transition in the former case is the first observation in a pure type II superconductor, and is noted by the universal jump in areal superfluid density, the algebraic decay law, and the square- root cusp in its exponent. Josephson coupling between grains in the film affects the overall magnitude of the effective penetration depth.