Vortex Depinning Critical Field in YBa sub 2 Cu sub 3 0 sub 7 Thin Films

An accurate determination of the magnetic field H dependence of the transition temperature T sub c (H) which identifies the phase boundary between the superconducting and normal states is a prerequisite to a meaningful characterization of any superconductor. For Type II bulk superconductors, the upper critical field H sub c2 delineates this phase boundary and is well described by the Ginzburg-Landau-Abrikosov-Gor'kov (GLAG) theory. In high-T sub c superconductors, however, the coherence lengths are on the order of atomic dimensions and thus the mean-field assumptions underlying GLAG theory may not be relevant.