2D-3D CROSSOVER EFFECTS ON THE VORTEX-GLASS PHASE-TRANSITION IN THIN YBA2CU3O7-DELTA FILMS

Nonlinear current-voltage characteristics have been measured for ultrathin (16-400 angstrom) YBa2Cu3O7-delta films in high magnetic fields. A scaling analysis of these data reveals deviations from the universal vortex-glass critical scaling behavior observed for thick films. This is argued to be a dimensionality effect: At large currents, one probes length scales smaller than the film thickness, i.e., the three-dimensional (3d) vortex-glass behavior, whereas at low currents the vortex excitations involve typical length scales which exceed the film thickness, hence the 2d behavior is exhibited. Further evidence for this picture is found from the 3d vortex-glass correlation length, which appears to be cut off by the film thickness.