FINITE-SIZE EFFECTS ON THE VORTEX-GLASS TRANSITION IN THIN YBA2CU3O7-DELTA FILMS

Nonlinear current-voltage characteristics have been measured at high m agnetic fields in YBa2Cu3O7-delta films of a thickness t ranging from 3000 down to 16 Angstrom. Critical-scaling analyses of the data for th e thinner films (t less than or equal to 400 Angstrom) reveal deviatio ns from the vortex-glass critical, scaling appropriate for three-dimen sional (3D) systems. This is argued to be a finite-size effect. At lar ge current densities J, the vortices are probed at length scales small er than the film thickness, i.e., 3D vortex-glass behavior is observed . At low J by contrast, the vortex excitations involve typical length scales exceeding the film thickness, resulting in 2D behavior. Further evidence for this picture is found directly from the 3D vortex-glass correlation length, which, upon approach of the glass transition tempe rature, appears to level off at the film thickness. The results indica te that a vortex-glass phase transition does occur at finite temperatu re in 3D systems, but not in 2D systems. In the latter an onset of 2D correlations occurs towards zero temperature. This is demonstrated in our thinnest film (16 Angstrom), which, in a magnetic field, displays a 2D vortex-glass correlation length which critically diverges at zero temperature.