Vortex pinning in YBa2Cu3O7-x films

Evidence that pinning on linear or planar defects dominates the vortex dynamics in YBa2Cu3O7-X (YBCO) films is provided by complex impedance measurements at temperature 8 K < T < T-c and magnetic field 0<B<6 T. Below the vortex lattice melting transition B-g(T) but above a threshold field B-p approximate to 8(1-T/T-c) T, the inductance of vortices increases as B-2, much less rapidly than predicted for collective pinning of vortices by point defects. Above the vortex melting line, critical scaling persists over the region B-g(T) < B < B*(T) where the vortex correlation length xi exceeds a characteristic length scale xi* = xi(B=B*) approximate to 450 Angstrom. The value of xi* is not sensitive to Al-doping in the Cu sites in the lattice and is close to the size of twin domains in the film. The nature of the observed crossovers is discussed in terms of available theoretical models for a glass-liquid transition at B-g.