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 thresh
old 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 pinn
ing of vortices by point defects. Above the vortex melting line, criti
cal scaling persists over the region B-g(T) < B < B(T) where the vort
ex correlation length xi exceeds a characteristic length scale xi = x
i(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.