Abstract: | 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 0g(T) but above a threshold field Bp≈8(1-T/T c ) T, the inductance of vortices increases as B2, much less rapidly than predicted for collective pinning of vortices by point defects. Above the vortex melting line, critical scaling persists over the region Bg(T*(T) where the vortex correlation length ξ exceeds a characteristic length scale ξ*≡ξ(B=B*)≈450?. The value of ξ* 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 Bg. |