VORTEX PINNING IN THE FROZEN VORTEX LATTICE IN YBA2CU3O7-X FILMS

The inductance of vortices L(V)(T,B) in YBa2Cu3O7-x films is studied f or temperature 8<T<85 K, and magnetic field 0<B<6 T with the goal of i dentifying the morphology of the vortex pinning sites and the type of the glass phase below the vortex lattice melting transition B-g(T). Th e key feature that distinguishes point, linear, and planar pinning sit es is how rapidly L(V) increases when thresholds in T and B are crosse d. We find that, below a threshold field B-p(T)<B-g(T), the effective pinning force on each vortex is independent of B. At B-p<B<B-g; L(V) i ncreases as B-2, whereas at B approximate to B-g, L(V) diverges faster than B-2 following a scaling law on f, consistent with a phase transi tion between a glass and a Liquid vortex state. Although the value of the threshold field, B-p approximate to 8(1-T/T-C) T, is consistent wi th any type of pinning defect, the increase of L(V) proportional to B- 2 for B>B-p is much less rapid than predicted for collective pinning o f vortices by point defects. Linear and planar defects are possible ex planations, but the necessary theoretical calculations are currently a bsent.