VORTEX DYNAMICS BELOW THE FLUX-LATTICE MELTING TRANSITION IN YBA2CU2O7-DELTA

We report measurements of the critical current, differential resistanc e, and voltage noise below the vortex-lattice melting transition in cl ean, untwinned YBa2CU3O7-delta crystals. The differential resistance a t dc current bias much greater than J(c) shows a sharp feature at the melting transition accompanied by a rapid drop in the voltage noise, d emonstrating that the melting transition is insensitive to the static pinning potential. In the solid phase, the measurements all show a mar ked change from the low-field, pinned lattice to the high-field vortex -glass contiguous with the tricritical point in this sample.