TRANSITION FROM INTACT TO SHORT DECOUPLED VORTICES IN THE VORTEX LIQUID OF YBA2CU3O7-DELTA

Superconducting transitions were studied in single-crystal YBa2Cu3O7-delta with magnetic fields in the range 0.02-8 T applied along the c axis and six contacts mounted on two ab-plane surfaces to measure voltages on opposite sides of the crystal. In the beginning of the transition curves, a change was observed at about 0.3 T from a high-field region, where vortices are preserved over the length of the crystal, to a low-field behavior where vortices move independently in different layers. In the high-field region a peak was observed in V(bot), which can be qualitatively understood in terms of breaking of long vortices by increased thermal fluctuations. At about 0.3 T we also observed a crossover in the magnetic-field dependence of an apparent activation energy from a 1/B behavior at high fields to -lnB at low fields. From the data in the low-field region new quantitative support is obtained for a Kosterlitz-Thouless-type transition where vortex-antivortex pairs dissociate and move independently in different layers of the crystal. The correlation length of these vortices is estimated to be 650 +/- 150 angstrom.