ON THE PEAK EFFECT AND VORTEX LATTICE MELTING IN YBA2CU3O7-DELTA

Recently numerous experiments have aimed at verifying the existence of a melting transition in the vortex system of high-temperature superco nductors. Direct experimental access to thermodynamic quantities of th e vortex system is hard to obtain. On the other hand, the interpretati on of experiments probing the vortex behaviour indirectly is also diff icult. Transport measurements in single-crystal YBa2Cu3O7-delta by Kwo k et al. (1994, Phys. Rev. Lett., 73, 2614) reveal a peak in the criti cal current density below H-c2 (T). Their field-dependent data dose to T-c indicate that the pinning behaviour of the magnetic flux system u ndergoes several marked changes with increasing field. A simple model, based on the assumption of a strongly pinned disordered vortex phase at low and at high magnetic fields, separated by a depinned or weakly pinned ordered vortex phase could explain these and other experimental observations.