Abstract
Recently numerous experiments have aimed at verifying the existence of a melting transition in the vortex system of high-temperature superconductors. Direct experimental access to thermodynamic quantities of the vortex system is hard to obtain. On the other hand, the interpretation of experiments probing the vortex behaviour indirectly is also difficult. Transport measurements in single-crystal YBa2Cu3O7-δ by Kwok et al. (1994, Phys. Rev. Lett., 73, 2614) reveal a peak in the critical current density below H c2 (T). Their field-dependent data close to T c indicate that the pinning behaviour of the magnetic flux system undergoes 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.