Vortex solid-solid phase transition in an untwinned YBa2Cu3O7+delta crystal

Local magnetic measurements vs temperature in an untwinned YBa2Cu3O7-delta crystal reveal an abrupt increase in the local magnetization at a field-dep endent temperature, well below the melting line. At the same field and temp erature a pronounced kink is observed in the local magnetizations field cur ves. The line B-k(T) describing the locations of these anomalies in the fie ld-temperature phase diagram divides the vortex solid phase into two region s characterized by weak and strong pinning. A recently developed model desc ribing the vortex solid-solid disorder-induced phase transition explains qu antitatively the observed behavior of B-k(T). From this behavior we infer t hat the microscopic origin of pinning in YBa2Cu3O7-delta is fluctuations in the charge-carrier mean free path. [S0163-1829(99)00922-4].