Vortex solid-solid phase transition in an untwinnedYBa2Cu3O7−δcrystal

Local magnetic measurements vs temperature in an untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ crystal reveal an abrupt increase in the local magnetization at a field-dependent temperature, well below the melting line. At the same field and temperature a pronounced kink is observed in the local magnetization vs field curves. The line $B_k\mathrm{}\left(T\right)\mathrm{}$ describing the locations of these anomalies in the field-temperature phase diagram divides the vortex solid phase into two regions characterized by weak and strong pinning. A recently developed model describing the vortex solid-solid disorder-induced phase transition explains quantitatively the observed behavior of $B_k\mathrm{}\left(T\right).$ From this behavior we infer that the microscopic origin of pinning in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is fluctuations in the charge-carrier mean free path.