PEAK EFFECT IN TWINNED SUPERCONDUCTORS

A sharp maximum in the critical current J(c) as a function of temperat ure just below the melting point of the Abrikosov flux lattice has rec ently been observed in both low- and high-temperature superconductors. This peak effect is strongest in twinned crystals for fields aligned with the twin planes. We propose that this peak signals the breakdown of the collective pinning regime and the crossover to strong pinning o f single vortices on the twin boundaries. This crossover is very sharp acid can account for the steep drop of the differential resistivity o bserved in experiments.