Peak effect, plateau effect, and fishtail anomaly:: The reentrant amorphization of vortex matter in 2H-NbSe2

The magnetic field dependence of the critical current is studied in single-crystal samples of the weak pinning type-II superconductor 2H-NbSe2 in the high-temperature and the low-field region of the (H,T) space. The experimental results demonstrate various pinning regimes: a collective pinned quasiordered solid in the intermediate-field range that is destabilized in favor of disordered vortex phases in both high fields near H-c2 and at low fields near H-c1. The temperature evolution of the pinning behavior demonstrates how the amorphous limit (where the correlation volume is nearly field independent) is approached around the so-called nose region of the reentrant peak-effect boundary. In the high-field regime the rapid approach to the amorphous limit naturally yields a peak effect, i.e., a peak in the critical current. In the low-field regime the crossover to the individual pinning regime gives rise to a "plateau effect." We show that with increasing effective pinning the peak effect shifts away from H-c2 and resembles a "fishtail" anomaly.