Peak effect, plateau effect, and fishtail anomaly: The reentrant amorphization of vortex matter in2H−NbSe2

The magnetic field dependence of the critical current is studied in single-crystal samples of the weak pinning type-II superconductor $2H\ensuremath{-}{\mathrm{NbSe}}_{2}$ 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.