Peak Effect, Fishtail Effect and Plateau Effect : The Reentrant Amorphization of Vortex Matter in 2H-NbSe_2

The magnetic field dependence of the critical current is studied in single crystal samples of the weak pinning type-II superconductor 2H-NbSe$_2$ in the high temperature and the low field region of the (H,T) phase space, in the vicinity of the reentrant peak effect. The experimental results demonstrate various pinning regimes : a collective pinned quasi-ordered solid in the intermediate field 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. Furthermore, the data show that the rapid approach to the amorphous limit naturally yields a peak effect, i.e., a peak in the critical current, in the high field regime, but yields a ``plateau effect'' instead in the low field regime in an analogous way. With increasing effective disorder the peak effect shifts away from H$_{c2}$ and resembles a ``fishtail'' anomaly.