A universal phase diagram for weakly pinned low-T c type-II superconductors is revisited and extended with new proposals. The low-temperature "Bragg glass" phase is argued to transform first into a disordered, glassy phase upon heating. This glassy phase, a continuation of the high-field equilibrium vortex glass phase, then melts at higher temperatures into a liquid. This proposal provides an explanation for the anomalies observed in the peak effect regime of 2H-NbSe 2 and several other low-T c materials which is independent of the * Corresponding Author, Email:menon@imsc.ernet.in † Corresponding Author, Email:shampa@mailhost.tifr.res.in 1 microscopic mechanisms of superconductivity in these systems.

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Magnetic screening response of the superconductor 2H-NbSe 2 with varying pinning reveals a rich evolution of the peak effect from a history-independent sharp anomaly to a broad and strongly history dependent effect with internal structure. The results display a stepwise disordering of the vortex lattice through transformations affected by both thermal fluctuations and quenched disorder. ͓S0163-1829͑99͒03309-3͔ PHYSICAL REVIEW B

The peak effect ͑PE͒ region in a single crystal of Ca 3 Rh 4 Sn 13 is shown to comprise two discontinuous first-order-like transitions located near its onset and peak positions, in accordance with a stepwise fracturing of the flux-line lattice. Magnetization response to thermal cycling across the onset position produces an open hysteresis loop, consistent with the notion of the fracturing. A thermomagnetic history dependence study shows that the critical current density J c (H,T) is path dependent over a large part of the (H,T) parameter space. This path dependence ceases above the peak position of the peak effect, suggesting a complete amorphization of the flux-line lattice at (T p ,H p ) line. A plausible vortex phase diagram has been constructed for Ca 3 Rh 4 Sn 13 in which phases like an elastic solid, a plastic solid, and pinned and unpinned amorphous states have been identified.