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.

We report on the observation of novel features in the minor hysteresis loops in a clean crystal of N bSe2 which displays a peak effect. The observed behavior can be explained in terms of a supercooling of the disordered vortex phase while cooling the superconductor in a field. Also, the extent of spatial order in a flux line lattice formed in ascending fields is different from (and larger than) that in the descending fields below the peak position of the peak effect; this is attributed to unequal degree of annealing of the state induced by a change of field in the two cases.

The peak effect (PE) has been observed in a twinned crystal of YBa 2 Cu 3 O 7−δ for H c in the low field range, close to the zero field superconducting transition temperature (T c (0)) . A sharp depinning transition succeeds the peak temperature T p of the PE. The PE phenomenon broadens and its internal structure smoothens out as the field is increased or decreased beyond the interval between 250 Oe and 1000 Oe. Moreover, the PE could not be observed above 10 kOe and below 20 Oe. The locus of the T p (H) values shows a reentrant characteristic with a nose like feature located at T p (H)/T c (0)≈0.99 and H≈100 Oe (where the FLL constant a 0 ≈penetration depth λ). The upper part of the PE curve (0.5 kOe

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