G. Ravikumar

Anomalous peak effect in CeRu2 and 2H-NbSe2: Fracturing of a flux line lattice

CeRu 2 and 2H-NbSe 2 display remarkable similarities in their magnetic response, reflecting the manner in which the weakly pinned flux line lattice (FLL) loses spatial order in the peak-effect (PE) regime. We present evidence for discontinuous changes in the screening response near the onset of the PE in these systems, and demonstrate history-dependent effects. We attribute these features to a disorder-induced fracturing and entanglement of the FLL, as an alternative to the appearance of a spatially modulated ground state for CeRu 2 .

Magnetisation hysteresis and critical current density

Abstract The hysteresis in magnetisation is extensively used to infer the critical current density of high T c superconductors. We use calculated magnetisation curves, obtained analytically for two particular forms of J c ( H ), to examine the validity of the method currently used. We show that this method is valid only at large fields, and that the region of validity depends on the sample size. We compare our results with existing data. We also examine the case of a general form of J c ( H ), and present a method for obtaining a zero field J c from magnetisation curves.

Supercooling of the disordered vortex phase via minor hysteresis loops in 2 H − NbSe 2

We report on the observation of novel features in the minor hysteresis loops in a clean crystal of

Metastability and switching in the vortex state of 2H-NbSe2

2\mathrm{H}\ensuremath{-}{\mathrm{NbSe}}_{2},

Transport Critical Current Density of Polycrystalline YBCO in Zero Field Cooled and Field Cooled States: A Comparison

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 a different degree of reorganization of the vortex state induced by changes in the field in the two cases.

Disordered type-II superconductors: a universal phase diagram for low-Tc systems

History-dependent metastable states with different bulk properties are formed in the vortex state of the type-II superconductor 2H-NbSe2. Magnetic measurements demonstrate the difference between the shielding responses of a field- and a zero-field-cooled state, and provide a procedure for switching the system from one state to the other.

Step change in equilibrium magnetization across the peak effect in 2H-NbSe2

Magnetic field dependence of transport critical current density (Jc) of a polycrystalline YBCO superconductor in field cooled and zero field cooled cases have been studied experimentally. The measured Jc value when the sample was cooled in a field (FC) was always larger than that measured for a zero field cooled (ZFC) case in the same magnetic field. We also observe (i) a peak in the field dependence of the ratio of Jc in FC case to that of ZFC case and (ii) an anomalous rise in Jc in ZFC case with increasing field. These observations are explained as a signature of two types of regions (intergrain and intragrain) in sintered specimens.

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

A universal phase diagram for weakly pinned low-Tc 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-NbSe2 and several other low-Tc materials which is independent of the

Effect of field inhomogeneity on the magnetisation measurements in the peak effect region of CeRu2 superconductor

Isothermal magnetization measurements across the peak effect (PE) region in a clean superconducting crystal of 2H-NbSe 2 reveal a step increase in the equilibrium magnetization suggestive of a first order phase transition. We relate this observation to the entropy change associated with the occurrence of order-disorder transformation across the PE in a weakly pinned flux line lattice (FLL).

Magnetic phase diagram of YNi2B2C

Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai-400005, Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai-400085, India Department of Physics, Indian Institute of Technology, Powai, Mumbai-400076, India Department of Physics, University of Warwick, Coventry, CV4 7AL, United Kingdom Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974 NEC Research Institute, 4 Independence Way, Princeton, New Jersey 08540 ~Received 21 June 1999; revised manuscript received 23 September 1999 !