D. Dasgupta

Amorphization of vortex matter and indication of a reentrant peak effect in YBa 2 Cu 3 O 7-δ

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 and 1000 Oe. Moreover, the PE could not be observed above 10 kOe and below 20 Oe. The locus of the Tp(H) values is indicative of the occurrence of a reentrant characteristic with a noselike feature located at T p (H)/T c (0)≈0.99 and H≈100 Oe (where the flux line lattice constant a 0 ≈penetration depth λ). The upper part of the PE curve (0.5 L ≈0.25. The vortex phase diagram near T c (0) determined from the characteristic features of the PE in YBa 2 Cu 3 O 7-δ (H||c) bears close resemblance to that in the 2H-NbSe 2 system, in which a reentrant PE had been distinctly observed earlier.

Comparison of the history effects in magnetization in weakly pinned crystals of high Tc and low Tc superconductors

A comparison of the history effects in weakly pinned single crystals of a high Tc YBa2Cu3O7−δ (for H || c) and a low Tc Ca3Rh4Sn13, which show anomalous variations in critical current density Jc(H), is presented via the tracings of the minor magnetization hysteresis loops using a vibrating sample magnetometer. The sample histories focussed are: (i) the field cooled (FC), (ii) the zero field cooled (ZFC) and (iii) an isothermal reversal of field from the normal state. An understanding of the results in terms of the modulation in the plastic deformation of the elastic vortex solid and supercooling across the order–disorder transitions is sought.

ULTRASONIC SPECTROMETERS FOR CONDENSED MATTER STUDIES AT VERY HIGH MAGNETIC FIELDS

Three ultrasonic spectrometers have been constructed for use with the continuous and pulsed magnetic field facilities of the NHMFL at the Tallahassee and Los Alamos sites. The first of these, designed for use with the dc magnets, is a heterodyne spectrometer with the capabilities of high velocity resolution and near-simultaneous measurements at several frequencies (typically near transducer harmonics). The other two spectrometers are designed for the short-pulsed magnets, with the capability of fast data acquisition. These spectrometers have been used in the study of metamagnetism in the heavy fermion materials.