J.R. Laverty

Non-destructive determination of the current-carrying length scale in superconducting crystals and thin films

Abstract The magnetisation of a disc-shaped superconductor in a perpendicular applied field is analysed. The differential susceptibility as the field is reduced slightly from its maximum value, is shown to give an unambiguous determination of both the critical current density and the length scale on which the current is carried. These results can be applied to thin films and also to single crystals of large aspect ratio, and allow the behaviour of internal “weak-links”, if present, to be studied non-destructively as a function of field and temperature. Magnetisation measurements on a single crystal, a thin film, and a thick film, all of YBa 2 Cu 3 O 7 , are presented. These confirm the validity of the analysis.

‘Strong’ grain boundaries in Ag-BSCCO(2223) tapes

Abstract The remarkable improvement of the superconducting properties of polycrystalline BSCCO(2223) when processed into Ag-clad tapes has been studied magnetically. Two factors contribute to the excellent tape performance: Over the investigated range of field and temperature, the grain boundaries show no evidence of being ‘weak-links’. Also, intra granular flux pinning is much stronger than might be expected from single crystal data, and appears to determine the tape critical current.

Dissipation in granular high-temperature superconductors

The current-voltage characteristics of bulk-sintered YBa2Cu3O7 have been studied in detail. The onset of dissipation indicates that an intergranular critical current can be defined. The magnitudes of the resistance of a single grain boundary and of its critical current are estimated. These estimates lead to values of the rnic(0) product for the intergranular weak-links that are of the order of 100 μV, well within the wide range of rnic(0) product measured on bi-crystal grain boundaries by Dimos et al., but two to three orders of magnitude less than that expected from “classical” models of superconducting weak-links.

High critical current densities in bulk Pb-doped BiSrCaCuO

Abstract Transport critical currents J ct and magnetisations of a range of Pb-doped Bi 2 Sr 2 Ca 2 Cu 3 O x materials have been measured. The best sample has J ct at 77K of 3×10 6 A m −2 in zero field, and shows substantial irreversible magnetisation.

Non-destructive measurement of the critical current and the current carrying length scale in superconducting crystals and films

Abstract In superconductors having a plate-like geometry, valuable information can be obtained from detailed examination of the magnetic hysteresis loop. The length scale Λ on which screening currents flow, and the sheet critical current density K c can be evaluated independently. Because of internal weak-links, Λ may be less than the sample dimension, and will change with field and temperature. Experimental results on crystals and films confirming the validity of the analysis are presented.

A new approach to the magnetic measurement of critical currents in superconducting discs and films

Unlike the situation in zero field, the critical state of a thin superconducting disc at fields large compared with the penetration field has a simple approximate solution. The differential susceptibility as the field is first decreased from its maximum value is a direct measure of the length scale Lambda on which screening currents circulate, so that both Lambda and the critical current density can be measured independently. If weak links are present, this approach allows their evolution with field and temperature to be followed.

Resonance photoemission from single crystalline Bi2Sr2CaCu2O8 at the Cu 3p absorption edge

Abstract Resonance photoemission measurements of the high- T c superconductor Bi 2 Sr 2 CaCu 2 O 8 at the Cu 3p absorption edge are reported. The resonance behaviour of valence states lying between 10 and 14 eV binding energy is similar to that observed from CuO, confirming their d 8 character. A resonance of the valence band states is also observed, although neither an enhancement nor a resonance is observed in Fermi level states. The latter effect is consistent with these final states having little d 8 character.

A photoemission study to confirm the second order nature of anomalous O 2s resonant enhancement of Bi2Sr2CaCu2O8(001) fermi level states

Abstract We have performed a synchrotron-excited angle resolved photoemission study of the valence band region of cleaved Bi 2 Sr 2 CaCu 2 O 8 (001) samples. By recording spectra between 16 and 30eV on two separate beamlines with differing second order light contributions, we have investigated the origin of previously reported “anomalous” O 2s resonant enhancement of Fermi level states, in which the features showing resonant enhancement apparently disperse with photon energy. Our results indicate that anomalous O 2s resonant enhancement may be explained by second order effects.

Transport critical currents, I-V characteristics and magnetisation of YBa2Cu3O7 thick films

YBa2Cu3O7 thick films doped with Ag have improved critical current performance. Magnetisation measurements and the I-V characteristics show that the Ag reduces the normal state resistance of the intergranular weak-links by about 2 orders of magnitude, and also removes their sensitivity to small magnetic fields.

Transport critical currents, I-V characteristics and magnetisation of Ag-sheathed BiSrCaCuO (2223) tapes

The grain-boundary weak-links in pressed Ag-sheathed BiSrCaCuO (2223) tapes have been studied with magnetic and transport techniques. Compared with unsheathed BiSrCaCuO, there is significant grain alignment, and about two orders of magnitude reduction in the weak-link resistance.