Electrical transport properties of epitaxial and granular oriented YBaℓCuℓOℓ[̲delta] thin films

Author

Edwin C. Jones

Date of Award

12-1992

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

James R. Thompson

Committee Members

David Christen, Gerald Mahan, Ollie Thomson, Philip Schaefer

Abstract

Strong correlations between the Hall coefficient R_H, the transition temperature T_c, and the critical current density J_c were established in a series of epitaxial YBa₂Cu₃O_7-δ thin films as a function of oxygen deficiency δ. Steady increases in R_H with δ suggests that deoxygenation reduces the density of states which, according to BCS theory, should lead to corresponding decreases in T_c In contrast, two well known plateaus occurring at 90K and 60K were observed in T_c, vs. δ. Others have ascribed these plateaus to either electronic phenomena or oxygen clustering. We find that in the 90K plateau, the critical current density J_c(δ,H=0) decreases with δ and extrapolates toward zero at the edge of the plateau, while the relative field dependence of J_c(δ,H) is independent of δ. Furthermore, a fluctuation analysis of the resistive transitions indicates a constant upper critical field B_c2(0) = 110T across this plateau. These observations suggest that the oxygen clustering/percolation scenario occurs on the 90K plateau. Moreover, computer simulations showed this oxygen clustering/percolation picture to be a plausible explanation for the occasional observation of a sign reversal of R_H near T_c. For large oxygen deficiencies (δ > 0.5) and for the granular oriented YBa₂Cu₃O_7-δ thin films, rapid decreases in J_c with applied field were observed which is reminiscent of the conventional granular alloys. In addition, the self-field critical current densities J_c behaved as SNS weak link systems in a Josephson mixed state. In sum, due to the short coherence length ξ in these materials, many properties formerly believed to be "intrinsic" in nature are apparently "extrinsic" in nature.

Recommended Citation

Jones, Edwin C., "Electrical transport properties of epitaxial and granular oriented YBaℓCuℓOℓ[̲delta] thin films. " PhD diss., University of Tennessee, 1992.
https://trace.tennessee.edu/utk_graddiss/10928