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

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.