Experiments concerning the connective nature of superconductivity in YBa2Cu3O7

Abstract

Samples of YBa₂Cu₃O₇ have been prepared with rather sharp inductive transitions having in the best cases breadths of 7 K and midpoint T_c values of 88 K. The resistive T_c midpoints are 92–95 K with transition widths of ±1–2 K. Flux shielding at 4.2 K is normally 100% and flux expulsion at 4.2 K reaches 95%. However, even small fields of order 1 mT decouple some 15%–20% of the volume, allowing flux to enter the samples. Resistive Hc₂ measurements suggest that Hc₂(0) varies from 300 T, depending on the criterion chosen. ac susceptibility measurements suggest that Hc₂(0) is ∼60 T. Magnetization current densities are relatively high (150–200 A/mm² at 1–10 T at 4.2 K) but measured transport current densities are small (≤1 A/mm²). Magnetization current densities at 77 K are about two orders of magnitude lower. The samples were seen to be heavily twinned by light microscopy (scale of 1–5 μm) and by transmission electron microscopy (scale of ∼250 nm). It is concluded that these measurements are consistent with a model of superconducting regions of reduced dimensionality coupled by tunneling.