STATIONARY PROPERTIES OF HIGH-CRITICAL-TEMPERATURE PROXIMITY EFFECT JOSEPHSON-JUNCTIONS

The discovery of superconductors with high critical temperatures (T-c) has led to a considerable effort to fabricate Josephson junctions ope rating at temperatures approaching, or even exceeding, 77 K for both s cientific investigations and potential applications. Superconductor-no rmal-superconductor (SNS) devices, with noble or oxide metals as norma l interlayers, are perhaps the most widely explored high-T-c junction type at present. Although demonstrations of individual high-T-c SNS de vices exhibiting excellent current-voltage characteristics, high criti cal current-resistance products, and low noise behaviour have been mad e, reproducible devices suitable for electronic applications are elusi ve. It is therefore important to ask how well these nominally SNS high -T-c junctions are understood. We review the available data, with emph asis on junction critical currents, and conclude that there is little evidence supporting a conventional proximity effect interpretation in the majority of reported high-T-c devices. The strongest candidates fo r SNS behaviour are junctions in which N is a superconductor above its transition temperature. We discuss the present experimental and theor etical understanding of SNS junctions with emphasis on the implication s for future research and development of these devices.