Stationary properties of high-critical-temperature proximity effect Josephson junctions

The discovery of superconductors with high critical temperatures (Tc) has led to a considerable effort to fabricate Josephson junctions operating at temperatures approaching, or even exceeding, 77 K for both scientific investigations and potential applications. Superconductor-normal-superconductor (SNS) devices, with noble or oxide metals as normal interlayers, are perhaps the most widely explored high-Tc junction type at present. Although demonstrations of individual high-Tc SNS devices exhibiting excellent current-voltage characteristics, high critical current-resistance products, and low noise behaviour have been made, reproducible devices suitable for electronic applications are elusive. It is therefore important to ask how well these nominally SNS high-Tc junctions are understood. We review the available data, with emphasis on junction critical currents, and conclude that there is little evidence supporting a conventional proximity effect interpretation in the majority of reported high-Tc devices. The strongest candidates for SNS behaviour are junctions in which N is a superconductor above its transition temperature. We discuss the present experimental and theoretical understanding of SNS junctions with emphasis on the implications for future research and development of these devices.