The discovery of superconductors with high critical temperatures (T c) 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-T c junction type at present. Although demonstrations of individual high-T c 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-T c 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-T c 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.
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