Proximity-coupled Josephson Junctions in HTS Materials
Bruce A. Davidson, Istituto di Cibernetica, CNR, via Toiano 6, I-80072 Arco Felice, Italy, Phone: +39-81-853-4142, Fax: +39-81-526-7654, email: email@example.com, WWW: http://fisica.cib.na.cnr.it
To date, two different mechanisms have been clearly identified that explain the Josephson effect in the high-temperature superconductors (HTS). These are 1) resonant tunneling, as observed in grain-boundary junctions and epitaxial edge junctions; and 2) proximity coupling, in which a normal metal becomes superconducting when sandwiched between two superconductors. Junctions fabricated by the electron-beam scribing technique are perhaps the best example of uniform, proximity-coupled HTS junctions in the literature, displaying characteristics that are explained well by conventional SNS theory without modification for the unusual symmetry of the order parameter in HTS materials. It is precisely this close fit to conventional theory that allows quantification of the interlayer's microscopic properties such as normal coherence length, quasiparticle mean-free path, and resistivity. Specifically, the interlayer exhibits properties of a dirty metal near the metal-insulator transition, with a Fermi surface area reduced an order of magnitude from the unirradiated film. Presence of an "excess" critical current and hysteresis at low temperatures provides evidence that the low-temperature dynamic properties of these junctions are dominated by nonequilibrium processes.
Given this situation, two general questions arise. First, what can be said about the usefulness of proximity coupling to study essential physical properties of HTS materials, such as d-wave symmetry or microscopic transport mechanisms underlying the Josephson effect in HTS junctions? Furthermore, can limits be determined for the device performance of HTS junctions based on the proximity effect? We show that the e-beam junctions described here allow the possibility to experimentally explore answers to both these questions.