Proximity DC squids in the long junction limit

摘要

We report the design and measurement ofSuperconducting/normal/superconducting (SNS) proximity DC squids in the longjunction limit, i.e. superconducting loops interrupted by two normal metalwires roughly a micrometer long. Thanks to the clean interface between themetals, at low temperature a large supercurrent flows through the device. Thedc squid-like geometry leads to an almost complete periodic modulation of thecritical current through the device by a magnetic flux, with a flux periodicityof a flux quantum h/2e through the SNS loop. In addition, we examine the entirefield dependence, notably the low and high field dependence of the maximumswitching current. In contrast with the well-known Fraunhoffer-typeoscillations typical of short wide junctions, we find a monotonous gaussianextinction of the critical current at high field. As shown in [15], thismonotonous dependence is typical of long and narrow diffusive junctions. Wealso find in some cases a puzzling reentrance at low field. In contrast, thetemperature dependence of the critical current is well described by theproximity effect theory, as found by Dubos {\it et al.} [16] on SNS wires inthe long junction limit. The switching current distributions and hysteretic IVcurves also suggest interesting dynamics of long SNS junctions with animportant role played by the diffusion time across the junction.