Proximity dc squids in the long-junction limit

Abstract

We report the design and measurement of superconducting/normal/superconducting (SNS) proximity dc squids in the long-junction limit, i.e., superconducting loops interrupted by two normal metal wires, which are roughly a micrometer long. Thanks to the clean interface between the metals, a large supercurrent flows through the device at low temperature. The dc squidlike geometry leads to an almost complete periodic modulation of the critical current through the device by a magnetic flux, with a flux periodicity of a flux quantum $h/2e$ through the SNS loop. In addition, we examine the entire field dependence, notably the low and high field dependences of the maximum switching current. In contrast to the well-known Fraunhofer-type oscillations typical of wide junctions, we find a monotonous Gaussian extinction of the critical current at high field. As shown by Cuevas and co-workers [Phys. Rev. B 76, 064514 (2007); Phys. Rev. Lett. 99, 217002 (2007)], this monotonous dependence is typical of long and narrow diffusive junctions. We also find in some cases a puzzling reentrance at low field. In contrast, the temperature dependence of the critical current is well described by the proximity effect theory, as found by Dubos et al. [Phys. Rev. B 63, 064502 (2001); Phys. Rev. Lett. 87, 206801 (2001)] on SNS wires in the long-junction limit. The switching current distributions and hysteretic $IV$ curves also suggest interesting dynamics of long SNS junctions with an important role played by the diffusion time across the junction.