Properties of n × n square arrays of proximity effect bridges

Abstract

We have studied the I-V characteristics, the dc resistance at the origin R(T), and the critical current I c (T) of arrays consisting of n × n (n = 10, 20, 30, and 40) In squares separated by Au/In regions. The superconducting squares have side d s (In) from 7.5 to 15 Μm and d N (Au/In) from 2.5 to 11 Μm. These devices show two transition temperatures in their R(T) curve; one at T cG near T c (In) and another one at T c * above T c (Au/In). The I-V curves for each temperature region resemble recent published results on NbN granular films. These arrays follow a formalism proposed by Wolf, Gubser, and Imry to describe the transition to a resistanceless state at T c *. The measured values of I c (T) are larger than expected from n noninteracting Bardeen-Johnson junctions in parallel, although the exponential behavior in temperature is followed. They are compatible with the idea of bands in the quasiparticle spectrum in the Au/In regions. Deviations in I c (T) from an exponential behavior in T near T c * have been analyzed by applying to SNS arrays ideas developed for phase transitions in SIS arrays. Evidence that at T c * we may be observing a phase transition at T cJ due to the Josephson coupling between In squares exists but it needs the development of theoretical work to be put on more solid grounds.