Approaching Zero-Temperature Metallic States in Mesoscopic Superconductor-Normal-Superconductor Arrays

TL;DR

This paper investigates how arrays of superconducting islands on normal metal films can approach zero-temperature metallic states, revealing complex transition behaviors and the influence of island spacing and coupling.

Contribution

It provides experimental characterization and a phenomenological model explaining the emergence of zero-temperature metallic states in mesoscopic superconductor-normal-superconductor arrays.

Findings

Two distinct superconducting transitions observed.

Transition temperatures strongly suppressed with increased island spacing.

Evidence suggests the system approaches zero-temperature metallic states.

Abstract

Systems of superconducting islands placed on normal metal films offer tunable realizations of two-dimensional (2D) superconductivity; they can thus elucidate open questions regarding the nature of 2D superconductors and competing states. In particular, island systems have been predicted to exhibit zero-temperature metallic states. Although evidence exists for such metallic states in some 2D systems, their character is not well understood: the conventional theory of metals cannot explain them, and their properties are difficult to tune. Here, we characterize the superconducting transitions in mesoscopic island-array systems as a function of island thickness and spacing. We observe two transitions in the progression to superconductivity; both transition temperatures exhibit unexpectedly strong depression for widely spaced islands. These depressions are consistent with the system approaching zero-temperature metallic states. The nature of the transitions and the state between them is explained using a phenomenological model involving the stabilization of superconductivity on each island via a weak coupling to and feedback from its neighbors.