Approaching zero-temperature metallic states in mesoscopic superconductor–normal–superconductor arrays

By S. Eley, S. Gopalakrishnan, P. Goldbart, N. Mason. Nature Physics, 2011

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It has long been debated whether it is possible to approach a zero-temperature metallic state in a two-dimensional system. A study of the electrical characteristics of arrays of superconducting islands of varying thickness and spacing on a normal metal film suggests it is. Systems of superconducting islands placed on normal metal films offer tunable realizations of two-dimensional (2D) superconductivity1,2; 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 states3,4,5. Although evidence exists for such metallic states in some 2D systems6,7, their character is not well understood: the conventional theory of metals cannot explain them8, and their properties are difficult to tune7,9. 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, consistent with the system approaching zero-temperature (T=0) metallic states. In particular, the first transition temperature seems to linearly approach T=0 for finite island spacing. The nature of the transitions is explained using a phenomenological model involving the stabilization of superconductivity on each island via a coupling to its neighbours.

Subfields: Atomic and Molecular Physics, and Optics, Condensed Matter Physics

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