Rare-Region Onset of Superconductivity in Granular Systems

TL;DR

This paper provides direct evidence that rare, large grains in granular superconductors initiate superconductivity, revealing how local disorder influences the phase transition and the onset of superconducting order.

Contribution

It demonstrates the role of rare regions in the onset of superconductivity in granular systems through experimental observations and analysis.

Findings

Superconductivity first appears in large grains within the granular islands.

Transition temperature fluctuations increase as grain size decreases.

Superconducting order spreads via proximity coupling from rare regions.

Abstract

The critical behavior of disordered systems-from metals (1) to magnets (2) and superconductors (3)-is often dominated by the behavior of rare regions of a correlated phase, which control the inception and dynamics of the phase transition. Yet, despite significant theoretical (3,4,5) and experimental (6,7,8,9) interest, there has been little direct evidence of the presence of these regions, or of their role in initiating transitions. Here, we provide direct evidence for rare-region effects at the onset of superconductivity in granular superconducting islands. By considering the strong diameter-dependence of the transition, as well as observations of large fluctuations in the transition temperature as island diameters decrease, we are able to show that superconducting order first appears in unusually large grains- i.e. rare regions- within each island and, due to proximity coupling, spreads to other grains. This work thus provides a quantitative, local understanding of the onset of correlated order in strongly disordered systems.