An inhomogeneous Josephson phase in thin-film and High-Tc superconductors

TL;DR

The paper proposes that inhomogeneous superconductors can exhibit a Josephson-coupled zero-resistance state before becoming metallic, explaining experimental observations in thin films and high-Tc materials through a simple coupling constant comparison.

Contribution

It introduces a model where inhomogeneities lead to a Josephson phase in superconductors, providing a new perspective on the metal-insulator transition and related experimental data.

Findings

Inhomogeneous regions can form a Josephson-coupled zero-resistance state.

Magnetic fields can quench the Josephson state, leading to insulating behavior.

Large-grain inhomogeneous models follow Uemura correlations, while small grains behave like dirty metals.

Abstract

In many cases inhomogeneities are known to exist near the metal (or superconductor)-insulator transition, as follows from well-known domain-wall arguments. If the conducting regions are large enough (i.e. when the T=0 superconducting gap is much larger than the single-electron level spacing), and if they have superconducting correlations, it becomes energetically favorable for the system to go into a Josephson-coupled zero-resistance state before (i.e. at higher resistance than) becoming a "real" metal. We show that this is plausible by a simple comparison of the relevant coupling constants. For small grains in the above sense, the electronic grain structure is washed out by delocalization and thus becomes irrelevant. When the proposed "Josephson state" is quenched by a magnetic field, an insulating, rather then a metallic, state should appear. This has been shown to be consistent with the existing data on oxide materials as well as ultra-thin films. We discuss the Uemura correlations versus the Homes law, and derive the former for the large-grain Josephson array (inhomogenous superconductor) model. The small-grain case behaves like a dirty homogenous metal. It should obey the Homes law provided that the system is in the dirty supeconductivity limit. A speculation why that is typically the case for d-wave superconductors is presented.