Superconductivity in dilute system of sites with strong electron-electron attraction

TL;DR

This paper investigates how strong electron-electron attraction in dilute negative U-centers can induce superconductivity, highlighting the formation of local Cooper pairs and the conditions for global coherence.

Contribution

It introduces a model for superconductivity mediated by negative U-centers in dilute systems, emphasizing the role of Andreev scattering in establishing global coherence.

Findings

Local Cooper pairs form when attraction exceeds a threshold

Superconducting transition temperature depends on attraction strength

Second critical magnetic field varies with attraction strength

Abstract

We consider the effect of strong electron-electron attraction on superconductivity in the dilute system of the negative U-centers. They couple to the conducting electronic states and mediate attraction between electrons. We predict the formation of the local Cooper pairs provided attraction exceeds a certain threshold value. The global coherent superconducting state appears only due to the Andreev scattering between the centers at low temperatures. The dependence of the superconducting transition temperature and the second critical magnetic field on attraction strength is calculated.