Scale-dependent competing interactions: sign reversal of the average persistent current

TL;DR

This paper investigates how competing interactions in a nanoscale superconducting ring can cause a reversal in the sign of the persistent current, highlighting the importance of size-dependent effects on magnetic response.

Contribution

It introduces a model analyzing the interplay of Coulomb and Fröhlich interactions affecting persistent currents in nanoscale superconductors, revealing conditions for sign reversal.

Findings

Diamagnetic response can become paramagnetic in small superconductors.

Sign reversal depends on the ratio of Thouless energy to Debye energy.

Finite-size effects critically influence magnetic properties.

Abstract

The interaction-induced orbital magnetic response of a nanoscale system, modeled by the persistent current in a ring geometry, is evaluated for a system which is a superconductor in the bulk. The interplay of the renormalized Coulomb and Fr\"{o}hlich interactions is crucial. The diamagnetic response of the large superconductor may become paramagnetic when the finite-size-determined Thouless energy is larger than or on the order of the Debye energy.