Shape-resonant superconductivity in nanofilms: from weak to strong coupling

TL;DR

This paper investigates how shape resonances influence superconductivity in nanofilms across weak to strong coupling regimes, revealing conditions for enhanced and uniform multigap superconducting states.

Contribution

It provides a comprehensive analysis of shape resonances in superconducting nanofilms considering variable coupling strengths and predicts the emergence of robust, uniform multigap superconductivity at resonance.

Findings

Shape resonances are most pronounced in weakly coupled superconductors.

Strong coupling leads to mixing of subbands, smoothing out shape resonances.

Superconducting gap becomes spatially uniform at resonance.

Abstract

Ultrathin superconductors of different materials are becoming a powerful platform to find mechanisms for enhancement of superconductivity, exploiting shape resonances in different superconducting properties. Here we evaluate the superconducting gap and its spatial profile, the multiple gap components, and the chemical potential, of generic superconducting nanofilms, considering the pairing attraction and its energy scale as tunable parameters, from weak to strong coupling, at fixed electron density. Superconducting properties are evaluated at mean field level as a function of the thickness of the nanofilm, in order to characterize the shape resonances in the superconducting gap. We find that the most pronounced shape resonances are generated for weakly coupled superconductors, while approaching the strong coupling regime the shape resonances are rounded by a mixing of the subbands due to the large energy gaps extending over large energy scales. Finally, we find that the spatial profile, transverse to the nanofilm, of the superconducting gap acquires a flat behavior in the shape resonance region, indicating that a robust and uniform multigap superconducting state can arise at resonance.