Superconducting nanofilms: molecule-like pairing induced by quantum confinement

TL;DR

This paper explores how quantum confinement in superconducting nanofilms induces a molecule-like pairing regime, significantly altering the nature of superconductivity and leading to a mixture of traditional and molecule-like Cooper pairs.

Contribution

It demonstrates the transition to molecule-like pairing in nanofilms near the Fermi level, revealing a crossover similar to BCS-BEC in ultracold fermions, driven by quantum confinement effects.

Findings

Up to half of Cooper pairs shrink to nanometer scale in Pb nanofilms.

Superconducting condensate becomes a mixture of extended and molecule-like pairs.

Quantum confinement causes significant variation in subband energies affecting pairing.

Abstract

Quantum confinement of the perpendicular motion of electrons in single-crystalline metallic superconducting nanofilms splits the conduction band into a series of single-electron subbands. A distinctive feature of such a nanoscale multi-band superconductor is that the energetic position of each subband can vary significantly with changing nanofilm thickness, substrate material, protection cover and other details of the fabrication process. It can occur that the bottom of one of the available subbands is situated in the vicinity of the Fermi level. We demonstrate that the character of the superconducting pairing in such a subband changes dramatically and exhibits a clear molecule-like trend, which is very similar to the well-known crossover from the Bardeen-Cooper-Schrieffer regime to Bose-Einstein condensation (BCS-BEC) observed in trapped ultracold fermions. For Pb nanofilms with thickness of 4 and 5 monolayers (ML) this will lead to a spectacular scenario: up to half of all the Cooper pairs nearly collapse, shrinking in the lateral size (parallel to the nanofilm) down to a few nanometers. As a result, the superconducting condensate will be a coherent mixture of almost molecule-like fermionic pairs with ordinary, extended Cooper pairs.