Superconducting nanofilms: molecule-like pairing induced by quantum confinement

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.

7 pages, 3 figures

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• Enhanced critical temperature, pairing fluctuation effects, and BCS-BEC crossover in a two-band Fermi gas
• Suppression of fluctuations in a two-band superconductor with a quasi-1D band
• Spin-orbit coupling controlling the superconducting dome of artificial superlattices of quantum wells
• Mechanism of screening or enhancing the pseudogap throughout the two-band Bardeen-Cooper-Schrieffer to Bose-Einstein condensate crossover
• High-Tc superconducting dome in artificial heterostructures made of nanoscale quantum building blocks
• Multigap superconductivity and interaction driven resonances in superconducting nanofilms with an inner potential barrier
• Tunable BCS-BEC crossover, reentrant, and hidden quantum phase transitions in two-band superconductors with tunable valence and conduction bands
• High-$T_c$ Berezinskii-Kosterlitz-Thouless transition in 2D superconducting systems with coupled deep and quasi-flat electronic bands with van Hove singularities
• Multifaceted impact of a surface step on superconductivity in atomically thin films
• Upper critical magnetic field and multiband superconductivity in artificial high-Tc superlattices of nano quantum wells
• Dimensional crossover in ultracold Fermi gases from Functional Renormalisation
• Periodicity of superconducting shape resonances in thin films