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, protective 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 thicknesses of 4 and 5 monolayers (MLs) 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.

Substrate-Induced Proximity Effect in Superconducting Niobium Nanofilms

“…We have explored the important role played by the substrate on the fundamental properties of the Nb nanofilms. Our study establishes the existence of an interplay between quantum size and proximity effects at the substrate interface [13,14].…”

Atomic-scale epitaxial aluminum film on GaAs substrate

“…1 With film thickness reduced from the nanometer scale to the atomic scale, a metal exhibits intriguing properties, due to so-called quantum size effect, and opens a window for fundamental science exploration and novel device development. [8][9][10][11][12] Unlike that in semiconductors, electronic quantum size effect in metals has been much more difficult to be observed because of its extremely short Fermi wavelength (< 1 nm). Driven by the huge potential of the quantized electronic/metallic states for various applications, searching for a practical and reliable method for preparing an atomic-scale metal film has been a long-standing challenge for material scientists and engineers.…”

Screening of pair fluctuations in superconductors with coupled shallow and deep bands: A route to higher-temperature superconductivity

“…Multi-band superconductivity can be also achieved in artificial inhomogeneous structures made of a single-band superconducting material -nanofilms, nanostripes or samples with spatially controlled impurity distributions [9][10][11][12].The phenomenon entangled with the multi-band superconductivity, important in this work, is the BCS-BEC crossover [13][14][15][16]. Proximity to this crossover in multiband materials with deep and shallow bands can give rise to a notable increase of superconducting gaps [17][18][19][20], which on the mean-field level leads to higher T c . The physical reason is the depletion of the Fermi motion in a shallow band, which yields short-sized pairs.…”

“…In such materials the superconducting state is a coherent mixture of a BCS condensate in deep bands and BCS-BEC-crossover or even nearly BEC condensate in shallow bands. This takes place in, e.g., in MgB 2 [19], many of iron-based superconductors [8,[21][22][23][24] and in nanoscale samples [17,18].However, the largest enemy of the high-T c superconductivity in such materials is superconducting fluctuations. They are significant for the same reason which leads to a higher T c -the depletion of the carrier motion in a shallow band that is associated with a low super-conducting stiffness.…”