Understanding and enhancing superconductivity in FeSe/STO by quantum size effects

Abstract

Superconductivity in one-atom-layer iron selenide (FeSe) on a strontium titanate (STO) substrate is enhanced by almost an order of magnitude with respect to bulk FeSe. There is recent experimental evidence suggesting that this enhancement persists in FeSe/STO nano-islands. More specifically, for sizes $L\sim 10$ nm, the superconducting gap is a highly non-monotonic function of $L$ with peaks well above the bulk gap value. This is the expected behavior only for weakly-coupled metallic superconductors such as Al or Sn. Here we develop a theoretical formalism to describe these experiments based on three ingredients: Eliashberg theory of superconductivity in the weak coupling limit, pairing dominated by forward scattering and periodic orbit theory to model spectral fluctuations. We obtain an explicit analytical expression for the size dependence of the gap that describes quantitatively the experimental results with no free parameters. This is a strong suggestion that superconductivity in FeSe/STO is mediated by STO phonons. We propose that, since FeSe/STO is still a weakly coupled superconductor, quantum size effects can be used to further enhance the bulk critical temperature in this interface.

20 pages, 2 figures, added references and corrected typos

References in corpus (7)

• Superconductivity in single-layer films of FeSe with a transition temperature above 100 K
• Observation of shell effects in superconducting nanoparticles of Sn
• Size dependence of the Tc and the superconducting energy gap in nanocrystalline thin films of Nb
• The gap amplification at a "shape resonance" in a superlattice of quantum stripes: a mechanism for high Tc
• Origin of the large phonon band-gap in SrTiO3 and the vibrational signatures of ferroelectricity in ATiO3 perovskite: First principles lattice dynamics and inelastic neutron scattering of PbTiO3, BaTiO3 and SrTiO3
• Shell Structure and Strengthening of Superconducting Pair Correlation in Nanoclusters
• Number theory, periodic orbits and superconductivity in nano-cubes

Cited by in corpus (1)

• The electron-phonon interaction with forward scattering peak is a relevant approach to high Tc superconductivity in FeSe films on SrTiO3 and TiO2