Superconductivity of the FeSe/SrTiO3 Interface in the View of BCS-BEC Crossover

TL;DR

This paper investigates the superconductivity at the FeSe/SrTiO3 interface within the BCS-BEC crossover framework, revealing how doping and substrate effects tune the pairing interactions and crossover position, with implications for high-temperature superconductivity.

Contribution

It extends the BCS-BEC crossover concept to FeSe-based superconductors and demonstrates how interface engineering and doping modulate their pairing regime.

Findings

FeSe/SrTiO3 interface is closer to the BCS-BEC crossover unitary point.

Charge doping shifts the superconductor's position in the crossover regime.

Hole doping can induce pre-paired electrons, enhancing tunability.

Abstract

In paired Fermi systems, strong many-body effects exhibit in the crossover regime between the Bardeen-Cooper-Schrieffer (BCS) and the Bose-Einstein condensation (BEC) limits. The concept of the BCS-BEC crossover, which is studied intensively in the research field of cold atoms, has been extended to condensed matters. Here, by analyzing the typical superconductors within the BCS-BEC phase diagram, we find that FeSe-based superconductors are prone to shift their positions in the BCS-BEC crossover regime by charge doping or substrate substitution, since their Fermi energies and the superconducting gap sizes are comparable. Especially at the interface of a single-layer FeSe on SrTiO3 substrate, the superconductivity is relocated closer to the crossover unitary than other doped FeSe-based materials, indicating that the pairing interaction is effectively modulated. We further show that hole-doping can drive the interfacial system into the phase with possible pre-paired electrons, demonstrating its flexible tunability within the BCS-BEC crossover regime.