BCS-BEC Crossover of the FeSe/SrTiO₃ Interface

Superconductivity can be described either in the Bardeen-Cooper-Schrieffer (BCS) limit or in the Bose-Einstein condensation (BEC) limit. Unlike the superconductivity in the BCS limit where the involved electron-boson interaction (EBI) is adiabatic, in the BEC limit, the low superfluid density and short coherence length are expected in the non-adiabatic EBI picture. Our previous studies have shown that the EBI in FeSe/SrTiO₃ systems exhibits an indispensable non-adiabatic nature, which leads to the superconductivity beyond the BCS limit. Here, through the analysis of BCS-BEC phase diagram, we find the superconductivity of single-layer FeSe on SrTiO₃ substrate locates close to the BCS-BEC crossover unitary. Moreover, the superconductivity can be tuned toward to the BEC limit by surface hole doping. Indeed the potential pseudogap was observed in our scanning tunneling spectroscopy study. These findings provide a rare platform to explore the many-body interactions in superconductors.