Superconductivity of the FeSe/SrTiO3 Interface in View of BCS–BEC Crossover**Supported by the National Key R&D Program of China under Grant Nos 2017YFA0303600, 2016YFA0300600 and 2016YFA0202300, the National Natural Science Foundation of China under Grant No 11634016, the Strategic Priority Research Program (B) of Chinese Academy of Sciences under Grant No XDB07030100, the Research Program of Beijing Academy of Quantum Information

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 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.