Decoupling Contributions to the Superconducting Enhancement Mechanism in Epitaxial FeSe Monolayers

The discovery of greatly increased superconducting Tc in FeSe/SrTiO3 monolayer films (>60 K vs 8 K in bulk) has drawn enormous interest to the prospect of interfacial high-Tc materials. The presence of the STO substrate has been shown to induce heavy electron doping and lattice strain on the adjacent FeSe layer, and is widely proposed to contribute additional enhancement via interfacial phonon coupling. In order to effectively decouple the contributions of these effects to the resultant high-Tc state, we systematically explore the evolution of superconductivity as measured unambiguously by in situ electrical resistivity under varied conditions of strain, surface doping concentration, and substrate interface condition. We show that the potassium surface dosing produces a heavily electron doped superconducting layer constrained to the film-vacuum interface, analogous to the monolayer FeSe/STO interface but lacking any STO phonon contribution. In contrast to observations from spectroscopic probes, we observe only modest discrepancies in the zero-resistance Tc for surface doped layers in comparison to monolayer films. We discuss possible explanations for this discrepancy and the implications of these results on the broader understanding of the FeSe/STO enhancement phenomenology.