Superconducting Fluctuations and Pairing Enhancement in Ultra-Thin FeSe/SrTiO3

A central challenge in understanding the Tc enhancement mechanism in FeSe/SrTiO3 monolayer films has been to decouple the myriad effects that the underlying substrate imposes on the superconductivity. Alkali surface-dosed multilayer films provide a natural comparison where the heavily electron-doped superconducting layer is constrained to the film-vacuum interface, analogous to the monolayer FeSe/STO interface but lacking any STO phonon contribution. To better understand the influence of the STO phonon contribution, we systematically explore the evolution of superconductivity as measured unambiguously by in situ electrical resistivity under varied conditions of film thickness, surface doping concentration, and substrate interface condition. In contrast to observations from spectroscopic probes which indicate a substantial enhancement in the pairing Tc, we observe only modest discrepancies in the zero-resistance temperature for surface doped layers in comparison to monolayer films. We demonstrate that this behavior is due to the shared influence of 2D fluctuation effects which act to suppress the resistive transition well below the cooper pair formation temperature, and discuss the implications of our results on the broader understanding of the FeSe/STO enhancement phenomenology.