Decoupling Interfacial Contributions to the Enhanced Superconductivity in FeSe/SrTiO₃

The discovery of greatly enhanced superconductivity at the interface between FeSe and SrTiO₃ has attracted enormous interest due to the potential of enhancing superconductivity through interfacial interactions. In our previous work, we confirmed the presence of intrinsic interfacial coupling of electrons in the FeSe layer to optical phonons in the adjacent SrTiO₃ substrate [1], but a clear causative connection between this phonon coupling and the apparent enhanced T_c enhancement remains unestablished. In order to better understand the true contribution of interface phonon coupling on the resultant high-T_c state, we consider a highly analogous system: alkali surface-doped FeSe films. This approach produces a controllably electron doped superconducting layer constrained to the film-vacuum interface, analogous to the FeSe/STO interface but lacking any phonon coupling effect. Here, using a simultaneous combination of in situ electrical resistivity and ARPES, we systematically explore the evolution of superconductivity as a function of surface doping concentration at this coupling-free interface. Interestingly, we observe only modest discrepancies in the superconducting T_c and high-temperature pseudogap signatures of optimally surface-doped layers in comparison to interfacial FeSe/STO films. We discuss the implications of this behavior on the broader understanding of FeSe/STO phenomenology.

[1] B.D. Faeth et al. Phys. Rev. Lett. 127, 016803 (2021)