Persistent Interfacial High-T_c Superconductivity in Multi-layer FeSe/SrTiO₃ Films

The interface between FeSe and SrTiO3 has attracted considerable attention owing primarily to the large enhancement of Tc (70 K vs 8 K in bulk). Most studies to date have utilized surface-sensitive spectroscopic measurements, necessitating the use of monolayer films to allow access to the interfacial superconductivity. The effect of additional FeSe layers on top of the initial monolayer remains an important but open question for understanding the nature of the Tc enhancement. As the air sensitivity of FeSe thin films precludes traditional ex situ transport measurements without potentially detrimental capping layers, we utilize an in situ four-point electrical contact probe to measure superconductivity at the buried FeSe/STO interface for films ranging in thickness from 1-20uc. We show that the Tc of the interfacial monolayer is robust and largely unaffected by additional FeSe layers, and that the earlier reported reduction of Tc in thicker films can be explained by a simple parallel conduction channel model which reproduces the features of our observed thickness dependence. This demonstrates that the contribution of additional FeSe layers does not affect superconductivity at the buried interface, placing important constraints on the mechanism of the Tc enhancement.