Pseudogap Behavior and Interfacial Electron-Phonon Coupling in Single-Layer FeSe/SrTiO₃

Single-layer FeSe grown on SrTiO₃ (FeSe/SrTiO₃) has attracted interest due to its unique characteristics as an atomically thin, interfacially enhanced high-T_c superconductor, exhibiting a spectroscopic gap-opening temperature nearly one order of magnitude higher than that of its bulk counterpart.  A better understanding of this enhanced superconducting state holds great potential for opening up a new frontier for high-temperature superconductivity through engineering atomic interfaces.  In this talk, I will discuss a series of experiments aimed at unveiling the nature and origin of the enhanced superconducting state of FeSe/SrTiO₃.  Using a first-ever combination of ARPES and in situ resistivity measurements, we reveal a striking dichotomy between the spectroscopic and transport properties of monolayer FeSe/SrTiO₃ - while spectroscopic measurements indicate the initial formation of a superconducting gap at temperatures as high as 70 K, a true zero-resistance state is not achieved until below 30 K. We show that this discrepancy is due to an unprecedentedly large pseudogap regime not previously observed in iron-based superconductors, but arising here from the intrinsic 2D nature of the system [1].  Additionally, we consider the potential influence of interfacial coupling between FeSe electrons and substrate phonons, widely believed to contribute to the enhanced T_c effect.  We perform a detailed analysis of the energy splittings and photon energy-dependent intensities of replica bands observed by ARPES in comparison to existing theoretical calculations, allowing us to confirm the origin of the observed replica bands as signatures of intrinsic electron-boson coupling and to directly quantify the strength of the coupling constant [2].