Mono-and multilayer FeSe thin films grown on SrTiO 3 and BiTiO 3 substrates exhibit a greatly enhanced superconductivity over that found in bulk FeSe. A number of proposals have been advanced for the mechanism of this enhancement. One possibility is the introduction of a cross-interface electronphonon (e-ph) interaction between the FeSe electrons and oxygen phonons in the substrates that is peaked in the forward scattering (small q) direction due to the two-dimensional nature of the interface system. Motivated by this, we explore the consequences of such an interaction on the superconducting state and electronic structure of a two-dimensional system using Migdal-Eliashberg theory. This interaction produces not only deviations from the expectations of conventional phonon-mediated pairing but also replica structures in the spectral function and density of states, as probed by angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and quasi-particle interference imaging. We also discuss the applicability of Migdal-Eliashberg theory for a situation where the e-ph interaction is peaked at small momentum transfer and in the FeSe/STO system.

We report the observation of multiple phonon satellite features in ultra thin superlattices of form nSrIrO3/mSrTiO3 using resonant inelastic x-ray scattering. As the values of n and m vary the energy loss spectra show a systematic evolution in the relative intensity of the phonon satellites. Using a closed-form solution for the RIXS cross section, we extract the variation in the electronphonon coupling strength as a function of n and m. Combined with the negligible carrier doping into the SrTiO3 layers, these results indicate that tuning of the electron-phonon coupling can be effectively decoupled from doping. This work showcases both a feasible method to extract the electron-phonon coupling in superlattices and unveils a potential route for tuning this coupling which is often associated with superconductivity in SrTiO3-based systems.Despite the discovery of several new classes of superconductors, a comprehensive understanding of superconductivity continues to evade the community, preventing attempts to systamtically control its behavior. The discovery of superconductivity at the interface of two insulating compounds, SrTiO 3 (STO) and LaAlO 3 , is particularly promising for expanding our understanding of superconductivity due to the myriad of control parameters introduced by the heterostructure morphology [1][2][3][4]. Furthermore, superconductivity in monolayer FeSe was recently found to be remarkably enhanced by an order of magnitude when interfaced with STO [5-7]. These findings point to heterostructuring as a promising route towards the rational engineering of the superconducting ground state.While debate remains, the coupling of the conduction electrons to the longitudinal optical (LO 4 ) phonon branch is routinely regarded as an essential ingredient in STO-based superconductors [8][9][10][11][12][13]. Recent angleresolved photoemission spectroscopy (ARPES) experiments observed the systematic tuning of the electron- * dmeyers@bnl.gov † tberlijn@gmail.com ‡

We study the temperature-filling phase diagram of the single-band Holstein model in two dimensions using the self-consistent Migdal approximation, where both the electron and phonon selfenergies are treated on an equal footing. By employing an efficient numerical algorithm utilizing fast Fourier transforms to evaluate momentum and Matsubara frequency summations, we determine the charge-density-wave (CDW) and superconducting transition temperatures in the thermodynamic limit using lattice sizes that are sufficient to eliminate significant finite size effects present at lower temperatures. We obtain the temperature-filling phase diagrams for a range of coupling strengths and phonon frequencies for the model defined on a square lattice with and without next-nearest neighbor hopping. We find the appearance of a superconducting dome with a critical temperature that decreases before reaching the qmax = ( , ) CDW phase boundary. For very low phonon frequencies, we also find an incommensurate CDW phase with the ordering vector qmax ≈ ( , ) appearing between the commensurate CDW and superconducting phases. Our numerical implementation can be easily extended to treat momentum-dependent electron-phonon coupling, as well as dispersive phonon branches, and has been made available to the public. arXiv:1811.03676v1 [cond-mat.supr-con]

Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial phonons in an iron-based superconductor Sr_{2}VO_{3}FeAs (T_{c}≈33  K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in the VO_{2} layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.