Role of optical phonons for normal and superconducting state properties of KTO*

Emergent superconductivity at the LaAlO$_3$/KTaO$_3$ interfaces exhibits a mysterious dependence on the KTaO$_3$ crystallographic orientations. Here by soft X-ray angle-resolved photoemission spectroscopy, we directly resolve the electronic structure of the LaAlO$_3$/KTaO$_3$ interfacial superconductors and the non-superconducting counterpart. We find that the mobile electrons that contribute to the interfacial superconductivity show strong $k_perp$ dispersion. Comparing the superconducting and non-superconducting interfaces, the quasi-three-dimensional electron gas with over 5.5~nm spatial distribution ubiquitously exists, while the main difference lies on the signature of electron-phonon coupling, which shows intriguingly different strengths depending on the interfacial orientations. Remarkably, the stronger electron-phonon coupling correlates with the higher superconducting transition temperature. Our observations offer a straightforward explanation for the orientation-dependent superconductivity and suggest that interfacial orientations can affect electron-phonon coupling strength over several nanometers, which may have profound implications for the applications of oxide interfaces in general.