Electron-phonon coupling and symmetry breaking in superconducting oxide interfaces near ferroelectric quantum criticality.

The origin of superconductivity in oxide interfaces and its relation to ferroelectricity remains an open question. At LaAlO3/SrTiO3 interfaces, quantum confinement and inversion symmetry breaking create a two-dimensional electron gas near a ferroelectric quantum critical point, yet direct evidence linking phonon dynamics to electron pairing has been lacking. Here we directly probe lattice vibrations and atomic structure at LaAlO3/SrTiO3 interfaces across the superconducting phase diagram using vibrational spectroscopy with momentum selectivity in a scanning transmission electron microscope. We find that superconductivity across the doping series correlates with inversion symmetry breaking and the appearance of high-frequency localized phonons. These tunable, polar vibrations-confined near the interface-exhibit strong electron-phonon coupling and evolve systematically with carrier density. Our findings establish a link between lattice instability, superconductivity and strong electron-phonon coupling mediated by tunable localized phonons, providing new insights into possible microscopic pairing pathways in quantum paraelectric systems.