Electron-Phonon Coupling and Symmetry-Breaking in Superconducting Oxide Interfaces Near Ferroelectric Quantum Criticality

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Abstract
The observation of unconventional superconductivity in oxide quantum paraelectrics–materials exhibiting incipient ferroelectricity at low temperatures– raises fundamental questions about the role of ferroelectricity in the pairing mechanism. In oxide interfaces, such as LaAlO3/SrTiO3 (LAO/STO), quantum confinement and inversion symmetry breaking give rise to intriguing electronic phases, with charge doping driving the system toward a ferroelectric quantum critical point. However, direct experimental evidence of coupling between confined electrons and active phonon modes in a polar lattice has remained elusive. Here, we use momentum-selective vibrational electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) to simultaneously probe the atomic structure and phonon spectra across a series of LAO/STO interfaces with varying carrier concentrations and superconducting behavior. We find that the emergence of superconductivity correlates with interfacial lattice polarization and the appearance of new high-frequency, localized phonon modes. These modes, primarily involving in-plane and out-of-plane vibrations of equatorial and apical oxygen atoms in the TiO2 and SrO layers, respectively, are highly sensitive to carrier concentration. Our results provide compelling evidence for the coexistence of inversion symmetry breaking, superconductivity in the dilute limit, and strong electron-phonon coupling mediated by localized phonons, offering new insights into the microscopic pairing mechanism in quantum paraelectric systems.

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