Oxygen-Stoichiometry-Driven Crossover from Polaronic Insulator to Superconductor in Orthorhombic Ti₉O₁₀ Films

The Bardeen-Cooper-Schrieffer theory of superconductivity is restricted to weak electron-phonon coupling. In the strong-coupling regime, charge carriers self-trap via local lattice distortions, forming polarons whose condensation provides an alternative mechanism for superconductivity. Titanium oxides serve as a model system for polaron physics, suggesting the possibility of polaronic superconductivity. Here we report Oxygen-Stoichiometry-Driven Crossover from Polaronic Insulator to Superconductor in epitaxial Ti₉O₁₀ thin films. Temperature-dependent Raman spectroscopy supports the films as a potential polaronic insulator, in which oxygen stoichiometry control reveals an anticorrelation between the polaronic insulating state and superconductivity. Tuning the oxygen pressure suppresses the polaronic insulating state and induces superconductivity, resulting in two distinct superconducting regions in the oxygen-pressure phase diagram. Our results provide a pathway toward understanding superconductivity in polaronic materials and offer a route for searching high-temperature superconductors.