Simultaneous <i>Q</i> ₀ and <i>E</i> _acc Enhancement in 1.3 GHz Niobium Cavities: Role of Carbon-Modulated Oxygen Segregation at the Atomic Scale

Superconducting radiofrequency (SRF) cavities are essential for high-energy particle accelerators rendering ultralow power dissipation and a high acceleration gradient. We demonstrate a superior performance in niobium-based 1.3 GHz cavities via medium-temperature (Mid-T) baking with remarkable values up to 5.5 × 10¹⁰ for the quality factor (<i>Q</i>₀) at 16 and 32.7 MV/m for the maximum acceleration gradient (<i>E</i>_acc). Through correlative in situ spectroscopy, mass spectrometry, and electron and tunneling microscopy, we establish that nanoscale spatial distributions of impurities (O, C, H) directly evidenced at the metal-oxide interface govern performance enhancement. In niobium, annealing at 300 °C drives uniform oxygen doping (10-100 nm depth) via diffusion from the native oxide, while an optimized Mid-T protocol in delivering high <i>E</i>_acc values suppresses interfacial NbO segregation through competitive C-O interactions. For the optimized protocol, post-treatment characterization reveals up to an 8.8% increased superconducting gap and 29.7% reduced quasiparticle broadening, corroborating strain-mediated defect-impurity interactions at proximity layers. These results provide a Mid-T baking recipe to simultaneously enhance <i>Q</i>₀ and maximum <i>E</i>_acc in SRF cavities via competitive impurity interactions at the metal-oxide interface.