Rf Surface Resistance Tuning Of Superconducting Niobium Via Thermal Diffusion Of Native Oxide

Recently, Nb superconducting radio frequency cavities vacuum heat treated between 300 and 400 degrees C for a few hours have exhibited very high quality factors (similar to 5 x 10(10) at 2.0 K). Secondary ion mass spectrometry measurements of O, N, and C show that this enhancement in RF surface conductivity is primarily associated with interstitial O alloying via dissolution and diffusion of the native oxide. We use a theory of oxide decomposition and O diffusion to quantify previously unknown parameters crucial in modeling this process. RF measurements of a vacuum heat-treated Nb superconducting radio frequency cavity confirm the minimized surface resistance (higher Q(0)) previously expected only from 800 degrees C diffusive alloying with N.