Effects of Interstitial Oxygen and Carbon on Niobium Superconducting   Cavities
  We present results on the effects of interstitial oxygen and carbon on a bulk-niobium superconducting radio-frequency cavity. Previous experiments have shown that high-temperature (~800 $^\circ\text{C}$) nitrogen-doping plays the dominant role in the reduction of the electron mean free path in the RF penetration layer of niobium that leads to a decrease in microwave surface resistance and a suppression the temperature-dependent component of the surface resistance with increasing accelerating gradient. In this work, we show that oxygen and carbon-doping has very similar effects on cavity performance, demonstrating that these effects are not unique to nitrogen. The preparation method used in the introduction of interstitial oxygen and carbon has the advantage that it is done at lower temperatures than that of high-temperature nitrogen-doping and does not require post-treatment electro-polishing. 
P. N. Koufalis, D. L. Hall, M. Liepe, J. T. Maniscalco
physics.acc-ph