Oxide Overlayers and the Superconducting Rf Properties of Yttrium-processed High-purity

. . Superconducting rf surface resistance measurements were made on oxidized Nb cavities that been warmed for a few minutes to temperatures between 250 and 300°C. Warmed oxide layers were further studied using x-ray photoelectron spectroscopy, Auger electron spectroscopy and secondary electron emission yield measurements. Warming usually reduced the temperature-dependent (BCS) part of the surface resistance by lo-20% while, for warming temperatures near 3OO”C, the low-temperature residual surface resistance increased by as much as 70 nano-ohms. Surface spectroscopy measurements showed that, at temperatures between 200 and 250°C, the oxide layer was chemically altered but remained surface-segregated; between 250 and 3OO”C, however, surface oxygen dissolved into the Nb bulk. Changes in the BCS re&tance were consistent with a model based on a shortening of the -electron mean free path by oxygen that had diffused into the Nb metal and based on the theoretical dependence of BCS resistance on electron mean free path. No specific cause for the increased residual resistance could be positively identified, although oxygen-induced surface roughening is a possibility. --