Oxygen enrichments in Nb surface layers and their apparent conductivity as observed by the superconducting penetration depth Δlambda (T,f,B/sub ac/)

Various measurements on superconducting Nb surfaces show strong deviations from BCS theory of homogeneous materials. To study causes for these deviations, penetration-depth Δlambda (t,f,B/sub ac/) and resistance ΔR (T,f,B/sub ac/) measurements were carried out. The observations prove that weak superconducting regions exist at Nb surfaces, with transition temperature T*/sub c/ below 7.2 K showing up in several aspects: (1) The transitions to the normal state sum up to Δlambda0(f,B/sub ac/). (2) The proximity above T*/sub c/ between normal and superconducting regions enhances lambda to lambda/sub eff/ causing a step Δlambda0(B/sub ac/) and a slope lambda0=dlambda/sub eff//dy enhanced over the BCS values. (3) The apparent resistance of shielding currents passing such normal regions embeded in a superconductor is low and decreasing further with B/sub ac/, even for B/sub ac/> or approx. =10-4 T, indicating overheating of quasiparticles. These normal regions with T*/sub c/'s below 7.2 K embeded in Nb with T/sub c/approx. =9.3 K give the first clear evidence for causes of, for example, lowered B/sub c//B/sub c/ above T*/sub c/, differences in mean free paths l obtained from lambda0 and B/sub c/, respectively, or the low rf breakdown fields B/sub crit/(f) and its dependence on frequency. The reason for the weak superconducting spots in Nb, which has been anodized or handled in air after UHV firing, is oxygen enrichment in a surface layer. The amount of the statistically distributed weak spots increases with cold working or electron impact. Possible mechanisms causing such O-enrichment or suboxide precipitation are discussed