On oxygen enrichments in Nb surface layers and their apparent conductivity as observed by the superconducting penetration depth Δλ (T,f,Bac)

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Abstract
Various measurements on superconducting Nb surfaces show strong deviations from BCS theory of homogeneous materials. To study causes for these deviations, penetration‐depth Δλ (t,f,Bac) and resistance ΔR (T,f,Bac) measurements were carried out. The observations prove that weak superconducting regions exist at Nb surfaces, with transition temperature T*c below 7.2 K showing up in several aspects: (1) The transitions to the normal state sum up to Δλ0(f,Bac). (2) The proximity above T*c between normal and superconducting regions enhances λ to λeff causing a step Δλ0(Bac) and a slope λ0=dλeff/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 Bac, even for Bac≳10−4 T, indicating overheating of quasiparticles. These normal regions with T*c’s below 7.2 K embeded in Nb with Tc?9.3 K give the first clear evidence for causes of, for example, lowered Bc3/Bc2 above T*c, differences in mean free paths l obtained from λ0 and Bc3, respectively, or the low rf breakdown fields Bcrit(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.