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

摘要

Various measurements on superconducting Nb surfaces show strong deviations from BCS theory of homogeneous materials. To study causes for these deviations, penetrationhyphen;depth Dgr;lgr; (t,f,Bac) and resistance Dgr;R(T,f,Bac) measurements were carried out. The observations prove that weak superconducting regions exist at Nb surfaces, with transition temperatureTast;cbelow 7.2 K showing up in several aspects: (1) The transitions to the normal state sum up to Dgr;lgr;0(f,Bac). (2) The proximity aboveTast;cbetween normal and superconducting regions enhances lgr; to lgr;effcausing a step Dgr;lgr;0(Bac) and a slope lgr;0=dlgr;eff/dyenhanced over the BCS values. (3) The apparent resistance of shielding currents passing such normal regions embeded in a superconductor is low and decreasing further withBac, even forBacgsim;10minus;4T, indicating overheating of quasiparticles. These normal regions withTast;crsquo;s below 7.2 K embeded in Nb withTcquest;9.3 K give the first clear evidence for causes of, for example, loweredBc3/Bc2aboveTast;c, differences in mean free pathslobtained from lgr;0andBc3, respectively, or the low rf breakdown fieldsBcrit(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 Ohyphen;enrichment or suboxide precipitation are discussed.