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Title Surface superconductivity of Niobium: Onset of long-range coherence
Type Poster SRF basics
Abstract Superconductivity above the upper critical field of Niobium cylinders with as-grown (i.e. rough), chemically, and electrolytically polished surfaces is investigated. Using the popular criterion for the surface critical field, i.e. the onset of a screening in the low-frequency (10 Hz) susceptibility chi, we find the ratio R between the surface critical field and the upper critical field to be larger than the classical Ginzburg-Landau based value of 1.69 and to increase from 1.8 to 2.0 by electropolishing the surface. This increase is tentatively related to a reduction of the correlation length near the surface due to impurities released from a dirty layer to the Nb due the polishing processes. We have also determined from the ac susceptibility the ac-conductivity, and find that the surface critical field marks only the appearance of a strongly increasing Ohmic conductance, accompanied by a rather small screening component. At a lower field, where the Ohmic conductivity tends to diverge, we observe a sharp increase of the screening component, i.e. of a long-range coherent surface superconductivity. The appearance of phase coherence at this coherent surface critical field is corroborated by the onset of a (inductively measured) finite critical surface current. The field and temperature variation of this current can be well explained in terms of the critical state model by J. Fink (Phys. Rev. Lett. 14, 853 (1965)). For the smoothest surface, also the magnitude of the surface critical current is close to the prediction of this GL-based model. Upon surface roughening, the current density is reduced, and along with the decrease of the ratio R this indicates a larger instability of surface superconductivity. Our analyses of the frequency variation of both conductivity components reveal the validity of dynamical scaling near the surface coherent field. The critical exponents suggest that the coherence is achieved by a percolation between superconducting patches nucleated at the surface critical field.

List of authors...

Principle author first.

Last (Family) Name First Name (Initials only) Affiliation or Organization (abbreviations if possible)
von Sawilski L Inst. of Appl. Phys., Univ. Hamburg
Casalbuoni S Inst. of Appl. Phys., Univ. Hamburg
Koetzler J Inst. of Appl. Phys., Univ. Hamburg

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