A simple model is developed which describes the reduction of the mean free path of conduction electrons in metals near a grain boundary. This leads to a decrease of the self-energy of flux lines in a layer which is considerably thicker than the perturbed zone of the boundary itself. The model yields pinning forces which agree, within an order of magnitude, with recent measurements on niobium bicrystals, and with observed values of grain boundary pinning in Nb3Sn.
The profiles of the magnetic induction near the surface of cylindrical niobium samples were measured by the method of differential susceptibility in minor hysteresis loops. The flux density gradient near the surface generally is considerably higher than in the bulk. This may be interpreted in terms of a surface barrier or of a surface pinning force. The experimental results can be summarized as follows: (1) Surface pinning always increased with surface roughness.(2) When the surface was polished the barrier was only observed in a plastically deformed sample. (3) The influence of the surface seems to extend down to a depth of some tenth of a millimeter.
I n A15 structures +(111) stacking-faults break the linear chains of A atoms. This affects the electronic structure resulting e.g. in changes of the superconducting critical temperature T,. Moreover the large pinning forces observed in these compounds are attributed to the described planar defects. f ( 111)-Stapelfehler in A16-Strukturen unterbrechen die linearen Ketten aus A-Atomen. Die dadurch verursachten Anderungen in der Elektronenstruktur konnen h d e r u n g e n z. B. der supraleitenden Sprungtemperatur T, zur Folge haben. Dariiber hinaus werden die in A15-Supraleitern beobachteten hohen FluOlinienhaftkrafte diesen flachenhaften Gitterfehlern zugeschrieben.
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