Effective volume of voids for flux pinning in superconductors

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Abstract
Flux pinning by voids in niobium and vanadium has been investigated. It is discussed how the collective pinning theory of Larkin and Ovchinnikov may be applied in the case where plastic defects mainly determine the disorder in the flux-line lattice. It is argued that the macroscopic pinning force above its maximum close to the upper critical field is described by the amorphous limit of the collective pinning theory. This provides a means for comparing the elementary interaction, deduced from experiment, with the recent nonlocal theory of Thuneberg et al. that predicts an appreciable enhancement of the effective void volume for pinning. Good agreement is obtained for niobium; the results of vanadium are satisfactory.