FRANK-READ SOURCE-ACTIVATED FLUX SHEAR IN TYPE-II SUPERCONDUCTORS

An expression is derived for the flux-pinning force density F(p) as a function of magnetic induction B and pinning defect center density rho, based on a model of flux-line-lattice shear activated by Frank-Read source dislocations. F(p)(b) peak positions shift with increasing rho to larger b, where b = B/B(c2) is the reduced induction and B(c2) is the upper critical field. For a given B there exists an optimum defect center density, rho-opt, for which F(p) becomes a maximum. The potential F(p) enhancement is a function of the initial defect concentration rho-i. At a constant B a larger F(p) increase is obtained for smaller rho-i, and at a constant rho-i the increase is larger for larger B. Quantitative agreement for neutron-irradiated Nb3Sn is obtained by fitting F(p)(b) data with three parameters rho-i, a pinning defect center generating linear rate constant alpha, and the third consisting of a function of the Ginzburg-Landau-kappa and B(c2). The major characteristic of F(p)(b) for Nb3Sn, which is F(p) peaking at or near b = 0.2, is also true for the copper-oxide-based high-T(c) superconductors, and the proposed model also applies to these materials.