Pinning and critical currents of heterogeneous superconductors in different structural states

The critical currents of a composite Nb3Sn-based superconductor are found to increase as a result of low intensity ultrasonic interactions. In terms of a mechanism for interactions of vortices with grain boundaries based on electron scattering at the boundaries, a reduction in the compressive forces in an Nb3Sn layer during ultrasonic interactions leads to an increase in the elementary pinning force because of a reduction in the coherence length and increases in the superconducting transition temperature and the electron specific heat. It is shown that for single-crystal niobium with a high density of uniformly distributed dislocations in fields close to Hc2, the field dependence of the volume pinning force corresponds to a system of effective point centers that satisfy a rarefaction criterion. The first order interaction of a vortex with these kinds of pinning centers greatly exceeds the characteristic for interactions of the vortex with single helical and edge dislocations.