Experimental study on the collective theory of flux pinning in a high-field superconductor

Homogenized ${\mathrm{V}}_3$Si crystals were irradiated with fast neutrons at conditions which result in a proportionality between the induced defect density and the neutron fluence. The critical current density is corrected for differences in the fluence dependence of the upper critical field and of the Ginzburg-Landau parameter. The resulting rise of the volume pinning force is investigated as a function of the density of the weak interacting defects. A second power law which points to a collective interaction is observed at low defect densities and at low fields. With increasing defect density and increasing field, this quadratic dependence vanishes abruptly and an almost linear relation dominates. The exponent of this first power law decreases continuously with further increase of field or defect density. These observations are compared with the collective pinning theory.