Direct observation of the critical state in low-κ type-II superconductors

Abstract

New results on the properties of niobium and vanadium in the mixed state obtained by the neutron depolarization method are presented. Magnetization measurements were taken on several samples differing in their degree of hysteresis to obtain the characteristic fields Hd, H + and Hc2 (H + denotes the field where the major hysteresis loop for one sense joins the initial magnetization curve). The depolarization is always observed to begin at Hcl and to disappear at H +. A theoretical reexamination of the critical state concept for low-x type-II superconductors leads to the conclusion that the maximum depolarization indicates the entrance of the entire sample into the critical state. The actual value of the depolarization is shown to be a measure of the flux line lattice distortions. Furthermore, evidence for an anisotropic flux density distribution in the mixed state is presented.