Observation of current-discontinuity lines in type-II superconductors

In this paper, magneto-optical observations and analyses of skeleton structures of flux distributions in type-II superconductors are presented. It will be shown that these structures are formed by lines where the critical current is bending. Such lines are a characteristic feature of a vortical vector field if the absolute value of the vector is constant as for the current distribution in a type-II superconductor in the full critical state. Thin samples offer enhanced sensitivity to observe these current discontinuity lines magneto-optically due to two facts: The Meissner phase contracts to these lines and remains there up to fields, which are higher than the full penetration field ${\mathit{H}}^{\mathrm{*}}$. At much higher magnetic fields, when the Meissner phase has vanished, the logarithmic infinity of the magnetic flux density at these lines leads to sufficiently high contrast. The positions of the discontinuity lines are not affected directly by particular defects, but are determined by the shape of the sample. Because the vortices are not able to cross these lines, the current discontinuity lines determine the flux distribution in the superconductor like a magnetic skeleton. An analysis of the observed structures yields characteristic parameters of the sample as anisotropy, inhomogeneity, or irradiation-induced enhancement of the critical current density. Additionally, the presence of defects allows the observation of the direction of flux motion and, thus, to visualize directly the current distribution. A strong concentration of flux motion near defects without any pecularities of the current and magnetic flux distribution can be predicted using the consideration presented in this paper.