OBSERVATION OF CURRENT-DISCONTINUITY LINES IN TYPE-II SUPERCONDUCTORS

In this paper, magneto-optical observations and analyses of skeleton s tructures of flux distributions in type-II superconductors are present ed. It will be shown that these structures are formed by lines where t he critical current is bending. Such lines are a characteristic featur e of a vortical vector field if the absolute value of the vector is co nstant as for the current distribution in a type-II superconductor in the full critical state. Thin samples offer enhanced sensitivity to ob serve these current discontinuity lines magneto-optically due to two f acts: The Meissner phase contracts to these lines and remains there up to fields, which are higher than the full penetration held H. At muc h higher magnetic fields, when the Meissner phase has vanished, the lo garithmic infinity of the magnetic flux density at these lines leads t o sufficiently high contrast. The positions of the discontinuity lines are not affected directly by particular defects, but are determined b y the shape of the sample. Because the vortices are not able to cross these lines, the current discontinuity lines determine the flux distri bution 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 t he critical current density. Additionally, the presence of defects all ows the observation of the direction of flux motion and, thus, to visu alize directly the current distribution. A strong concentration of flu x motion near defects without any peculiarities of the current and mag netic flux distribution can be predicted using the consideration prese nted in this paper.