ANISOTROPY OF PERPENDICULAR FIELD PENETRATION INTO HIGH-T(C) SUPERCONDUCTORS INDUCED BY STRONG LONGITUDINAL-FIELD

The geometry of penetration of a magnetic field oriented perpendicular to the surface of a superconductor is changed by the presence of a lo ngitudinal field. This is studied directly by means of the magneto-opt ical technique in different high-T(c) materials. In YBa2Cu3O7 single c rystals Brandt's mode of magnetization is observed, namely, penetratio n of an AC field only along the longitudinal field even if the corresp onding dimension of the sample is much longer than the other two. This anisotropy is the evidence that the force-free configuration of curre nt and vortices cannot be destroyed by cutting and reconnection of the vortices in this 3D superconductor. On the contrary, the absence of t he induced anisotropy in Bi2Sr2CaCu2O8 single crystals reveals the ind ependence of pancake vortices in CuO layers from longitudinal field wh ich makes a realization of the force-free configuration impossible in the layered 2D superconductor. An intermediate effect in thin films in dicates that vortices are almost normal to the film surface even if th e applied field is nearly parallel to it. The described experiment can be used as a new tool to study the internal vortex structure in super conductors.