ANOMALOUS FIELD-DEPENDENCE OF THE CRITICAL-CURRENT AND FLUX-CREEP RATE IN PB GE MULTILAYERS/

Critical current densities J(c) and normalized flux creep rate S(= dln M/dln t) have been measured as a function of the perpendicular field H(perpendicular-to) in low-T(c) Pb/Ge multilayers, which can be used t o model the superconducting properties of the layered high-T(c) cuprat es. By taking a proper Ge separator thickness a field-induced crossove r between two different pinning regimes can be observed. This transiti on leads to an anomalous behavior of J(c)(H(perpendicular-to)) and S(H (perpendicular-to)) in Pb/Ge multilayers, while a single Pb film demon strates a monotonic suppression of J(c) by H(perpendicular-to). These data have been analyzed in the framework of the collective pinning (CP ) theory of magnetically coupled superconducting planes: the J(c)(H(pe rpendicular-to)) anomaly is interpreted as a crossover from three-dime nsional (3D) flux lines in low fields to two-dimensional (2D) pancake vortices in higher fields. This interpretation has also been confirmed by the suppression of the J(c)(H(perpendicular)) anomaly in the Pb/Ge multilayers with a square lattice of holes, forming additional defect s for the confinement of the flux lines and for the expansion of the 3 D regime.