Magnetic field decoupling and 3D-2D crossover in Nb/Cu multilayers

Transport properties of Nb/Cu multilayers were measured along and across layers. It is shown that not only the temperature but also the magnetic field parallel to layers can effectively decouple layers and cause the three-to-two-dimensional (3D-2D) crossover. As a consequence of the 3D-2D crossover, sharpening of the resistive transition with current along layers occurs due to the appearance of a strong intrinsic pinning in the 2D state. Evidence for the intrinsic Josephson effect in the 2D state is provided both by the periodic modulation of the dynamic resistance across layers versus the parallel magnetic field and by the multiply branched $I-V$ curves caused by flux-flow of Josephson vortices in the stacked superconductor-normal-metal-superconductor junctions composing the multilayer. By measurements across layers the "breaking field" at which the proximity induced superconductivity in the normal layers of superconductor-normal-metal (Nb/Cu) multilayers is destroyed was observed directly. A dimensionality diagram in the ($H-T$) plane was deduced from our data. Reasons for complication of the "Fraunhofer pattern," $I_c\mathrm{}\left(H\right)\mathrm{}$, in "long" multilayers are discussed.