Josephson properties of Nb/Cu multilayers

We report direct experimental observations of the Josephson properties of Nb/Cu multilayers achieved by measurements across layers. Multilayered sandwiches with a small cross section (20 μm in diameter) consisting of 10 Nb/Cu/Nb junctions in series were fabricated for such measurements. Applying RF power we observed Shapiro steps in the current-voltage characteristics. Changing the temperature we have seen the sequential change in the principal Shapiro-step voltage representing the change in the number of the phase-locked junctions. This phenomenon is direct evidence for the dimensional 3D-2D crossover responsible for the subdivision of the sample in the individual layers. Simultaneously with the change of the RF step voltage, the behavior of the I-V characteristics change. For T<${\mathit{T}}_{2\mathrm{D}}$ hysteresis appears in the I-V characteristics and the slope of the temperature dependence of the Josephson critical current, ${\mathit{I}}_{\mathit{c}}$(T) , changes.