Field penetration into proximity-coupled superconducting multilayers

The penetration depth ${\lambda }_{\mathrm{eff}}$ of a magnetic field into multilayered superconductors is studied for the V-Ag proximity system. It is shown that ${\lambda }_{\mathrm{eff}}$ first decreases with increasing multilayer period, but then increases. This peculiar behavior is explained as the crossover of the system from a single superconductor to a composite one. With regard to the proximity effect, the nonmonotonic variation in ${\lambda }_{\mathrm{eff}}$ reveals a novel feature of thin normal films. This feature is understood by considering the fact that the normal-layer thickness restricts the electron mean free path.