Magnetic penetration depth in Nb/Al and Nb/Cu superconducting proximity bilayers

Magnetic penetration depth lambda in Nb/Al and Nb/Cu superconducting bilaye rs, its absolute values and temperature dependencies under the variations o f material and sample parameters are systematically investigated. Measureme nts of lambda are performed by using the two-coil mutual inductance techniq ue and the results are compared with a recent microscopic theory [Phys. Rev . B 59, 14 630 (1999)] based upon Eilenberger-Usadel's quasiclassical equat ions of superconductivity. Our results show that the lambda>(*) over bar * (T) characteristics at low temperatures usually follow a power law, or a li near temperature dependence, if the sublayer thicknesses are less than seve ral times of their coherence length and the interface resistance is small. A two-gap, steplike feature in the lambda>(*) over bar * (T) curve of the N b/Al bilayer system will develop around the sublayer transition temperature of Al when the interface resistance increases or when the Al layer thickne ss becomes much larger than its coherence length. These results can be well described within the framework of the microscopic theory.