NMR study of super conducting Nb/Cu multilayrs

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The multilayr of superconducting and normal metals, Nb/Cu, provides a good opportunity for the study of superconducivity induced by the proximity effect. NMR is especially suitable for the study of multilayrs, since it can selectively measure the thermal excitation spectra of the superconducting quasi-particles in the respecive layrs. We have carried out ^<93>Nb and ^<63>Cu NMR in Nb/Cu superconducting thin film down to 0.1K,and measured the nuclear spin lattice relaxation rate, 1/T_1, in zero field by using the field cycling method. The temperature dependence of 1/T_1 has shown that the existence of superconducting energy gap with the magnitude of 2DELTA=3.5K_BT_C for Nb layrs, and a smaller gap of about 2.8K_BT_C for Cu layrs. The 1/T_1 of ^<63>Cu measured at low temperatures has also shown the existence of the residual density of states (about 50% of the value in the normal states) in the superconducting Cu layrs. The corresponding residual density of states, even if exist, is very small in the superconducting Nb layrs.