NMR Study of the Proximity Effect in Nb-Cu Multilayers

A series of measurements on the nuclear spin-lattice relaxation rate, T₁ ^-1 of ⁶³Cu in superconducting multilayer Nb-Cu has been performed over a temperature range from 4.2 K down to 0.3 K, using the field cycling method. T₁ ^-1 decreases rapidly after an initial increment just below T_c At low temperatures, however, the rate of the decrease slows down, and T₁^-1 decreases in proportion to temperature. It has been concluded that such behavior of T₁ ^-1 originates from a distribution of the energy gap of the Cooper pairs in Cu. The electrons which are propagated nearly normal to the S-N interface have a finite energy gap predicted by McMillan, while the electrons propagated nearly parallel to the interface are gapless and behave like normal electrons.