NMR Study of the Proximity Effect in Nb–Cu Multilayers

Abstract

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.