Nuclear Spin Relaxation in Normal and Superconducting Aluminum

Nuclear spin-lattice relaxation times have been measured in normal and superconducting Al from 0. 94^ to 4. 2^, and as a function of static field in the normal state. In the normal state the relaxation rate is proportional to temperature as predicted by Redfield and others. The field dependence is somewhat greater than predicted. Relaxation in the superconductor was studied by a field cycling method which allowed the measurements to be made in the normal state but relaxation to occur in the superconductor. The results disagree with a simple two-fluid model, but are explained by the theory of Bardeen, Cooper, and Schrieffer. The contrast between the temperature dependence of nuclear relaxation and ultrasonic absorption confirms the central feature of the Bardeen-Cooper-Schrieffer theory that electrons of opposite spin and momentum are correlated.