ABSTRACT
This chapter aims to bring together the experimental and theoretical work concerning the four types of experiments, which show considerable departure from predictions of the older two-fluid model and excellent confirmation of the spin-momentum pairing introduced in the BCS theory. The four most important types of measurements were of infrared and microwave absorption, ultrasonic attenuation, nuclear spin-lattice relaxation, and thermal conductivity. At the same time that the BCS theory was being formulated, measurements of nonequilibrium properties of superconductors were being carried out which showed important new features characteristic of superconductivity. The chapter shows that no one-electron theory can simultaneously explain the results of the nuclear relaxation rate and infrared absorption, on the one hand, and the results concerning ultrasonic absorption measurements, on the other. Consequently, comparison of the two groups of measurements provides important verification of paired electron spin-momentum correlation, which is the basis of the BCS theory.
