Alternative Superconducting Ground States

Abstract

A comparison is made between (a) the trial ground state composed of a linear combination of normal state configurations in which individual electron states of opposite momentum and equal spin projection (k t, -k t) and (k t, -k t), abbreviated (kcr, -kcr), are both either occupied or unoccupied, and (b) the trial ground state with (k t , -k t ) pairing proposed by Bardeen, Cooper, and Schrieffer. The two trial states lead to identical thermodynamics as long as an average matrix element -V = < Vkle' >av is treated as a disposable parameter. They lead to similar coherence effects for ultrasonic attenuation and nuclear spin relaxation. They differ in that spin paramagnetism is predicted for (kcr, -kcr) pairing. Detailed knowledge of the electron-phonon interaction in superconductors seems at present inadequate to rule out either possibility for all superconductors.