A first‐principles approach to high‐ T c superconductivity. II. The superconducting ground state

We show that a consistent N ‐electron theory of a solid can within the Born–Oppenheimer ( BO ) approximation lead only to normal conductivity or to an insulating state. If one leaves the BO approximation by turning on electron–phonon interaction, one can establish a lower‐lying new ground state that has the characteristic properties of the Bardeen–Cooper–Schrieffer ( BSC ) ground state. We demonstrate that an ( N + N n )‐particle Schrödinger equation that describes the motion of N electrons and N n nuclei of a solid can lead only to phonon‐induced superconductivity. © 1993 John Wiley & Sons, Inc.