Bardeen–Cooper–Schrieffer (BCS) theory and Yang's concept of off-diagonal long-range order (ODLRO)

Abstract

In this contribution, we pay tribute to the scientific achievements of Professor Rodney J. Bartlett on account of his seminal contributions to the many-body electron correlation problem. We are here concerned with strongly correlated situations as met in the theory of superconductivity. In condensed matter physics, one often makes use of the famous Bardeen–Cooper–Schrieffer (BCS) formulation, while quantum chemists often instigate an approach that originates in Yang's concept of off-diagonal long-range order (ODLRO), and the Coleman–Sasaki extreme state. Our aim is to demonstrate that both approaches are essentially equivalent by deriving the BCS gap equation from the assumption of the presence of ODLRO.