A generalized BEC picture of superconductors

A recent re‐examination of BCS theory leads one to devise a generalized BEC formalism (GBEC), that is, essentially a boson‐fermion (BF) model containing three new ingredients: (i) Cooper pairs (CPs), in contrast to BCS pairs, as real bosons; (ii) BF vertex interactions (analogous to electron‐phonon vertices); and (iii) two‐hole CPs (2hCPs) accounted for along with two‐electron CPs (2eCPs). In addition to the usual normal phase at high‐enough temperatures T , three condensed phases emerge at lower temperatures with substantially higher T c s than BCS theory. These are: two pure BEC phases of 2eCPs and of 2hCPs as well as a mixed phase with both kinds of CPs. BCS theory is precisely recovered with a mixed phase of equal numbers of both kinds of CPs. In contrast to the well‐known BCS exponential rise from zero of T c as function of carrier‐number density the GBEC scheme exhibits the linear rise as function of doping as eminently observed in high‐ T c underdoped superconductors such as YBCO. © 2012 Wiley Periodicals, Inc.