Theoretical and Condensed Matter Physics

Biography

Biography: I Chávez

Abstract

Applying the generalized Bose-Einstein conden-sation (GBEC)1 theory we extend the BCS-Bose crossover theory by explicitly including hole Cooper pairs (2hCPs). GBEC hinges on three separate new ingredients, it: a) treats CPs as actual bosons which as distinct from BCS pairs which strictly speaking are not bosons; b) includes 2hCPs on an equal footing with two-electron ones (2eCPs); and c) in the resulting ternary ideal boson-fermion (BF) gas natur-ally incorporates BF vertex interactions that drive forma-tion/disintegration processes of CPs. This leads to a phase diagram with two pure phases, one with 2hCPs and the other with 2eCPs, plus a mixed phase with arbitrary proportions of 2eCPs and 2hCPs. The special-case phase when there is perfect symmetry, i.e., with ideal 50-50 proportions between 2eCPs and 2hCPs, gives the usual unextended BCS-Bose crossover. But the extended crossover predicts Tc/TF values (with Tc and TF the critical and the Fermi temperatures) for some well-known conventional superconductors comparing quite well with experiment and, notably, much better than BCS predictions. In turn, these results are compared with theoretical curves associated with the extended crossover for the special case of perfect symmetry holding at n/nf = 1, where n is the total number particle density and nf is the number density of unbound electrons at T = 0. Remarkably, for 50-50 symmetry all extended-crossover results lie below the Bogoliubov et al. upper limit λBCS ≤ ½, where λBCS is the dimensionless BCS coupling constant; this affords corroboration of their limit.