Bose-Einstein condensation of nonzero-center-of-mass-momentum Cooper pairs

Cooper pair (CP) binding with both zero and nonzero center-of-mass momenta (CMM) is studied with a set of renormalized equations assuming a short-rang ed (attractive) pairwise interfermion interaction. Expanding the associated dispersion relation in 2D in powers of the CMM, in weak-to-moderate coupli ng a term linear in the CMM dominates the pair excitation energy, while the quadratic behavior usually assumed in Bose-Einstein (BE) condensation stud ies prevails for any coupling only in the limit of zero Fermi velocity when the Fermi sea disappears, i.e., in vacuum. In 3D this same behavior is obs erved numerically. The linear term, moreover. exhibits CP breakup beyond a threshold CMM value which vanishes with coupling. This makes all the excite d (nonzero-CMM) BE levels with preformed CPs collapse into a single ground level so that a BCS condensate (where only zero CMM CPs are usually allowed ) appears in zero coupling to be a special case in either 2D or 3D of the B E condensate of linear-dispersion-relation CPs. (C) 2001 Elsevier Science B .V. All rights reserved.