ov 2 00 3 Cooper Pairing Revisited

Doubtlessly the most central notion in superconductivity, for both low and high transition temperatures Tc, is that of Cooper pairs (CPs) [1] that form among the underlying electron (or hole) charge carriers of the many-electron system. And yet, it is perhaps the least understood concept. Shortly after the publication of the BCS theory [2] of superconductivity, charged pairs observed in magnetic flux quantization experiments with 3D conventional [3][4], and much later with quasi-2D cuprate [5] superconductors, suggested CPs as an indispensable ingredient, regardless of the inability of BCS theory per se to describe high-Tc superconductivity, as now seems an almost universal consensus. Cooper pairing is no less central, albeit with a different interfermion interaction, to neutral-atom superfluidity as in liquid He [6], and presumably also to ultracold trapped alkali Fermi gases such as Li [7] and K [8] where pairing is expected to occur as well. More recently, studies of quantum degenerate Fermi gases consisting of neutral K atoms and their so-called Feshbach “resonance superfluidity” have appeared [9]-[13], but assuming quadratically-dispersive CPs and ignoring hole pairs—two fundamental shortcomings as we now illustrate. In Sec. 2 we recall how the Bethe-Salpeter (BS) many-body equation (in the ladder approximation), treating both 2p and 2h pairs on an equal footing, implies that the ordinary CP problem [based on an ideal Fermi gas (IFG)