BCS-Bose Crossover in Color Superconductivity

It is shown that the onset of the color superconducting phase occurs in the BCS-BE crossover region. During the last 2-3 years color superconductivity became one of the QCD focal points –see review papers [1]-[3]. To a large extent (but not completely) the basic ideas of the subject are traced back to the BCS theory of superconductivity and its later development. It is known that the discovery of high temperature superconductors (HTSC) gave rise to new ideas and approaches and revealed interest to the problem of the transition from the BCS regime to the Bose-Einstein (BE) condensation. The BCS-BE crossover is important for the physics of HTSC since the underlying distinction of HTSC from ordinary superconductors is that they are characterized by much smaller value of the dimensionless parameter ξn, where ξ is the coherence length and n is the carrier density. In the BCS, or weak coupling regime, ξn ∼ (108− 10) while in the opposite strong coupling case ξn < ∼ 1 and we are dealing with the compact pairs of composite bosons, which may undergo BE condensation. It has been suggested (see e.g. [4]) that the description of the HTSC might require an intermediate approach between the BCS and BE limits. The evolution from weak to strong coupling was theoretically investigated [5] before the discovery of HTSC. It was shown that the transition proceeds via a smooth crossover though the two limits are physically quite different (see also [6]).