Superconducting condensate formation in quasi-2D systems with arbitrary carrier density

A phase diagram for a quasi-2D metal with variable carrier density has been derived. The phases present are the normal phase, where the order paramete r is zero; the pseudogap phase where the absolute value of the order parame ter is non-zero but its phase is random, and a superconducting phase with a crossover quasi-2D Berezinskii-Kosterlitz-Thouless (BKT) region. The cross over region is bounded by the quasi-2D BKT temperature and the temperature for the onset of conventional long-range order (CLRO), The practical observ ation of these regions, however, critically depends on the carrier density. At high densities both the pseudogap and BKT regions vanish asymptotically , i.e., one obtains the standard BCS picture. At intermediate densities the pseudogap phase is large but the BKT region is negligible. Finally, at ver y low densities both the pseudogap and BKT regions are sizeable. An attempt is made to explain the behaviour observed in underdoped (intermediate dens ities) and optimally doped high-ir, superconducting compounds above their c ritical temperature. The transition to the pseudogap phase should also be r egarded as a crossover. (C) 1999 Elsevier Science B.V. All rights reserved.