RELATIONSHIP BETWEEN THE PSEUDOCONDUCTING AND SUPERCONDUCTING GAPS - EFFECTS OF RESIDUAL PAIRING CORRELATIONS BELOW T-C

The existence of a normal-state spectral gap in underdoped cuprates ra ises important questions about the associated superconducting phase. F or example, how does this pseudogap evolve into its below T-c counterp art? In this paper we characterize this unusual superconductor by inve stigating the nature of the ''residual'' pseudogap below T-c, and find that it leads to an important distinction between the superconducting excitation gap and order parameter. Our approach is based on a conser ving diagrammatic BCS Bose-Einstein crossover theory which yields the precise BCS result in weak coupling at any T < T-c and reproduces Legg ett's results in the T = 0 limit. We explore the resulting experimenta l implications.