THEORY OF QUASI-PARTICLES IN THE UNDERDOPED HIGH-T-C SUPERCONDUCTING STATE

The microscopic theory of the superconducting state in the SU(2) slave -boson model is developed. We show how the pseudogap and Fermi surface segments in the normal state develop into a d-wave gap in the superco nducting state. Even though the superfluid density is of order x (the doping concentration), the physical properties of the low lying quasip articles are found to resemble those in BCS theory. Thus the microscop ic theory lays the foundation for our earlier phenomenological discuss ion of the unusual superconducting properties in the underdoped cuprat es.