THE FERMI-SURFACE OF UNDERDOPED HIGH-T-C SUPERCONDUCTING CUPRATES

The coexistence of pi-flux state and d-wave RVB state is considered in this paper within the slave boson approach. A critical value of dopin g concentration delta(c) is found, below which the coexisting pi-flux and d-wave RVB state is favoured in energy. The pseudo Fermi surface o f spinons and the physical electron spectral function are calculated. A clear Fermi-level crossing is found along the (0,0) to (pi, pi) dire ction, but no such crossing is detected along the (pi, 0) to (pi, pi) direction. Also, an energy gap of d-wave symmetry appears at the Fermi level in our calculation. The above results are in agreement with the angle-resolved photoemission experiments which indicate at a d-wave p seudogap and a half-pocket-like Fermi surface in underdoped cuprates.