Fermi surface of underdoped high-T-c superconducting cuprates

The coexistence of a pi-flux stare and a d-wave resonant-valance-bond (RVB) state is considered in this paper within the slave-boson approach. A critical value of doping-concentration delta(c) is found, below which the coexisting pi-flux and d-wave RVB state is favored in energy. The pseudo-Fermi surface of spinons and the physical electron spectral function are calculated. A clear Fermi-level crossing is found along the (0,0) to (pi, pi) direction, 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 pseudogap and a half-pocket-like Fermi surface in underdoped cuprates.