Fermi-liquid behavior and spin-charge separation in underdoped cuprates:: Gaussian fluctuations in the U(1) slave-boson mean-field theory of the t-J model

The Gaussian fluctuations in slave-boson mean-field theory of the t-J model are analyzed in this paper in the low-doping regime where the superconducting to normal (pseudogap phase ) transition is driven by vanishing of bose-condensation amplitude. By eliminating the boson and constraint fields exactly in the linear-response regime, we show that the Gaussian theory describes an unconventional Fermi liquid superconductor where the superconducting to normal transition is a new type of phase transition characterized by a change of Landau parameter from F-1 > -1 in the superconducting phase to F-1 = -1 in the pseudo-gap phase, corresponding to a state with spin-charge separation. The consequences of this proposal are discussed.