  A preformed-pair model was considered. The quantum disordered phase of the
d-wave superconducting state was obtained by turning on an on-site repulsion.
The low energy effective theory of the quantum disordered phase was derived,
which was found to be qualitatively equivalent to the low energy effective
theory of the SU(2) slave-boson theory in the staggered-flux phase. Many
physical properties of the disordered phase at low energies were obtained, such
as the Mott insulator property at zero doping, spin gap and low superfluid
density at low doping.
