  Based on an improved SU(2) slave-boson approach showing coupling between the
cha rge and spin degrees of freedom, we derive a phase diagram of high $T_c$
cuprate s which displays both the superconducting and pseudogap phases in the
plane of t emperature vs. hole doping rate. It is shown that phase fluctuations
in the order parameters results in a closer agreement with the observed phase
diagram of an arch shape, by manifesting the p resence of an optimal doping
rate.
