Importance of Coupling between the Charge and Spin Degrees of Freedom in High Tc Superconductivity

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.