  We examine the roles of massless Dirac spinon and spin singlet pair
excitations on the phase transition in $d-wave$ superconductors. Although the
massless spinon excitations in the presence of the spin singlet pair
excitations do not alter the nature of the phase transition at $T = 0$, that
is, the XY universality class, they are seen to induce an additional attractive
interaction potential between vortices, further stabilizing vortex-antivortex
pairs at low temperature for lightly doped high $T_c$ samples.
