  We study the stability of the highest symmetric solution (Wigner-solution) of
Dyson-Schwinger equations in chiral limit and at zero temperature. Our results
confirm that if the chemical potential is not very large, the QCD vacuum is in
the chiral symmetry breaking phase and the quantum phase-transition of the
chiral symmetry restoration is in first order. Meanwhile it seems that there is
not competition between chiral symmetry breaking phase and color
superconductivity phase since the color superconductivity phase appears only if
the chemical potential is very large. Moreover, we propose that chiral symmetry
breaking arises from the positive feedback with respect to the mass
perturbation.
