Restoration and breakdown of the phi ->-phi symmetry in the (1+1)-dimensional massive sine-Gordon field theory by quantum and finite-temperature effects

Within the framework of the Gaussian wavefunctional approach, we investigat e the quantum and finite-temperature influences on the Z(2)-symmetry (phi - -> -phi) of the (1 + 1)-dimensional massive sine-Gordon field theory. It is explicitly demonstrated that by quantum effects the Z(2)-symmetry can be b roken in one region of the parameter space and restored in another region. Moreover, a finite-temperature effect can only restore the Z(2)-symmetry fu rther.