Phase Transitions in Quantum Field Theory
  We discuss here phase transitions in quantum field theory in the context of vacuum realignment through an explicit construction. Vacuum destabilisation may occur through a scalar attaining a nonzero expectation value, or through a condensate mechanism as in superconductivity and for chiral symmetry breaking, or by some other mechanism not taken here. Phase transition is defined as vacuum realignment with a discontinuity in some observables which correspond to the classical order parameters. The method is nonperturbative and variational, using equal time algebra and an ansatz for the construction of the physical vacuum. 
S.P. Misra