PHASE STRUCTURES OF U(1) AND SU(2) LATTICE GAUGE-MODELS AT FINITE-TEMPERATURE

With the variationl-cumulant expansion (VCE) method, the phase structures of the U(1) and SU(2) lattice gauge models at finite temperature are studied. The order parameter Polyakov line [L] with N-tau = 2 is calculated to the fourth order. For the U(1) and SU(2) lattice gauge models in D = 3 + 1 and D = 2 + 1 at finite temperature, the results show the existence of the second order phase transition.