PHASE-STRUCTURE OF U(1) LATTICE GAUGE-THEORY BY VARIATIONAL STUDY WITH INDEPENDENT PLAQUETTE EFFECTIVE ACTION

In order to compare independent plaquette effective action with link e ffective action in variational cumulant expansion, as a simple example , we calculate the plaquette internal energy E(p) of U(1) LGT in three dimensions with independent plaquette effective action in variational cumulant expansion approach. The E(p) - beta curve is continous, whic h is consistent with MC data. It shows U(1) theory has only one confin e phase. It also shows the independent plaquete effective action is be tter than the link effective action.