Vacuum structure and effective potential at finite temperature: a variational approach

We compute the effective potential for φ4 theory with a squeezed coherent-state type of construct for the ground state. The method essentially consists of optimizing the basis at zero and finite temperatures. The gap equation becomes identical to resumming the infinite series of daisy and superdaisy graphs while the effective potential includes multiloop effects and agrees with that obtained through the composite operator formalism at finite temperature. (author)