GAUSSIAN EFFECTIVE POTENTIAL AND ANTIFERROMAGNETISM IN THE HUBBARD MODEL

The Gaussian Effective Potential (GEP) is shown to be a useful variational tool for the study of the magnetic properties of strongly correlated electronic systems. The GEP is derived for a single band Hubbard model on a two-dimensional bi-partite square lattice in the strong coupling regime. At half-filling the antiferromagnetic order parameter emerges as the minimum of the effective potential with an accuracy which improves over RPA calculations and is very close to that achieved by Monte Carlo simulations. Extensions to other magnetic systems are discussed.

NONPERTURBATIVE EFFECTIVE MODEL FOR THE HIGGS SECTOR OF THE STANDARD MODEL

A nonperturbative effective model is derived for the Higgs sector of the Standard Model which is described by a simple scalar theory. The renormalized couplings are determined by the derivatives of the Gaussian effective potential that are known to be the sum of infinite bubble graphs contributing to the vertex functions. A good agreement has been found with strong coupling lattice simulations when a comparison can be made.

OPTIMIZED POST GAUSSIAN APPROXIMATION IN THE BACKGROUND FIELD METHOD

We have extended the variational perturbative theory based on the background field method to include the optimized expansion of Okopinska and the post Gaussian effective potential of Stansu and Stevenson. This new method provides a much simpler way to compute the correction terms to the Gaussian effective action (or potential). We have also renormalized the effective potential in 3+1 dimensions by introducing appropriate counterterms in the Lagrangian.