On the functional integral method applied to models with Hubbard type interaction

B. Lorenz, Holger Fehske

Eberhard O. Tüngler, Thilo Kopp

Hideo Hasegawa

A slave-boson functional-integral method has been developed for the Hubbard model with arbitrary, orbital degeneracy $D$. Its saddle-point mean-field theory is equivalent to the Gutzwiller approximation, as in the case of single-band Hubbard model. Our theory is applied to the doubly degenerate ($D = 2$) model, and numerical calculations have been performed for this model in the paramagnetic states. The effect of the exchange interaction on the metal-insulator (MI) transition is discussed. The critical interaction for the MI transition is analytically calculated as functions of orbital degeneracy and electron occupancy.

Yu. A. Izyumov, N. I. Chaschin, D. S. Alexeev +1 more

Cláudio Andrade Macedo, Mauriaacio D. Coutinho-Filho

We present a careful analysis of the functional-integral approach as applied to the Hubbard model in the framework of diagrammatic perturbation theory. It is demonstrated that a proper comprehension of the formalism can be attained and that the functional-integral method is a very systematic technique, free of ambiguity. In particular, additional exact relations among physical quantities of interest are derived in the context of the formalism.

A. P. Kampf

We use a functional-integral representation for the Hubbard model to describe the formation of local moments and their magnetic correlations. In the absence of long-range magnetic order, the presence of well-established local moments alone is sufficient to produce a gap in the electronic spectrum. The local moments are treated within a coherent-potential approximation, and we apply an embedded-cluster method to incorporate their interactions.