Functional integration method for the Hubbard model with large negative U and other local pairing models

Abstract

We formulate a constrained field theory as a representation of the Hubbard model with strong attractive ('negative U') coupling. The superconducting mean-field solutions are correctly reproduced after unusual antiperiodicity conditions on grassmann fields were imposed. In the exact action we find a new doubly-local symmetry, which together with Elitzur's theorem predicts strong corrections to mean field approximations by quantum fluctuations. We compare results for the large negative U Hubbard model with those for other local pairing models, including disorder too. A new subclassification scheme of type II superconductivity is proposed. For disordered models, we discuss implications of Elitzur's theorem for the existence of superconducting glass phases. © 1989 Springer-Verlag.