Analytical solution of extended Hubbard models on three- and four-site clusters

摘要

The thermodynamical potential of Hubbard models extended by either nearest-neighbour Coulomb correlation and/or nearest-neighbour Heisenberg exchange is calculated analytically for a triangle and tetrahedron. It is shown that the various degeneracies of the grandcanonical energy levels are partially lifted by the exchange interaction and completely by the Falicov term. The influence of the additional interactions on the specific heat has been calculated for the tetrahedron. (c) 2007 Elsevier B.V. All rights reserved.