Hubbard III approach with hopping interaction and intersite kinetic correlations

Abstract

We analyze the Hubbard model with added hopping interaction within the full Hubbard III approximation. In the Green's-function decoupling process, the intersite kinetic correlation functions are included. This is an extension of our previous paper [G. G\'orski and J. Mizia, Phys. Rev. B 79, 064414 (2009)] in which the basic Hubbard model with the intersite kinetic correlations was analyzed in the framework of the coherent potential approximation (CPA). In the CPA method, the up-spin electrons propagated in the lattice of frozen down-spin electrons. The full Hubbard III solution used now takes into account the itinerancy of down-spin electrons. The combined effect of the hopping interaction and intersite kinetic correlation leaves the position of spin bands unaffected, but it deforms the density of states (DOS) of electrons, changing in this way the average electron energy. It is the main driving force behind the ferromagnetism as opposed to the rigid shift of the entire band, which takes place in the conventional Stoner magnetism. In the numerical calculations, we have used the bands with symmetrical DOS (semielliptic or bcc-like DOS) and also with asymmetrical DOS resembling the fcc DOS. The spontaneous ferromagnetic transition was obtained under the combined action of the hopping interaction and the intersite correlation in the systems that contain even a moderately strong peak in the DOS, such as the bcc- and fcc-like DOS.