ON THE ELECTRON SPECTRUM OF THE HUBBARD-MODEL INCLUDING INTERACTIONS WITH LOCAL ANHARMONIC VIBRATIONS

This paper presents a self-consistent calculation of the single-electr on excitation spectrum within the framework of an Hubbard model includ ing interaction with pseudospin degrees of freedom due to the anharmon ic vibrations of the apex oxygen (O(4)) in high-temperature supercondu ctors of the YBaCuO-type (Muller model). The spectra are calculated fo r different electron concentrations using Green's function equations w ith a decoupling analogous to a Hubbard-I approximation. We use an app roach which is based on the exact solution of the single-site problem. In contrast to the Hubbard model, the spectra show additional subband s due to the interaction of the electrons with the anharmonic vibratio ns (pseudospins). Those interactions result in the destruction of the electron-hole symmetry and the system can show hole conductivity even at half filling.