On the electron spectrum of the Hubbard model including interactions with local anharmonic vibrations

This paper presents a self-consistent calculation of the single-electron excitation spectrum within the framework of an Hubbard model including interaction with pseudospin degrees of freedom due to the anharmonic vibrations of the apex oxygen (O(4)) in high-temperature superconductors of the YBaCuO-type (Müller model). The spectra are calculated for different electron concentrations using Green's function equations with a decoupling analogous to a Hubbard-I approximation. We use an approach which is based on the exact solution of the single-site problem. In contrast to the Hubbard model, the spectra show additional subbands due to the interaction of the electrons with the anharmonic vibrations (pseudospins). Those interactions result in the destruction of the electron-hole symmetry and the system can show hole conductivity even at half filling.