Electronic spectra and possible Jahn-Teller distortions in La-(Ba,Sr)-Cu-Osuperconducting systems

We study the possibility for the existence of Jahn-Teller distortions in th e La-(Ba,Sr)-Cu-O system. The Jahn-Teller analysis is based on the calculat ion and correspondingly group theory classification for the electron spectr a of the CuO6 elongated octahedra cluster, having the symmetry group D-4h. The motivation to study this model is that the origin of the apical deforma tion is not a pure Jahn-Teller effect, having therefore a non a priori cond ition to remove all the degeneracy of the ground state electronic wave func tion of the regular octahedron with O-h symmetry. This is based on an ab in itio calculation of the electronic ground state of the cluster representing the above mentioned system, using an open shell restricted and unrestricte d Hartree-Fock (RHF-OS,UHF) wave functions for several basis sets. The syst em is formed with a total of 87 electrons, with 44 and 43 occupied alpha an d beta orbitals respectively for the UHF calculation. We present a comparat ive analysis of the symmetry classified electron spectra for each of the ab ove mentioned calculations including polarized states. It is shown that in all cases the HOMO states for partially filled levels are not degenerate gi ven therefore a nondegenerate total electronic ground state wave function. Based on this analysis we do not find, within the cluster approximation and the use of HF wave functions, the necessary condition for the fulfillment of the Jahn-Teller theorem.