LATTICE-MEDIATED INTERACTION OF CU-2+ JAHN-TELLER IONS IN INSULATING CUPRATES

The interaction of the copper d orbitals with the lattice is investigated with the goal of finding the lattice and orbital ground state in La2CuO4. Due to the considerable tetragonality of the crystal, the Kanamori interaction is strongly anisotropic. This results in a tendency to a lattice instability with rhombic distortion of the oxygen squares around the Cu ions. In the present approach, the experimentally observed partial occupation of d3z2-r2 orbitals can be accounted for by the Jahn-Teller effect. The renormalization of the phonon spectrum due to virtual Cu2+ d-d orbital transitions is calculated using appropriate electronic and phononic model parameters. A qualitative agreement with some features in the experimental phonon spectra is obtained.