Shell model description of the phonon dispersion in La₂CuO₄

Abstract

The phonon dispersion in La2CuO4, a base compound for high-Tc superconductors, is analysed in terms of a shell model. Shell charges and core-shell interactions are considered as anisotropic tensors. Short-range potentials of Born-Mayer type are taken to act between shells of different ions. An anisotropic screening of the Coulomb interaction is incorporated in order to describe the observed degeneracy of LO and TO modes polarized parallel to the CuO2 planes. The phonon energies measured for wavevectors along the three principal symmetry directions are adjusted for the first time with a shell model and a good simulation is achieved with a relatively low number of parameters. Copper-oxygen and lanthanum-oxygen interactions turn out to be of comparable magnitude and much stronger than the oxygen-oxygen interaction. The soft mode that leads to the tetragonal-orthorhombic transition is found to be most sensitive to the La-O interaction. This suggests that anharmonicity in this interaction may be responsible for the lattice instability.