Infrared spectra from a shell model anharmonic theory in La2CuO4

S. KOVAL; R. MIGONI

Bled

Workshop; International Workshop on "Anharmonic Properties of High-Tc Cuprates"; 1994

Abstract:The infrared reflectivity (IR) of La2CuO4 is calculated by means of an anharmonic shell-model (SM) theory developed recently. The phonon frequencies and eigenvectors are taken from a SM fit of the experimental data in the principal symmetry directions. The phonon linewidths Γ are obtained first from the anharmonic terms of the potentials determined by fitting the phonon frequencies. The corresponding IR spectra show fairly good agreement with measurements, but this is remarkably improved by suitably varying the anharmonic terms. The frequency dependence of Γ explains certain shapes of the reflectivity bands, which led to erroneous assignments of phonons in a previous analysis of the spectra with ad hoc fitted Γ. The measurements show a low-frequency shoulder of the IR band corresponding to the stretching mode in the Cu-O2 plane (≃ 672 cm-1). We conclude that this feature cannot be interpreted as a peculiar shape of the band due to the frequency dependence of Γ, and should be ascribed to an additional IR-active, possibly polaronic excitation.