ANHARMONICITY IN THE C-DIRECTION OF HIGH-T-C OXIDES

The observation of various structural anomalies correlated with the superconducting transition temperature Tc provide a motivation to investigate anharmonic phonon-phonon and electron-phonon interactions, and their effect on normal state and superconducting properties. We specifically investigate c-axis-related anharmonic lattice fluctuations coupled to the electron density, which provide a charge transfer and interplanar coupling mechanism. Due to the strongly nonlinear phonon fluctuations, the effective interplanar electron (hole) hopping integral becomes temperature dependent and leads to (bi-)polaron formation at high temperatures, followed by a spin density wave instability with decreasing T, and finally interplane pair tunneling in the superconducting state. The c-axis-related infrared absorption is calculated and it is shown that due to anharmonic effects strong increases in the oscillator strengths, line shape asymmetries and broadening effects are obtained consistent with experimental data. © 1995 The American Physical Society.