Anharmonic effects in the apex O phonon in relation to the background continuum in high-Tc cuprates

The observed temperature dependence of the apex O phonon self-energy has been found to be described well by a static aharmonicity in the apex O vibrational potential in YBa2Cu3O6.9, TlBa2CaCu2O7 and Tl2Ba2CaCu2O8. The anharmonicity (of apex Os) thus appears to be present in all the cuprates investigated so far. The significant contribution from three- and four-phonon vorteces in the phonon self energy suggest that the asymmetric lineshape of the apex O lines, which is especially clearly visible in TlBa2CaCu2O7, arises due to anharmonic coupling, rather than coupling of the phonon to an electronic continuum. Given that multi-phonon effects are strong, we can infer that scattering ''continuum'' consists not only of electronic scattering, but contains also a multi-phonon scattering background. (orig.)