ANHARMONIC EFFECTS AND THE 2-PARTICLE CONTINUUM IN THE RAMAN-SPECTRA OF YBA2CU3O6.9, TLBA2CACU2O7, AND TL2BA2CACU2O8

Anharmonic interactions of the apex 0 vibrations in YBa2Cu3O6.9, TlBa2CaCu2O7, and T12Ba2CaCu2O8 single crystals are investigated by Raman spectroscopy. The temperature dependence of the 0 phonon frequency and linewidth is fit using the results of a perturbation calculation of Raman scattering from anharmonic vibrations with very good results, provided both third- and fourth-order terms in the phonon self-energy are included. Temperature-dependent asymmetric Fano line shapes of the apex 0 modes, which are observed especially well in TlBa2CaCu2O7, suggest Significant coupling of these modes to the background continuum. From the temperature dependence of the background intensity we deduce that the Raman scattering continuum in the region 100-1000 cm-1 contains significant two-particle scattering. The zz polarization of the observed scattering suggests the background is due to two-phonon rather than two-magnon scattering. The Fano line shape and the temperature dependence of the asymmetry can then be attributed to anharmonic interactions.