Anharmonic effects and the two-particle continuum in the Raman spectra of YBa2Cu3O6.9, TlBa2CaCu2O7, and Tl2Ba2CaCu2O8

Abstract

Anharmonic interactions of the apex O vibrations in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$, ${\mathrm{TlBa}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_7$, and ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ single crystals are investigated by Raman spectroscopy. The temperature dependence of the O 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 O modes, which are observed especially well in ${\mathrm{TlBa}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_7$, 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 ${\mathrm{cm}}^{\mathrm{-}1}$ 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.