Temperature dependence of the phonon frequencies, linewidths, and Raman-continuum scattering of single-domainY0.56Pr0.44Ba2Cu3O7

We report detailed Raman-scattering measurements from a single-domain crystal of composition ${\mathrm{Y}}_{0.56}$ ${\mathrm{Pr}}_{0.44}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (${\mathit{T}}_{\mathit{c}}$=40 K). Through a polarization analysis, 13 of the 15 Raman-active phonons are unambiguously identified. The frequencies and linewidths of the five ${\mathit{A}}_{\mathit{g}}$ phonons are studied as a function of temperature. Unlike ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$, there are no pronounced superconductivity-induced changes in phonon frequency and linewidth. At all temperatures, the ${\mathit{B}}_{1\mathit{g}}$-like phonon at 320 ${\mathrm{cm}}^{\mathrm{-}1}$ is intrinsically much broader than in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. The linewidths of the 440- and 515-${\mathrm{cm}}^{\mathrm{-}1}$ phonons decrease sharply with cooling, to an extent that cannot be explained by the simplest model of anharmonic decay. We identify Raman-scattering continua for photons polarized both within and perpendicular to the ${\mathrm{CuO}}_2$ planes. Like ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$, the ${\mathrm{CuO}}_2$-plane continuum interferes strongly with the phonon scattering at 130 ${\mathrm{cm}}^{\mathrm{-}1}$. Unlike ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$, the ${\mathrm{CuO}}_2$-plane continuum of ${\mathrm{Y}}_{0.56}$ ${\mathrm{Pr}}_{0.44}$ ${\mathrm{Ba}}_2$ ${\mathrm{Co}}^3$ ${\mathrm{O}}_7$ has a low-frequency tail that disappears upon cooling.