Temperature dependence of the phonon frequencies, linewidths, and Raman-continuum scattering of single-domain Y0.56Pr0.44Ba2Cu3O7.

Abstract

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 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ 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{\ensuremath{-}}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 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. 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.