Raman line shapes from sputtered thin films of Y(Pr)Ba2Cu3O6+δ: Fine structures and oxygen ordering

Abstract

A temperature dependence study of Raman line shapes taken from superconducting and insulating Y(Pr)${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ thin films with well-characterized electrophysical properties and morphologies (a-axis or c-axis orientation) has been carried out using the excitation energies at 1.83 eV and 2.41 eV. Ba and Cu bands show doublets, with 113\char21{}119 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 146\char21{}152 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ components and have been calculated using a Green's function model. The shapes of the 113 and 152 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ lines are well described by Lorentzians whereas those of the other two components can be satisfactorily reproduced in the assumption that two phonons of bare frequencies 119 and 146 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ interact with a common continuum of excitations. The temperature dependence of the phonon-continuum coupling parameters allows us to ascribe the scattering continuum to low-energy electronic interband transitions. The model is able to explain qualitatively the overestimated mixing between pure Ba and plane Cu ${\mathrm{A}}_{\mathrm{g}}$ vibrational modes found in previous first-principles calculations. We also find that for the 1.83 eV exciting line, the ${\mathrm{B}}_{1\mathrm{g}}$ oxygen phonon at 335 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ presents a strong Fano effect and anomalous temperature dependence. We link Raman features occuring at \ensuremath{\approx}220\char21{}240, 270\char21{}290, and 560\char21{}596 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to the existence of local variations in oxygen composition resulting from the conditions of deposition. These lines could originate from chain defects and be activated by a coupling between ${\mathrm{A}}_{\mathrm{g}}$ and ${\mathrm{B}}_{2\mathrm{g}}$-${\mathrm{B}}_{3\mathrm{g}}$ modes. The presence of microphases of oxygen-depleted domains (ortho-II phase) reproducing macroscopic oxygen disorder is proposed.