Light scattering in the high-temperature superconductor Ba(Pb1-xBix)O3.

Abstract

We report a comprehensive inelastic light scattering study of the Ba(${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$)${\mathrm{O}}_{3}$ system as a function of temperature (1.7--300 K) and composition (0\ensuremath{\le}x\ensuremath{\le}1.0), covering both the Brillouin (0--2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) and Raman (2--1000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) frequency regimes. Special attention is given to the compositions (x\ensuremath{\sim}0.25) where relatively sharp and high-temperature (\ensuremath{\sim}12 K) superconducting transitions are observed. Significant dependence of the spectra on temperature, composition, and scattering geometry are observed. However, no singular behavior occurs near ${T}_{c}$. The dominant room-temperature Raman spectral feature is a mode at 110 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which (for x\ensuremath{\sim}0.25) hardens and narrows as temperature is reduced. Its temperature dependence corresponds to a soft-mode behavior extrapolating to a structural transition near 700 K. No low-frequency structure corresponding to earlier inelastic tunneling results is observed in the low-temperature Raman spectrum. The Brillouin spectrum exhibits acoustic mode scattering, with the expected broadening due to the absorption present in the material. The sound velocity extracted from the latter spectra is 5.1\ifmmode\pm\else\textpm\fi{}0.3 km/sec at room temperature.