Landau damping and lifting of vibrational degeneracy in metallic potassium fulleride

Abstract

Raman spectra are presented for single crystal metallic potassium fulleride at low temperatures. As compared to the undoped material dramatic changes in line position (\ensuremath{\Delta}\ensuremath{\omega}\ensuremath{\ge}80 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) and linewidth (\ensuremath{\gamma}\ensuremath{\ge}60 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$), particularly for the two highest and the two lowest ${\mathit{H}}_{\mathit{g}}$ modes, were observed. The lowest frequency ${\mathit{H}}_{\mathit{g}}$ modes were found to be asymmetric but structured and could be fitted with five components which exhibit a line shift-linewidth relation. The experimental results are discussed in a model of single-particle excitation (Landau damping). Electron-phonon coupling constants ${\ensuremath{\lambda}}_{\mathit{i}}$ were determined for all eight ${\mathit{H}}_{\mathit{g}}$ modes using Allen's formula. The total for all ${\ensuremath{\lambda}}_{\mathit{i}}$ was found to be 0.9 for a density of states at the Fermi level of 12 ${\mathrm{eV}}^{\mathrm{\ensuremath{-}}1}$. \textcopyright{} 1996 The American Physical Society.