Crystallographic disorder and electron-phonon coupling in Fe_1−xCo_xSi single crystals: Raman spectroscopy study

A comprehensive concentration and temperature dependent Raman spectroscopy study of Fe_1−xCo_xSi with x between 0 and 1 is presented. All the nine Raman active modes predicted by the factor group analysis are observed in our experiments. A detailed analysis of the symmetry of the Raman lines and a comparison with IR spectroscopy helps assign the observed peaks to modes with A, E, or T symmetry. Local density approximation calculations of the phonon frequencies in FeSi and CoSi match very well the experiments. The concentration dependence of the Raman linewidth with a maximum around x=0.5 points to crystallographic disorder in the substituted systems. For the semiconducting compounds (x≲0.1), the temperature dependence of the Raman linewidth reveals strong electron-phonon interactions. The fit of this temperature dependence gives an activation energy of 30 meV in accordance with the semiconducting gap of FeSi.