Phonons in ${\mathrm{MgB}}_2$ by polarized Raman scattering on single crystals

This paper presents a detailed Raman scattering study of the unusually broad $E_{2g}$ phonon mode in ${\mathrm{MgB}}_2$ crystal. It is shown by polarized Raman scattering on a few-micron-size crystallites with natural faces that the observed broad Raman feature really does obey the selection rules of an $E_{2g}$ mode. Raman spectra on high quality polycrystalline superconducting ${\mathrm{MgB}}_2$ wires reveal a very symmetric $E_{2g}$ phonon line near $615\hspace{0.5em}{\mathrm{cm}}^{-1}$ with the room temperature linewidth of $260\hspace{0.5em}{\mathrm{cm}}^{-1}$ only. Additional scattering of different polarization dependence, observed in certain crystallites, is interpreted as weighted phonon density of states induced by lattice imperfections.