Electronic Raman scattering across the unconventional charge gap in FeSi

The unusual charge gap in FeSi is investigated using Raman scattering. These studies reveal (i) a rapid suppression of low-frequency (${\mathrm{cm}}^{\mathrm{-}1}$) electronic scattering below ∼250 K, (ii) a redistribution of low-frequency electronic scattering strength to interband transitions near ${\mathrm{\Delta }}_0$∼1200 ${\mathrm{cm}}^{\mathrm{-}1}$, and (iii) a dramatic decrease in phonon linewidths below 250 K, indicating a strong coupling between low-frequency charge excitations and the lattice in FeSi.