Tunneling spectroscopy on the correlation effects in FeSi

We have performed electron tunneling spectroscopy on FeSi single crystals in the temperature range 4--300 K by using a scanning tunneling microscope. The differential conductance $(dI/dV),$ when corrected for Schottky barrier effects, exhibits two strongly temperature-dependent peaks on either side of the Fermi level that emerge below ≈200 K and that are separated by a (pseudo)gap of ≈50 meV. Our observations can be ascribed to the formation of quasiparticle density of states caused by d-electron correlation. The tunneling spectra are in good agreement with photoemission spectroscopy as both techniques probe the correlated d-electron density of states (DOS). Our results are also consistent with optical reflectivity data and Raman spectroscopy, which, in contrast, are sensitive to the conduction $(c)$ electron DOS.