Angle-resolved photoemission study of the strongly correlated semiconductor FeSi

The temperature-dependent electronic states of FeSi have been studied by using high-resolution angle-resolved photoemission spectroscopy (ARPES) and using low-energy tunable photons. At low temperatures, a peak indicating the valence-band maximum (VBM) exists at a binding energy of similar to 20 meV along the Gamma R direction. The observed dispersional width of the energy band!; is narrower than that given by the band-structure calculation, and the width of the ARPES peak near the VBM rapidly broadens as the binding energy increases. Analysis of a model self-energy reveals the importance of electron correlation, especially near the VBM. We observed an unusual temperature dependence of the ARPES spectral features near the Fermi level (E(F)): Below similar to 100 K, the peak at the VBM and the energy gap structures are almost unchanged, while at similar to 100-350 K, the peak gradually moves toward E(F) and the gap is filled. The present results indicate that FeSi is a strongly correlated semiconductor, with a renormalized band near E(F) being responsible for the rapid collapse of the peak and the coherent energy gap upon heating.