Angle-resolved photoemission study of the strongly correlated semiconductor FeSi

Abstract

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 $\sim 20\text{meV}$ along the $\Gamma R$ direction. The observed dispersional width of the energy bands 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 $\left(E_F\right)$: Below $\sim 100\text{K}$, the peak at the VBM and the energy gap structures are almost unchanged, while at $\sim 100–350\text{K}$, 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.