Direct observation of a narrow band near the gap edge of FeSi.

We report a high-resolution angle-resolved photoemission investigation of FeSi. The high angle and energy resolutions enable us to identify a band with spectacularly sharp features near the valence-band edge. This band shows very small dispersion but remarkable temperature dependence. The small dispersion constitutes a direct observation of a narrow band in FeSi. This finding supports a theoretical picture involving two narrow bands at the edges of a narrow gap, to account for the unusual temperature dependence of the magnetic susceptibility that makes FeSi a fascinating material.