  IV-VI semiconductor SnSe has been known as the material with record high
thermoelectric performance.The multiple close-to-degenerate valence bands in
the electronic band structure has been one of the key factors contributing to
the high power factor and thus figure of merit in the SnSe single crystal. To
date, there have been primarily theoretical calculations of this particular
electronic band structure. In this paper, however, using angle-resolved
photoemission spectroscopy, we perform a systematic investigation of the
electronic structure of SnSe. We directly observe three predicted hole bands
with small energy differences between their band tops and relatively small
in-plane effective masses, in good agreement with the ab initio calculations
and critical for the enhancement of the Seebeck coefficient while keeping high
electrical conductivity. Our results reveal the complete band structure of SnSe
and help to provide a deeper understanding of the electronic origin of the
excellent thermoelectric performances in SnSe.
