  We have performed high-resolution angle-resolved photoemission spectroscopy
on FeSe superconductor (Tc ~ 8 K), which exhibits a tetragonal-to-orthorhombic
structural transition at Ts ~ 90 K. At low temperature we found splitting of
the energy bands as large as 50 meV at the M point in the Brillouin zone,
likely caused by the formation of electronically driven nematic states. This
band splitting persists up to T ~ 110 K, slightly above Ts, suggesting that the
structural transition is triggered by the electronic nematicity. We have also
revealed that at low temperature the band splitting gives rise to a van Hove
singularity within 5 meV of the Fermi energy. The present result strongly
suggests that this unusual electronic state is responsible for the
unconventional superconductivity in FeSe.
