Reconstruction of Band Structure Induced by Electronic Nematicity in an FeSe Superconductor

We have performed high-resolution angle-resolved photoemission spectroscopy on an FeSe superconductor (T-c similar to 8 K), which exhibits a tetragonal-to-orthorhombic structural transition at T-s similar to 90 K. At low temperature, we found splitting of the energy bands as large as 50 meVat the M point in the Brillouin zone, likely caused by the formation of electronically driven nematic states. This band splitting persists up to T similar to 110 K, slightly above T-s, 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.