Electron correlation in the FeSe superconductor studied by bulk-sensitive photoemission spectroscopy

We have investigated the electronic structures of recently discovered superconductor FeSe by soft-x-ray and hard-x-ray photoemission spectroscopy with high-bulk sensitivity. The large $\text{Fe}\text{ }3d$ spectral weight is located in the vicinity of the Fermi level $({E}_{F})$, which is demonstrated to be a coherent quasiparticle peak. Compared with the results of the band-structure calculation with local-density approximation, $\text{Fe}\text{ }3d$ band narrowing and the energy shift of the band toward ${E}_{F}$ are found, suggesting an importance of the electron correlation effect in FeSe. The self-energy correction provides the larger mass enhancement value $({Z}^{\ensuremath{-}1}\ensuremath{\simeq}3.6)$ than in Fe-As superconductors and enables us to separate an incoherent part from the spectrum. These features are quite consistent with the results of recent dynamical mean-field calculations, in which the incoherent part is attributed to the lower Hubbard band.