Electronic structure of Co-substituted FeSe superconductor probed by soft x-ray spectroscopy and density functional theory

We study the crystalline and electronic properties of the Fe1-x Co-x Se system (x = 0, 0.25, 0.5, 0.75, and 1.0) using x-ray diffraction, x-ray spectroscopy, and density functional theory. We show that the introduction of Co 3d states in FeSe relaxes the bond strengths and induces a structural transition from tetragonal to hexagonal whose crossover takes place at x approximate to 0.38. This structural transition in turn modifies the magnetic order, which can be related to the spin state. Using resonant inelastic x-ray scattering, we estimate the spin state of the system; FeSe is found to be in a high-spin state (S = 2), but Fe is reduced to a low-spin state upon Co substitution of x <= 0.25, well below the structural transition. Finally, we show evidence that FeSe is a moderately correlated system but the introduction of Co into the host lattice weakens the correlation strength for x >= 0.25. These novel findings are important to unravel the mechanisms responsible for the superconducting state in iron-chalcogenide superconductors.