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

Abstract

We study the crystalline and electronic properties of the Fe1−xCoxSe 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≈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.

Source-reported materials — not catalogue approval

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