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 ${\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Se}$ 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 $x\ensuremath{\approx}0.38$. 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 $x\ensuremath{\le}0.25$, 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 $x\ensuremath{\ge}0.25$. These novel findings are important to unravel the mechanisms responsible for the superconducting state in iron-chalcogenide superconductors.