The electronic structure of Co-substituted $\textrm{Fe}\textrm{Se}$ superconductor probed by soft X-ray spectroscopy and density functional theory

TL;DR

This study investigates how Co substitution affects the structural, magnetic, and electronic properties of FeSe superconductors using spectroscopy and density functional theory, revealing a structural transition and changes in spin state and electron correlations.

Contribution

It provides new insights into the effects of Co doping on FeSe's structure, magnetism, and electronic correlations, advancing understanding of iron-chalcogenide superconductors.

Findings

Structural transition from tetragonal to hexagonal at x≈0.38

Fe in FeSe is high spin (S=2), reduced to low spin with Co doping

Co doping weakens electronic correlations for x≥0.25

Abstract

We study the crystalline and electronic properties of the $\textrm{Fe}_{1-x}\textrm{Co}_x\textrm{Se}$ 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 $x\approx0.38$. This structural transition in turn modifies the magnetic order which can be related to the spin state. Using resonant inelastic X-ray spectroscopy 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 $x \le 0.25$, 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 $x\ge0.25$. These novel findings are important to unravel the mechanisms responsible for the superconducting state in iron-chalcogenide superconductors.