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 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≤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.
@article{arxiv.1407.0746,
title = {The electronic structure of Co-substituted $\textrm{Fe}\textrm{Se}$ superconductor probed by soft X-ray spectroscopy and density functional theory},
author = {I. Perez and J. A. McLeod and R. J. Green and R. Escamilla and V. Ortiz and A. Moewes},
journal= {arXiv preprint arXiv:1407.0746},
year = {2014}
}