Probing the role of Co substitution in the electronic structure of iron-pnictides

TL;DR

This study investigates how cobalt substitution affects the electronic structure of CaFe2As2 using spectroscopy and theoretical calculations, revealing complex interactions that challenge simple models.

Contribution

It provides new insights into the electronic role of Co in iron-pnictides, showing that Co electrons participate in the Fermi surface and challenging the rigid-band shift approximation.

Findings

Co 3d states are at higher binding energy than Fe's by 250 meV.

Co electrons are part of the Fermi sea, influencing the Fermi surface.

Rigid-band shift models are inadequate for describing Co substitution effects.

Abstract

The role of Co substitution in the low-energy electronic structure of Ca(Fe$_{0.944}$Co$_{0.056}$)$_2$As$_2$ is investigated by resonant photoemission spectroscopy and density functional theory. The Co 3d-state center-of-mass is observed at 250 meV higher binding energy than Fe's, indicating that Co posses one extra valence electron, and that Fe and Co are in the same 2+ oxidation state. Yet, significant Co character is detected for the Bloch wavefunctions at the chemical potential, revealing that the Co 3d electrons are part of the Fermi sea determining the Fermi surface. This establishes the complex role of Co substitution in CaFe2As2, and the inadequacy of a rigid-band shift description.