Electronic structure of single-crystalline Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ probed by x-ray absorption spectroscopy: Evidence for isovalent substitution of Fe$^{2+}$ by Co$^{2+}$

TL;DR

This study uses x-ray absorption spectroscopy to show that Co substitution in Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ is effectively isovalent, challenging the assumption of electron doping and highlighting the importance of hybridization and covalency in its electronic structure.

Contribution

The paper provides direct spectroscopic evidence that Co substitutes Fe as Co$^{2+}$ without changing valence, contrasting with the common electron-doping assumption.

Findings

Co substitution is isovalent, not electron-doping.

Fe valence remains unchanged across doping levels.

Hybridization and covalency influence the band structure.

Abstract

The substitutional dependence of valence and spin-state configurations of Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($x =$ 0, 0.05, 0.11, 0.17, and 0.38) is investigated with near-edge x-ray absorption fine structure at the $L_{2,3}$ edges of Fe, Co, and As. The present data provide direct spectroscopic evidence for an effectively isovalent substitution of Fe$^{2+}$ by Co$^{2+}$, which is in contrast to the widely assumed Co-induced electron-doping effect. Moreover, the data reveal that not only does the Fe valency remain completely unaffected across the entire doping range, but so do the Co and As valencies as well. The data underline a prominent role of the hybridization between (Fe,Co) 3$d_{xy}$, $d_{xz}$, $d_{yz}$ orbitals and As $4s/4p$ states for the band structure in $A$(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and suggest that the covalency of the (Fe,Co)-As bond is a key parameter for the interplay between magnetism and superconductivity.