Electronic structure of single-crystalline Sr(Fe1-xCox)2As2 probed by x-ray absorption spectroscopy: Evidence for effectively isovalent substitution of Fe2+ by Co2+

The substitutional dependence of valence and spin-state configurations of Sr(Fe1-xCox)(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,L-3 edges of Fe, Co, and As. The present data provide direct spectroscopic evidence for an effectively isovalent substitution of Fe2+ by Co2+, 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) 3d(xy), d(xz), d(yz) orbitals and As 4s/4p states for the band structure in A(Fe1-xCox)(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.