The electronic structure of BaFe(2)As(2) doped with Co, Ni and Cu has been studied by a variety of experimental and theoretical methods, but a clear picture of the dopant 3d states has not yet emerged. Herein we provide experimental evidence of the distribution of Co, Ni and Cu 3d states in the valence band. We conclude that the Co and Ni 3d states provide additional free carriers to the Fermi level, while the Cu 3d states are found at the bottom of the valence band in a localized 3d(10) shell. These findings help shed light on why superconductivity can occur in BaFe(2)As(2) doped with Co and Ni but not Cu.

We present density functional theory (DFT) calculations and soft x-ray spectra (soft x-ray spectroscopy and x-ray photoelectron spectra) measurements of single-crystal ${\text{CaFe}}_2{\text{As}}_2$. The experimental valence-band spectra are consistent with our DFT calculations. Both theory and experiment show that the $\text{Fe}3d$ states dominate the Fermi level and hybridize with $\text{Ca}3d$ states. The simple shape of x-ray photoelectron $\text{Fe}2p$ core level spectrum (without any satellite structure typical for correlated systems) suggests that the $\text{Fe}3d$ electrons are weakly or at most moderately correlated.

Natural 12CaO$·$7Al${}_2$O${}_3$ (C12A7) is a wide band gap insulator, but conductivity can be realized by introducing oxygen deficiency. Currently, there are two competing models explaining conductivity in oxygen-deficient C12A7, one involving the electron transfer via a “cage conduction band” inside the nominal band gap, the other involving electron hopping along framework lattice sites. To help resolve this debate, we probe insulating and conducting C12A7 with x-ray emission, x-ray absorption, and x-ray photoemission spectroscopy, which provide a full picture of both the valence and conduction band edges in these materials. These measurements suggest the existence of a narrow conduction band between the main conduction and valence bands common in both conducting and insulating C12A7 and support the theory that free electrons in oxygen-deficient C12A7 occupy the low-energy states of this narrow band. Our measurements are corroborated with density functional theory calculations.