High-resolution, hard x-ray photoemission investigation ofBaFe2As2: Moderate influence of the surface and evidence for a low degree ofFe 3d–As 4phybridization of electronic states near the Fermi energy

Photoemission data taken with hard x-ray radiation on cleaved single crystals of the barium parent compound of the $M{\text{Fe}}_{2}{\text{As}}_{2}$ pnictide high-temperature superconductor family are presented. Making use of the increased bulk sensitivity upon hard x-ray excitation, and comparing the results to data taken at conventional vacuum ultraviolet photoemission excitation energies, it is shown that the ${\text{BaFe}}_{2}{\text{As}}_{2}$ cleavage surface provides an electrostatic environment that is slightly different to the bulk, most likely in the form of a modified Madelung potential. However, as the data argue against a different surface doping level, and the surface-related features in the spectra are by no means as dominating as seen in systems such as ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$, we can conclude that the itinerant, near-${E}_{F}$ electronic states are almost unaffected by the existence of the cleavage surface. Furthermore, exploiting the strong changes in photoionization cross section between the Fe and As states across the wide photon energy range employed, it is shown that the degree of energetic overlap between the iron $3d$ and arsenic $4p$ valence bands is particularly small at the Fermi level, which can only mean a very low degree of hybridization between the $\text{Fe}\text{ }3d$ and $\text{As}\text{ }4p$ states near and at ${E}_{F}$. Consequently, this means that the itinerancy of the charge carriers in this group of materials involves mainly the $\text{Fe}\text{ }3d\text{-Fe}\text{ }3d$ overlap integrals with at best a minor role for the $\text{Fe}\text{ }3d\text{-As}\text{ }4p$ hopping parameters and that the states which support superconductivity upon doping are essentially of $\text{Fe}\text{ }3d$ character.