Sensitivity of As K-edge absorption to rare earth (RE) doping in [formula omitted]: A first principles study

Abstract

Systematic density functional theory-based first principles studies on As K-edge electron energy loss near edge structures are presented for rare earth (RE)-doped iron-based superconducting materials Ca1−xRExFeAs2 (known as 112 compounds). The As K-edge electron energy losses near edge differ for two different types of As atoms (belonging to Fe–As plane and As2 chains between the planes). The As K-edge spectra (of both As atoms in chain and plane) are very sensitive to the nature of RE doping (RE = La, Ce, Pr, Nd, Sm, Gd). For example, the As K-edge shifts to higher energies successively with decreasing RE size (La → Gd), indicating larger binding energies and hence stability of the corresponding compound. In contrast, the white-line peak successively shifts to lower energies for La → Gd doping, resulting in complete suppression of another low energy absorption peak for Gd. These are in essence an artifact of heavy electron doping caused by RE atoms. Interrelationship between the electron doping and corresponding shift in the white-line feature for various RE-doped samples is established through Bader charge analysis. Each absorption spectra consists of several peaked features of particular significance; they are thoroughly analyzed in terms of the unoccupied partial density of states. To emphasize the influence of the “core-hole” effect, absorption spectra in the presence and absence of core-holes are presented separately. Relevance of our results with respect to experimental scenarios are also discussed.