Angular dependence of x-ray absorption spectrum for field-aligned Fe-based superconductors*

Anisotropic Fe $K$-edge and As $K$-edge x-ray absorption near-edge spectrum measurements on superconducting ($T_c$=52 K) (Sm$_{0.95}$La$_{0.05}$)FeAs(O$_{0.85}$F$_{0.15}$) field-aligned microcrystalline powder are presented. The angular dependence of Fe pre-edge peak (dipole transition of Fe $1s$ electrons to Fe $3d$/As $4p$ hybrid bands) relative to the tetragonal $ab$ plane of aligned powder indicates larger density of state along the $c$ axis and is consistent with the local-density approximation band-structure calculation. The anisotropic Fe $K$-edge spectra exhibit a chemical shift to lower energy compared to FeO which are closely related to the itinerant character of Fe$^{2+}$ $3d^6$ orbitals. The anisotropic As $K$-edge spectra are more or less the mirror images of Fe $K$-edge due to the symmetrical Fe-As hybridization in the FeAs layer. Angular dependence of As main peak (dipole transition of As $1s$ electrons to higher-energy hybrid bands) was observed suggesting character of As $4d$ $e_g$ orbitals.