Influence of the Nd $4f$ states on the magnetic behavior and the electric field gradient of the oxypnictides superconductors NdFeAsO${}_{1-x}$F${}_x$

The structural, electronic, and magnetic properties of the superconducting NdFeAsO${}_{1-x}$F${}_x$ phases ($T_C=43\mathrm{K}$ for $x=0.15$) have been investigated experimentally by high-resolution synchrotron x-ray powder diffraction, magnetization and ${}^{75}$As NMR measurements. Density-functional calculations were performed to calculate and analyze the electric field gradient and the density of states. Compared to LaFeAsO family, the NdFeAsO family shows a contraction of the lattice parameters with shorter rare-earth (RE)-As bond distances, an increased thickness of the As-Fe${}_2$-As layer, and less distorted Fe-As${}_4$ tetrahedra. The ${}^{75}$As quadrupole frequencies are enhanced with respect to the La analogs. This is due to a more prolate As $4p$ electron distribution mainly caused by the reduced lattice parameters and not by the presence of Nd $4f$ electrons. A non-negligible hyperfine coupling between the ${}^{75}$As nuclei and the Nd $4f$ states indicates a weak coupling between the REO and FeAs layer and possibly opens the channel for a Ruderman-Kittel-Kasuya-Yosida (RKKY)-type interaction between localized Nd $4f$ moments mediated by itinerant Fe 3d and/or Nd $5d$ states.