Influence of the Nd 4f states on the magnetic behavior and the electric field gradient of the oxypnictides superconductors NdFeAsO1-xFx

The structural, electronic, and magnetic properties of the superconducting NdFeAsO1-xFx phases (T-C=43 K for x=0.15) have been investigated experimentally by high-resolution synchrotron x-ray powder diffraction, magnetization and As-75 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 As-75 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 As-75 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.