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

The structural, electronic, and magnetic properties of the superconducting ${\text{NdFeAsO}}_{1-x}{\text{F}}_x$ phases ($T_C=43\text{K}$ for $x=0.15$) have been investigated experimentally by high-resolution synchrotron x-ray powder diffraction, magnetization and ${}^{75}\text{A}\text{s}$ 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 ${\text{As-Fe}}_2\text{-As}$ layer, and less distorted ${\text{Fe-As}}_4$ tetrahedra. The ${}^{75}\text{A}\text{s}$ quadrupole frequencies are enhanced with respect to the La analogs. This is due to a more prolate $\text{As}4p$ electron distribution mainly caused by the reduced lattice parameters and not by the presence of $\text{Nd}4f$ electrons. A non-negligible hyperfine coupling between the ${}^{75}\text{A}\text{s}$ nuclei and the $\text{Nd}4f$ 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 $\text{Nd}4f$ moments mediated by itinerant $\text{Fe}3d$ and/or $\text{Nd}5d$ states.

Figure 1

(Color online) Measured (○) and calculated (−) synchrotron x-ray powder diffraction ($\lambda =0.309952\text{Å}$, room temperature) profiles for NdFeAsO. The lower solid line shows the difference profile, and the tick marks show the reflection positions (top: NdFeAsO; bottom: ${\text{Nd}}_2{\text{O}}_3$ impurity). The inset illustrates the $T\text{−}O$ structural phase transition.

Figure 2

(a) Temperature dependence of the ZFC susceptibility measured in 10 kOe applied field for NdFeAsO powder. Inset: temperature dependence of the inverse ZFC susceptibility for NdFeAsO powder. (b) Temperature dependence of the ZFC magnetization for ${\text{NdFeAsO}}_{0.85}{\text{F}}_{0.15}$ with the superconducting transition temperature of 43 K and a shielding fraction of 40.6%. The magnetic field was set to 20 Oe.

Figure 3

Representative ${}^{75}\text{A}\text{s}$ NMR spectra measured in (a) the parent NdFeAsO and (b) the F-doped ${\text{NdFeAsO}}_{0.85}{\text{F}}_{0.15}$ powder samples. Inset: structure around the As ion showing its room-temperature distances to nearest-neighboring Fe (black spheres) and Nd ions (white spheres).

Figure 4

(Color online) Calculated $V_{zz}$ obtained from the virtual crystal approximation using the experimental As $z=0.65735\left(9\right)$ position (empty squares), the optimized As $z=0.6515$ (filled diamonds) position, and the ${\text{NdFeAsO}}_{0.85}{\text{F}}_{0.15}$ structure at room temperature (black circle marked by an arrow). The measured EFGs for the pure and the 15% F-doped compound are shown by error bars. Inset: dependence of ${}^{75}\text{A}\text{s}$ $V_{zz}$ on the $\text{As}z$ position. The optimized $\text{As}z$ position is marked by an arrow. The experimental $V_{zz}$ is represented by the shaded bar.

Figure 5

(Color online) Temperature dependence of the ${}^{75}\text{A}\text{s}$ NMR shift measured for undoped NdFeAsO (open circles) and F-doped ${\text{NdFeAsO}}_{0.85}{\text{F}}_{0.15}$ powder samples (solid circles). For comparison we show the corresponding shift measured in ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ (solid triangles) (Ref. 17). Inset: the ${}^{75}\text{A}\text{s}$ NMR line-shape simulations [Eq. (1)] for NdFeAsO sample above $\left(T=180\text{K}\right)$ and below $\left(T=80\text{K}\right)$ the AF ordering transition $T_N=140\text{K}$.

Figure 6

(Color online) Total density of states for LaFeAsO (shaded red) and NdFeAsO (full light blue line) using the experimental room-temperature structural data. The density of states of the fictitious NdFeAsO (labeled ${\text{NdFeAsO}}^{\text{La}}$, using set 1 in Table ) is shown by the dashed dark blue line. Inset: the density of states of the $\text{Fe}3d$ states for the same three structures.