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

TL;DR

This study investigates how Nd 4f states influence the structural, electronic, and magnetic properties of NdFeAsO$_{1-x}$F$_x$ superconductors, revealing effects on lattice parameters, electron distribution, and magnetic interactions.

Contribution

It provides new insights into the role of Nd 4f electrons in the magnetic behavior and electric field gradient of NdFeAsO$_{1-x}$F$_x$, combining experimental and computational analyses.

Findings

Shorter RE-As bonds compared to LaFeAsO.

Enhanced $^{75}$As quadrupole frequencies due to lattice contraction.

Weak coupling between Nd 4f states and the FeAs layer, suggesting RKKY interactions.

Abstract

The structural, electronic, and magnetic properties of the superconducting NdFeAsO$_{1-x}$F$_{x}$ phases ($T_C=43 {\rm K}$ 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.