Electronic properties of LaOFFeAs in the normal state probed by NMR/NQR

TL;DR

This study uses NMR and NQR techniques to investigate the electronic and magnetic properties of LaOFFeAs superconductors, revealing a uniform magnetic response and structural effects influencing the electric field gradient.

Contribution

It provides the first detailed NMR/NQR analysis of LaOFFeAs, showing a lack of q-space structure in spin susceptibility and highlighting the importance of structural changes.

Findings

Knight shift scales with susceptibility, indicating a single magnetic degree of freedom.

Spin lattice relaxation rates are correlated across nuclei, suggesting simple quasiparticle scattering.

Electric field gradient at As site is affected by structural changes, not captured by LDA calculations.

Abstract

We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on powders of the new LaO1-xFxFeAs superconductor for x = 0 and x = 0.1 at temperatures up to 480 K, and compare our measured NQR spectra with local density approximation (LDA) calculations. For all three nuclei in the x = 0.1 material, it is found that the local Knight shift increases monotonically with an increase in temperature, and scales with the macroscopic susceptibility, suggesting a single magnetic degree of freedom. Surprisingly, the spin lattice relaxation rates for all nuclei also scale with one another, despite the fact that the form factors for each site sample different regions of q-space. This result suggests a lack of any q-space structure in the dynamical spin susceptibility that might be expected in the presence of antiferromagnetic correlations. Rather, our results are more compatible with simple quasi-particle scattering. Furthermore, we find that the increase in the electric field gradient at the As cannot be accounted for by LDA calculations, suggesting that structural changes, in particular the position of the As in the unit cell, dominate the NQR response.