NMR/NQR investigation of superconductive ferropnictides

The recent discovery of superconductivity in the layered ferropnictides RO1-xFxFeAs (R=rare earth) has raised great interest within the solid state community. We have performed 139La, 57Fe, and 75As NMR and NQR measurements on the LaO1-xFxFeAs oriented powders at temperatures up to 480 K. For all three nuclei in the x=0.1 material, it is found that the local Knight shift increases monotonically with increasing temperature, and scales with the macroscopic susceptibility, suggesting a single magnetic degree of freedom. The spin lattice relaxation rate for all nuclei also scale with one another. 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. Our measurements reveal a strong anisotropy of the spin lattice relaxation rate, which suggest that superconducting vortices contribute to the relaxation rate when the field is parallel to the c-axis but not for the perpendicular direction. In the superconducting state, we find evidence for line nodes in the superconducting gap and spin- singlet pairing. This presentation was supported by FP7-REGPOT 229390 (Solid state NMR laboratory, Zagreb).