${}^{75}\mathrm{As}$ NMR Studies of Superconducting $\mathrm{LaFeAs}{\mathrm{O}}_{0.9}{\mathrm{F}}_{0.1}$

We have performed ${}^{75}\mathrm{As}$ nuclear magnetic resonance measurements on aligned powders of the new $\mathrm{LaFeAs}{\mathrm{O}}_{0.9}{\mathrm{F}}_{0.1}$ superconductor. In the normal state, we find a strong temperature dependence of the spin shift and Korringa behavior of the spin lattice relaxation rate. In the superconducting state, we find evidence for line nodes in the superconducting gap and spin-singlet pairing. Our measurements reveal a strong anisotropy of the spin lattice relaxation rate, which suggests that superconducting vortices contribute to the relaxation rate when the field is parallel to the $c$ axis but not for the perpendicular direction.

Figure 1

Frequency swept ${}^{75}\mathrm{As}$ NMR powder spectra of $\mathrm{LaFeAs}{\mathrm{O}}_{0.9}{\mathrm{F}}_{0.1}$ measured at $H_0=7.0494\mathrm{T}$. The thick solid red line is a simulation of the powder spectrum as described in the text.

Figure 2

${}^{75}\mathrm{As}$ NMR spectra of the oriented powder measured with ${\mathbf{H}}_0\perp c$ and $H_0=7.0494\mathrm{T}$.

Figure 3

(a) Magnetic shift $K_{ab}$ versus temperature in the aligned sample. The dashed line indicates $K_{\mathrm{orb}}$. (b) $K_{ab}$ versus ${\chi }_{\mathrm{powder}}$, the magnetic susceptibility of the powder sample. The solid line is a fit as described in the text.

Figure 4

(a) The ${}^{75}\mathrm{As}$ $(T_{1}T{)}^{-1}$ versus temperature in $\mathrm{LaFeAs}{\mathrm{O}}_{0.9}{\mathrm{F}}_{0.1}$ measured at the upper horn (■) ($\theta =90°$), the lower horn (blue ▵) ($\theta \approx 41.8°$), and in the aligned sample (red ○) ($\theta =90°$). The solid line is given by $K_s^2(T)/\alpha \kappa$ as described in the text. (b) $T_1^{-1}$ versus temperature. The arrow indicates $T_c=23.5\mathrm{K}$ at 7 T, and the solid line indicates $T_1^{-1}\sim T^3$, indicative of line nodes. Inset: Magnetization recovery and fits for two temperatures.