Unusual disorder effects in superconducting ${\text{LaFeAs}}_{1-\delta }{\text{O}}_{0.9}{\text{F}}_{0.1}$ as revealed by ${}^{75}\text{A}\text{s}$ NMR spectroscopy

We report ${}^{75}\text{A}\text{s}$ nuclear magnetic resonance measurements of the spin-lattice relaxation in the superconducting (SC) state of ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ and As-deficient ${\text{LaFeAs}}_{1-\delta }{\text{O}}_{0.9}{\text{F}}_{0.1}$. The temperature behavior of $1/T_1$ below $T_c$ changes drastically from a $T^3$ dependence for ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ to a $T^5$ dependence for the As-deficient sample. These results, together with the previously reported unexpected increase in $T_c$ and the slope of the upper critical field near $T_c$ for the As-deficient sample, are discussed in terms of nonuniversal SC gaps in Fe pnictides and the effect of As deficiency as an exotic case where nonmagnetic “smart” impurities even stabilize an $s_{\pm }$-wave superconductor or within a scenario of a disorder-driven change to $s_{++}$ superconductivity.

Figure 1

(Color online) Recovery curves for $T=100\text{K}$ (red points), $T=22\text{K}$ (green diamonds), and $T=7\text{K}$ (blue squares) in $H_0=7.01\text{T}$. Normalization corresponds to a division by the prefactor $f\left(=1.7–2\right)$ of Eq. (1). The lines are examples for the different fitting functions containing a single $T_1$ component (solid line), a distribution around one $T_1$ component with a stretching parameter $\lambda$ (dashed line) and two components $T_{1\text{SC}}$ and $T_{1s}$ (dotted line).

Figure 2

(Color online)${}^{75}\text{A}\text{s}$ SLRR for ${\text{LaFeAs}}_{1-\delta }{\text{O}}_{0.9}{\text{F}}_{0.1}$ in 7.01 T (red squares) and 15.77 T (blue circles) compared to ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ in 7.05 T [gray crossed squares (Ref. 29), new data points for $T\le 4.2\text{K}$]. The dotted line illustrates the $T^3$ behavior of $T_1^{-1}$ for ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$, the solid line indicates the $T^5$ behavior observed for ${\text{LaFeAs}}_{1-\delta }{\text{O}}_{0.9}{\text{F}}_{0.1}$.

Figure 3

(Color online) Comparison of the ${}^{75}\text{A}\text{s}$ NSLRR for our ${\text{LaFeAs}}_{1-\delta }{\text{O}}_{0.9}{\text{F}}_{0.1}$ (red diamonds) with pure and Co-doped ${\text{LaFeAsO}}_{0.89}{\text{F}}_{0.11}$ from Ref. 15 (open and filled circles).