We report ${}^{75}\text{A}\text{s}$ nuclear quadrupole resonance studies on superconducting oxypnictide ${\text{LaFeAsO}}_{0.92}{\text{F}}_{0.08}$ $\left(T_c=23\text{K}\right)$. The temperature dependence of the spin-lattice relaxation rate $\left(1/T_1\right)$ decreases below $T_c$ without a coherence (Hebel-Slichter) peak and shows a temperature dependence that is not simple power law nor exponential. We show that the result can be understood in terms of two superconducting gaps of either $d$- or $\pm s$-wave symmetry, with the larger gap ${\Delta }_1\sim 4k_B{T}_c$ and the smaller one ${\Delta }_2\sim 1.5k_B{T}_c$. Our result suggests that the multiple-gap feature is universal in the oxypnictide superconductors, which is probably associated with the multiple electronic band structures in this class of materials. We also find that $1/T_{1}T$ above $T_c$ increases with decreasing temperature, which suggests weak magnetic fluctuations in the normal state.

• Received 9 October 2008

DOI:https://doi.org/10.1103/PhysRevB.78.220506