Two superconducting gaps in LaFeAsO_0.92F_0.08 revealed by ⁷⁵As nuclear quadrupole resonance

We report A 75 s nuclear quadrupole resonance studies on superconducting oxypnictide LaFeAsO0.92 F0.08 (Tc =23 K). The temperature dependence of the spin-lattice relaxation rate (1/ T1) decreases below Tc 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 ±s -wave symmetry, with the larger gap Δ1 ∼4 kB Tc and the smaller one Δ2 ∼1.5 kB Tc. 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/ T1 T above Tc increases with decreasing temperature, which suggests weak magnetic fluctuations in the normal state.