Two superconducting gaps inLaFeAsO0.92F0.08revealed byA75snuclear quadrupole resonance

Abstract

We report $^{75}\text{A}\text{s}$ nuclear quadrupole resonance studies on superconducting oxypnictide ${\text{LaFeAsO}}_{0.92}{\text{F}}_{0.08}$ $({T}_{c}=23\text{ }\text{K})$. The temperature dependence of the spin-lattice relaxation rate $(1/{T}_{1})$ 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 $\ifmmode\pm\else\textpm\fi{}s$-wave symmetry, with the larger gap ${\ensuremath{\Delta}}_{1}\ensuremath{\sim}4{k}_{B}{T}_{c}$ and the smaller one ${\ensuremath{\Delta}}_{2}\ensuremath{\sim}1.5{k}_{B}{T}_{c}$. 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.