Two superconducting gaps in LaFeAsO0.92F0.08 revealed by 75As nuclear quadrupole resonance

Abstract

We report ${}^{75}$As nuclear quadrupole resonance (NQR) studies on superconducting oxypnictide LaFeAsO${}_{0.92}$F${}_{0.08}$ ($T_{\mathrm{c}}$ = 23 K). The temperature dependence of the spin lattice relaxation rate (1/$T_1$) decreases below $T_{\mathrm{c}}$ 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 ${\Delta }_1\sim 4k_{\mathrm{B}}{T}_{\mathrm{c}}$ and the smaller one ${\Delta }_2\sim 1.5k_{\mathrm{B}}{T}_{\mathrm{c}}$. Our result suggests that the multiple-gaps feature is universal in the oxypnictides superconductors, which is probably associated with the multiple electronic bands structure in this new class of materials. We also find that 1/$T_{1}T$ above $T_{\mathrm{c}}$ increases with decreasing temperature, which suggests weak magnetic fluctuations in the normal state.

4pages, 4figures, accepted for publication in Phys. Rev. B (rapid Communication)

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• Upper critical field, superconducting energy gaps, and Seebeck coefficient in La0.8Th0.2OFeAs
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• Effect of inelastic scattering on the nuclear magnetic relaxation rate 1/T1T in iron-based superconductors
• Critical fields and fluctuations determined from specific heat and magnetoresistance in the same nanogram SmFeAs(O,F) single crystal