Physics – Condensed Matter – Superconductivity
Scientific paper
2008-10-10
Phys. Rev. B 78, 220506(R) (2008)
Physics
Condensed Matter
Superconductivity
4pages, 4figures, accepted for publication in Phys. Rev. B (rapid Communication)
Scientific paper
10.1103/PhysRevB.78.220506
We report $^{75}$As nuclear quadrupole resonance (NQR) studies on superconducting oxypnictide LaFeAsO$_{0.92}$F$_{0.08}$ ($T_{\rm c}$ = 23 K). The temperature dependence of the spin lattice relaxation rate (1/$T_1$) decreases below $T_{\rm c}$ 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 $\Delta_1\sim 4 k_{\rm B}T_{\rm c}$ and the smaller one $\Delta_2 \sim 1.5 k_{\rm B}T_{\rm c}$. 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/$T_1T$ above $T_{\rm c}$ increases with decreasing temperature, which suggests weak magnetic fluctuations in the normal state.
Chen Guo-Feng
Kawasaki Shigeo
Luo Jian-Lin
Shimada Kenya
Wang Nan-Lin
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