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

TL;DR

This study uses $^{75}$As NQR to reveal two distinct superconducting gaps in LaFeAsO$_{0.92}$F$_{0.08}$, indicating a multiband superconductivity likely common in oxypnictides, with implications for understanding their pairing mechanisms.

Contribution

The paper demonstrates the existence of two superconducting gaps with different magnitudes and symmetries in LaFeAsO$_{0.92}$F$_{0.08}$, highlighting the multiband nature of superconductivity in oxypnictides.

Findings

Two superconducting gaps with different sizes and symmetries.

Absence of coherence peak in relaxation rate below $T_c$.

Weak magnetic fluctuations in the normal state.

Abstract

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.