Coherence Effect of Sign-Reversing $s_{\pm}$-Wave Cooper Pair State in Heavily Overdoped LaFeAsO-based Superconductor: 75As-Nuclear Quadrupole Resonance

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We report an 75As-nuclear quadrupole resonance (NQR) study on heavily electron-doped LaFeAsO1-xFx (La1111) with $T_{\text{c}}=5$ K. The measurement of the nuclear spin relaxation rate ($1/T_{1}$) has revealed that a Hebel–Slichter (HS) peak partially recovers in heavily electron-overdoped regimes where the nesting condition of hole and electron Fermi surfaces (FSs) becomes significantly worse. This is in contrast to previous results reported for optimally doped La1111 with $T_{\text{c}}=28$ K that there is a lack of the HS peak. These results indicate that the interband scattering between the hole and electron FSs is strongly suppressed by an almost vanishing hole FS because of the heavy electron-overdoping. Our findings strongly suggest that the sign reversal of the gap functions on different FSs, that is, the $s_{\pm}$-wave state, is realized in La1111 compounds. We remark that interband scattering on well-nested FSs is essential for stabilizing the $s_{\pm}$-wave state and enhancing $T_{\text{c}}$ up to 28 K in LaFeAsO-based superconductors.
2010-11-15