  We report an 75As-nuclear quadrupole resonance (NQR) study on heavily
electron-doped LaFeAsO_1-xF_x(La1111) with Tc=5 K. Nuclear spin relaxation rate
(1/T1) measurement 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 in optimally doped La1111 with Tc=28 K
where a lack of the HS peak was reported. It indicates that the interband
scattering between the hole and electron FSs is strongly suppressed by an
almost vanishing hole FS through the heavily electron-overdoping. Our findings
strongly suggest that the sign reversal of the gap functions on the different
FSs, that is, s(+-)-wave state is realized in La1111 compounds. We remark that
interband scattering on well-nested FSs is essential for stabilizing the
s(+-)-wave state and enhancing the Tc up to 28 K in LaFeAsO-based
superconductors.
