Antiferromagnetic spin fluctuations above the dome-shaped and full-gap superconducting states of LaFeAsO _1-xF _x revealed by As75-nuclear quadrupole resonance

We report a systematic study by As75 nuclear-quadrupole resonance in LaFeAsO _1-xF _x. The antiferromagnetic spin fluctuation found above the magnetic ordering temperature T _N=58K for x=0.03 persists in the regime 0.04≤x≤0.08, where superconductivity sets in. A dome-shaped x dependence of the superconducting transition temperature T _c is found, with the highest T _c=27K at x=0.06, which is realized under significant antiferromagnetic spin fluctuation. With increasing x further, the antiferromagnetic spin fluctuation decreases, and so does T _c. These features resemble closely the cuprates La _2-xSr _xCuO ₄. In x=0.06, the spin-lattice relaxation rate (1/T ₁) below T _c decreases exponentially down to 0.13T _c, which unambiguously indicates that the energy gaps are fully opened. The temperature variation of 1/T ₁ below T _c is rendered nonexponential for other x by impurity scattering.