Structural phase transition, antiferromagnetism and two superconducting domes in LaFeAsO1-xFx(0＜x≤0.75)

We report75As nuclear magnetic resonance(NMR)/nuclear quadrupole resonance(NQR) and transmission electron microscopy(TEM) studies on LaFeAsO1-xFx. There are two superconducting domes in this material. The first one appears at 0.03 ≤ x ≤0.2 with Tcmax= 27 K, and the second one at 0.25 ≤x≤0.75 with Tcmax= 30 K. By NMR and TEM, we demonstrate that a C4-to-C2 structural phase transition(SPT) takes place above both domes, with the transition temperature Tsvarying strongly with x. In the first dome, the SPT is followed by an antiferromagnetic(AF) transition, but neither AF order nor low-energy spin fluctuations are found in the second dome. By75As nuclear spin-lattice relaxation rate(1/T1) measurements, we find that AF order and superconductivity coexist microscopically in LaFeAsO0.97F0.03. In the coexisting region, 1/T1decreases at Tcbut becomes proportional to T below 0.6 Tc, indicating gapless excitations. Therefore, in contrast to the early reports, the obtained phase diagram for x ≤ 0.2 is quite similar to the doped BaFe2As2system. The electrical resistivity p in the second dome can be fitted by ρ = ρ0 + AT~n with n = 1 and a maximal coefficient A at around xopt = 0.5-0.55 at which Tsextrapolates to zero and Tc is the maximal, which suggests the importance of quantum critical fluctuations associated with the SPT. We have constructed a complete phase diagram of LaFeAsO1-xFx, which provides insight into the relationship between SPT, antiferromagnetism and superconductivity.