Structural phase transition, antiferromagnetism and two superconducting domes in LaFeAsO_1-xF_x (0 < x ≤ 0.75)

We report ⁷⁵As nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) and transmission electron microscopy (TEM) studies on LaFeAsO_1−xF_x. There are two superconducting domes in this material. The first one appears at 0.03 ≤ x ≤ 0.2 with T_c ^max = 27 K, and the second one at 0.25 ≤ x ≤ 0.75 with T_c ^max = 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 T_s varying 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. By ⁷⁵As nuclear spin-lattice relaxation rate (1/T₁) measurements, we find that AF order and superconductivity coexist microscopically in LaFeAsO_0.97F_0.03. In the coexisting region, 1/T₁ decreases at T_c but becomes proportional to T below 0.6T_c, indicating gapless excitations. Therefore, in contrast to the early reports, the obtained phase diagram for x ≤ 0.2 is quite similar to the doped BaFe₂As₂ system. The electrical resistivity ρ in the second dome can be fitted by ρ = ρ₀ + ATⁿ with n = 1 and a maximal coefficient A at around x_opt = 0.5-0.55 at which T_s extrapolates to zero and T_c is the maximal, which suggests the importance of quantum critical fluctuations associated with the SPT. We have constructed a complete phase diagram of LaFeAsO_1−xF_x, which provides insight into the relationship between SPT, antiferromagnetism and superconductivity.