Structural phase transition, antiferromagnetism and two superconducting domes in LaFeAsO 1- x F x (0 < x ≤ 0.75)

We report 75 As nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) and transmission electron microscopy (TEM) studies on LaFeAsO 1− x F 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 C 4-to- C 2 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 75 As nuclear spin-lattice relaxation rate (1/ T 1 ) measurements, we find that AF order and superconductivity coexist microscopically in LaFeAsO 0.97 F 0.03 . In the coexisting region, 1/ T 1 decreases at T c but becomes proportional to T below 0.6 T 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 BaFe 2 As 2 system. The electrical resistivity ρ in the second dome can be fitted by ρ = ρ 0 + AT n 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− x F x , which provides insight into the relationship between SPT, antiferromagnetism and superconductivity.