### Abstract

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 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 ^{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 0.6T_{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 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.

Original language | English |
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Article number | 117411 |

Journal | Science China: Physics, Mechanics and Astronomy |

Volume | 61 |

Issue number | 11 |

DOIs | |

Publication status | Published - Nov 1 2018 |

### Keywords

- antiferromagnetism
- nuclear magnetic resonance
- structural phase transition
- superconductivity

### ASJC Scopus subject areas

- Physics and Astronomy(all)

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## Cite this

_{1-x}F

_{x}(0 < x ≤ 0.75).

*Science China: Physics, Mechanics and Astronomy*,

*61*(11), [117411]. https://doi.org/10.1007/s11433-018-9240-0