### 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 |
---|---|

Article number | 117411 |

Journal | Science China: Physics, Mechanics and Astronomy |

Volume | 61 |

Issue number | 11 |

DOIs | |

Publication status | Published - Nov 1 2018 |

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### Keywords

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

### ASJC Scopus subject areas

- Physics and Astronomy(all)

### 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

**Structural phase transition, antiferromagnetism and two superconducting domes in LaFeAsO _{1-x}F_{x} (0 < x ≤ 0.75).** / Yang, Jie; Oka, Toshihide; Li, Zheng; Yang, Huai Xin; Li, Jian Qi; Chen, Gen Fu; Zheng, Guo-Qing.

Research output: Contribution to journal › Article

_{1-x}F

_{x}(0 < x ≤ 0.75)',

*Science China: Physics, Mechanics and Astronomy*, vol. 61, no. 11, 117411. https://doi.org/10.1007/s11433-018-9240-0

_{1-x}F

_{x}(0 < x ≤ 0.75). Science China: Physics, Mechanics and Astronomy. 2018 Nov 1;61(11). 117411. https://doi.org/10.1007/s11433-018-9240-0

}

TY - JOUR

T1 - Structural phase transition, antiferromagnetism and two superconducting domes in LaFeAsO1-xFx (0 < x ≤ 0.75)

AU - Yang, Jie

AU - Oka, Toshihide

AU - Li, Zheng

AU - Yang, Huai Xin

AU - Li, Jian Qi

AU - Chen, Gen Fu

AU - Zheng, Guo-Qing

PY - 2018/11/1

Y1 - 2018/11/1

N2 - We report 75As 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 Tc max = 27 K, and the second one at 0.25 ≤ x ≤ 0.75 with Tc 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 Ts 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 LaFeAsO0.97F0.03. In the coexisting region, 1/T1 decreases at Tc but becomes proportional to T below 0.6Tc, 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 xopt = 0.5-0.55 at which Ts extrapolates 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.

AB - We report 75As 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 Tc max = 27 K, and the second one at 0.25 ≤ x ≤ 0.75 with Tc 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 Ts 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 LaFeAsO0.97F0.03. In the coexisting region, 1/T1 decreases at Tc but becomes proportional to T below 0.6Tc, 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 xopt = 0.5-0.55 at which Ts extrapolates 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.

KW - antiferromagnetism

KW - nuclear magnetic resonance

KW - structural phase transition

KW - superconductivity

UR - http://www.scopus.com/inward/record.url?scp=85049327334&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85049327334&partnerID=8YFLogxK

U2 - 10.1007/s11433-018-9240-0

DO - 10.1007/s11433-018-9240-0

M3 - Article

AN - SCOPUS:85049327334

VL - 61

JO - Science China: Physics, Mechanics and Astronomy

JF - Science China: Physics, Mechanics and Astronomy

SN - 1674-7348

IS - 11

M1 - 117411

ER -