Structural phase transition, antiferromagnetism and two superconducting domes in LaFeAsO1-xFx (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 ≤...

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Bibliographic Details
Published in:Science China. Physics, mechanics & astronomy mechanics & astronomy, 2018-11, Vol.61 (11), p.117411, Article 117411
Main Authors: Yang, Jie, Oka, Toshihide, Li, Zheng, Yang, HuaiXin, Li, JianQi, Chen, GenFu, Zheng, Guo-Qing
Format: Article
Language:English
Subjects:
Antiferromagnetism
Astronomy
Classical and Continuum Physics
Domes
Electrical resistivity
Electrons
Fluctuations
NMR
Nuclear magnetic resonance
Nuclear quadrupole resonance
Nuclear spin
Observations and Techniques
Phase diagrams
Phase transitions
Physics
Physics and Astronomy
Quadrupoles
Spin-lattice relaxation
Superconductivity
Transition temperature
Transition temperatures
Transmission electron microscopy
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Summary: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.
ISSN:1674-7348
1869-1927
DOI:10.1007/s11433-018-9240-0