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

Jie Yang, Toshihide Oka, Zheng Li, Huai Xin Yang, Jian Qi Li, Gen Fu Chen, Guo-Qing Zheng

Research output: Contribution to journalArticle

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Abstract

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.

Original languageEnglish
Article number117411
JournalScience China: Physics, Mechanics and Astronomy
Volume61
Issue number11
DOIs
Publication statusPublished - Nov 1 2018

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antiferromagnetism
domes
superconductivity
phase diagrams
nuclear magnetic resonance
transmission electron microscopy
nuclear quadrupole resonance
spin-lattice relaxation
nuclear spin
transition temperature
electrical resistivity
coefficients
excitation
energy

Keywords

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

ASJC Scopus subject areas

  • Physics and Astronomy(all)

Cite this

Structural phase transition, antiferromagnetism and two superconducting domes in LaFeAsO1-xFx (0 < x ≤ 0.75). / Yang, Jie; Oka, Toshihide; Li, Zheng; Yang, Huai Xin; Li, Jian Qi; Chen, Gen Fu; Zheng, Guo-Qing.

In: Science China: Physics, Mechanics and Astronomy, Vol. 61, No. 11, 117411, 01.11.2018.

Research output: Contribution to journalArticle

Yang, Jie ; Oka, Toshihide ; Li, Zheng ; Yang, Huai Xin ; Li, Jian Qi ; Chen, Gen Fu ; Zheng, Guo-Qing. / Structural phase transition, antiferromagnetism and two superconducting domes in LaFeAsO1-xFx (0 < x ≤ 0.75). In: Science China: Physics, Mechanics and Astronomy. 2018 ; Vol. 61, No. 11.
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AU - Yang, Jie

AU - Oka, Toshihide

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AU - Li, Jian Qi

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AU - Zheng, Guo-Qing

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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.

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