Mechanism of the high transition temperature for the 1111-type iron-based superconductors $R\mathrm{FeAsO}$ $(R=\mathrm{rare}\phantom{\rule{0.28em}{0ex}}\mathrm{earth})$: Synergistic effects of local structures and $4f$ electrons

Mechanism of the high transition temperature for the 1111-type iron-based superconductors $R FeAsO$ $(R = rare earth)$ : Synergistic effects of local structures and $4 f$ electrons

Lifang Zhang, Junling Meng, Xiaojuan Liu, Fen Yao, Jian Meng, and Hongjie Zhang

Phys. Rev. B 96, 045114 – Published 14 July 2017

Abstract

Among the iron-based superconductors, the 1111-type Fe-As–based superconductors $REFeAs O_{1 − x} F_{x}$ (RE = rare earth) exhibit high transition temperatures $(T_{c})$ above 40 K. We perform first-principles calculations based on density functional theory with the consideration of both electronic correlations and spin-orbit couplings on rare earths and Fe ions to study the underlying mechanism as the microscopic structural distortions in REFeAsO tuned by both lanthanide contraction and external strain. The electronic structures evolve similarly in both cases. It is found that there exist an optimal structural regime that will not only initialize but also optimize the orbital fluctuations due to the competing Fe-As and Fe-Fe crystal fields. We also find that the key structural features in REFeAsO, such as As-Fe-As bond angle, intrinsically induce the modification of the Fermi surface and dynamic spin fluctuation. These results suggest that the superconductivity is mediated by antiferromagnetic spin fluctuations. Simultaneously, we show that the rare-earth $4 f$ electrons play important roles on the high transition temperature whose behavior might be analogous to that of the heavy-fermion superconductors. The superconductivity of these 1111-type iron-based superconductors with high- $T_{c}$ is considered to originate from the synergistic effects of local structures and $4 f$ electrons.

Received 19 January 2017
Revised 21 June 2017

DOI:https://doi.org/10.1103/PhysRevB.96.045114

Physics Subject Headings (PhySH)

Superconductivity

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Lifang Zhang^1,2, Junling Meng¹, Xiaojuan Liu^1,*, Fen Yao^1,3, Jian Meng¹, and Hongjie Zhang¹

¹State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China
²University of Science and Technology of China, Hefei, Anhui 230026, China
³University of Chinese Academy of Sciences, Beijing, 10049, China

^*lxjuan@ciac.ac.cn

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Vol. 96, Iss. 4 — 15 July 2017

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Figure 1
(a) Crystal structure of REFeAsO with P4/nmm space group. (b) The checkerboard collinear antiferromagnetic order schematic of the Fe-As layer, the pointers represent the magnetic orientation of each Fe-ion and Fe1, Fe2 are labeled.
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Figure 2
(a) Lanthanide contraction dependence of the normalized lattice parameter (NLPM) variation $(a_{0} = 4.02752, c_{0} = 8.60606 Å)$ . (b) Varying pattern of the bond angle $(α_{(Fe − As − Fe)} β_{(As − Fe 1 − Fe 2)})$ with the lanthanide substitution. (c)–(e) The interatomic distances $(d_{(Fe − As)}, d_{(Fe − RE)}$ and $d_{(Fe − Fe)})$ changing tendencies along with the lanthanide contraction. (f)–(j) Structural parameters evolution with the external strain: (f) normalized LPMs, (g) bond angles, (h)–(j) interatomic distances.
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Figure 3
(a) The density of states (DOS) of parent compound LaFeAsO, the pink shadow highlights Fe1 total orbital distribution. (b) The partial DOS of $Fe 1 3 d$ electrons within LaFeAsO. (c, d) Each iron $3 d$ electrons $(n_{v})$ occupancy on the normal state as functions of the lanthanide contraction and the external strain. (e, f) The evolution of $N (E_{F})$ with the lanthanide contraction and external strain, in which $N (E_{F})$ means the DOS on the Fermi level; the light red area indicates the spin fluctuation region. (g, h) Dependencies of Fe1 and Fe2 magnetic moment on the lanthanide contraction and external strain, AFM fluctuation area is highlighted with the light red.
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Figure 4
(a) The electronic band structure of LaFeAsO, in which iron $3 d$ contributed bands are labeled by numbers. (b, c) The 2D and 3D Fermi-surface of LaFeAsO, high symmetric point M, $Γ$ and Z in the Brillouin zone are marked in the schematic. The figures are divided into two columns, the left column graphs show the effect of lanthanide contraction and the right ones demonstrate the one of external strain. (d, e) The mobile hole at $Γ$ -Z path change with structural distortion, the upper insert is the partial band structure (the color of each band is consistent with $H_{1}, H_{2}$ , and $H_{3})$ , and the bottom one is the magnified $H_{3}$ mobility. (f, g) The electron-mobility variation at M high symmetric point, the insert is the partial band structure in M point. (h, i) The transition ranges of hole-pocket and electron-pocket at Fermi surface, blue spheres indicate the volume of hole-pocket and green ones represent the electrons.
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Figure 5
(a) The magnetic order schematics of REFeAsO with the magnetic order of RE were set ferromagnetically coupled to Fe1. (b) The magnetic order of RE were set ferromagnetically coupled Fe2. (c, d) The charge density of Fe1 and Fe2 at the different magnetic directions of RE along Pr, Nd, Sm, Gd, and Tb. (e, f) The magnetic moments of Fe1 and Fe2 at the different magnetic directions of RE along Pr, Nd, Sm, Gd, and Tb.
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Figure 6
(a) Fe2- $3 d$ partial DOS in SmFeAsO obtained without the effect of $4 f$ electrons and (b) Fe2- $3 d$ partial DOS in SmFeAsO obtained with the effect of $4 f$ electrons. (c) DOS of Sm 4f orbitals in SmFeAsO obtained at $GGA + U + SOC$ level. (d) Fermi-surface of SmFeAsO without $4 f$ electrons. (e) Fermi-surface of SmFeAsO taking account of the $4 f$ electrons.
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Physical Review B

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Mechanism of the high transition temperature for the 1111-type iron-based superconductors $R FeAsO$ $(R = rare earth)$ : Synergistic effects of local structures and $4 f$ electrons

Lifang Zhang, Junling Meng, Xiaojuan Liu, Fen Yao, Jian Meng, and Hongjie Zhang

Phys. Rev. B 96, 045114 – Published 14 July 2017

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Physical Review B

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Mechanism of the high transition temperature for the 1111-type iron-based superconductors RFeAsO (R=rareearth): Synergistic effects of local structures and 4f electrons

Lifang Zhang, Junling Meng, Xiaojuan Liu, Fen Yao, Jian Meng, and Hongjie Zhang

Phys. Rev. B 96, 045114 – Published 14 July 2017

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Mechanism of the high transition temperature for the 1111-type iron-based superconductors $R FeAsO$ $(R = rare earth)$ : Synergistic effects of local structures and $4 f$ electrons