Orbital fluctuation theory in iron-based superconductors: s-wave superconductivity, structure transition, and impurity-induced nematic order

Abstract

The main features in iron-based superconductors would be (i) the orthorhombic transition accompanied by remarkable softening of shear modulus, (ii) high-Tc superconductivity close to the orthorhombic phase, and (iii) nematic transition in the tetragonal phase. In this paper, we present a unified explanation for them, based on the orbital fluctuation theory, considering both the e-ph and the Coulomb interaction. It is found that a small e-ph coupling constant ($\lambda 0.2$) is enough to produce large orbital (=charge quadrupole $O_{xz/yz}$) fluctuations, which causes the s-wave superconductivity without sign reversal ($s_{++}$-wave state). The derived orbital fluctuations also cause the instability toward the structure transition due to the bound state formation of two orbitons with opposite momenta, which is called the "two-orbiton process". Moreover, impurity-induced non-local orbital order with $C_2$-symmetry is obtained when the orbital fluctuations are strong. This "impurity-induced nematic state" explains the in-plane anisotropy of resistivity in detwinned samples. We stress that (i)-(iii) are reproducible only when orbital fluctuations with respect to $O_{xz}$ and $O_{yz}$ charge quadrupoles are the most divergent. This fact ensures the reliability of the present model Hamiltonian and calculation.

13 pages, 9 figures, an article in a special issue on Iron Pnictide Superconductors in Solid State Communications

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• Scaling between magnetic and lattice fluctuations in iron-pnictide superconductors
• Magnetic interactions in iron superconductors: A review
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• Emergence of Orbital Nematicity in the Tetragonal Phase of BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$
• Revisit of the Orbital-Fluctuation-Mediated Superconductivity in LiFeAs: Nontrivial Spin-Orbit Interaction Effects on the Bandstructure and Superconducting Gap Function
• Study of phase diagram and superconducting states in LaFeAsO${}_{1-x}$H${}_x$ based on the multiorbital extended Hubbard model
• Raman scattering as a probe of nematic correlations
• Spin-Fluctuation-Driven Orbital Nematic Order in Ru-Oxides: Self-Consistent Vertex Correction Analysis for Two-Orbital Model
• Strange Inter-layer Properties of $Ba(F{e}_{1-x}C{o}_x)2A{s}_2$ Appearing in Ultrasonic Measurements
• Diverse fluctuations and anisotropic Gr{\" u}neisen parameter behavior in iron-based superconductor Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ and their correlation with superconductivity
• Doping dependence of the anisotropic quasiparticle interference in NaFe1-xCoxAs iron-based superconductors
• Diverging Nematic Susceptibility, Physical Meaning of T* scale, and Pseudogap in the Spin Fermion Model for Pnictides
• Magnetic neutron scattering studies on the Fe-based superconductor system Fe${}_{1+y}$Te${}_{1-x}$Se${}_x$
• Isotropic Quenched Disorder Triggers a Robust Nematic State in Electron-Doped Pnictides
• Quasi-one Dimensional Nanostructures as Sign of Nematicity in Iron Pnictides and Chalcogenides
• Zigzag chain structure transition and orbital fluctuations in Ni-based superconductors
• Single 20meV boson mode in KFe2As2 detected by point-contact spectroscopy
• Slope of the upper critical field at $T_c$ in two-band superconductors with non-magnetic disorder: $s_{++}$ superconductivity in ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$
• Effect of inelastic scattering on the nuclear magnetic relaxation rate 1/T1T in iron-based superconductors