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

The main features in iron-based superconductors would be (i) the orthorhombic transition accompanied by remarkable softening of shear modulus, (ii) high-T-c 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-phonon coupling constant (lambda similar to 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. (C) 2012 Elsevier Ltd. All rights reserved.