s-wave superconductivity with orbital-dependent sign change in checkerboard models of iron-based superconductors

Abstract

We study three different multi-orbital models for iron-based superconductors (iron-SCs) in the solvable limit of weakly coupled square plaquettes. The strongest superconducting (SC) pairing is in the A1g s-wave channel and its development is correlated with the emergence of the next-nearestneighbour antiferromagnetism (NNN-AFM). For the models with more than three orbitals, this study suggests that the signs of the intra-orbital pairing order parameters of the dxy and the dxz (or dyz) orbitals must be opposite. Such sign difference stems from the intrinsic symmetry properties of inter-orbital hoppings and might, ultimately, lead to the sign-change of the SC orders between the hole Fermi pockets at the Γ point and produce anisotropic or even gapless SC gaps in the electron Fermi pockets around the M point in reciprocal space, as restoring back to the homogeneous limit.

Robustness ofs-Wave Pairing in Electron-OverdopedA1−yFe2−xSe2Fang1, Wu2, Thomale3 et al. 2011Phys. Rev. XSelf Cite9751141View full textAdd to dashboardCite

“…It is, however, different from the result in the very-strong-coupling limit in Ref. [57], where the strong interorbital repulsion favors different signs between the d xy intraorbital and the d xz; d yz intraorbital pairings. Some quantitative differences between Figs.…”

S4Symmetric Microscopic Model for Iron-Based Superconductors

“…In contrast, in a multiorbital model, many theoretical calculations based on weak-coupling approaches have shown that the gap functions are very sensitive to detailed band structures and vary significantly when the doping changes [6,[24][25][26][27][28]. The robustness of the form factor has therefore been argued to favor strong-coupling approaches, which emphasize electron-electron correlation or the effective next-nearest-neighbor (NNN) antiferromagnetic (AF) exchange coupling J 2 [29][30][31][32][33][34][35] as a primary source of the pairing force. However, realistically, it is very difficult to imagine that such a local exchange interaction remains identical between all d-orbital electrons if a multiple-d-orbital model is considered.…”

Magnetic excitations in iron chalcogenide superconductors

“…The disappearance of hole pockets at the point yields the exclusion of s ± wave for the SC symmetry, because the nesting vector is modified from ( , 0) (unfolded Brillouin zone notation in this section). The nodeless d wave and nodal d wave have been proposed by a nesting scenario [11][12][13][14][15], while the robustness of s-wave has been pointed out by a strong-coupling scenario taking into account the next-nearest neighbor exchange interaction [16][17][18]. On the other hand, an s ++ wave mediated by the orbital fluctuations is another candidate [14].…”

“…Such variety in materials is a characteristic of Fe-based superconductors. Their superconductivity mechanism is still controversial; the spin-fluctuation scenario yields s ± symmetry in most of Fe-based superconductors [7][8][9][10]; however, for instance in K x Fe 2−y Se 2 , different superconducting (SC) symmetries have been proposed by different theoretical approaches [11][12][13][14][15][16][17][18]. On the other hand, the orbital-fluctuation scenario gives s ++ symmetry [19,20].…”