Superconductivity at the border of electron localization and itinerancy

Abstract

The superconducting state of iron pnictides and chalcogenides exists at the border of antiferromagnetic order. Consequently, these materials could provide clues about the relationship between magnetism and unconventional superconductivity. One explanation, motivated by the so-called bad metal behaviour of these materials proposes that magnetism and superconductivity develop out of quasi-localized magnetic moments that are generated by strong electron-electron correlations. Another suggests that these phenomena are the result of weakly interacting electron states that lie on nested Fermi surfaces. Here we address the issue by comparing the newly discovered alkaline iron selenide superconductors, which exhibit no Fermi-surface nesting, to their iron pnictide counterparts. We show that the strongcoupling approach leads to similar pairing amplitudes in these materials, despite their different Fermi surfaces. We also find that the pairing amplitudes are largest at the boundary between electronic localization and itinerancy, suggesting that new superconductors might be found in materials with similar characteristics.

Fermiology, orbital order, orbital fluctuations, and Cooper pairing in iron-based superconductors

“…The fact that after removing the hole pockets the symmetry of the gap function changes is reminiscent to the change of spin alignment from coplanar to collinear after removing a spin to unfrustrate the AF Heisenberg interaction on a triangle. In the literature [31,32] it is noted that there is a near degeneracy between the s ± and d x 2 −y 2 pairing symmetry for values of the interaction parameters yielding significant pairing strength. We claim this degeneracy is due to the pairing frustration discussed above.…”

Competing superconducting channels in iron pnictides from the strong coupling theory with biquadratic spin interactions

“…Neutron scattering measurements on these compounds observe a clear spin resonance mode in the superconducting state [28][29][30][31] . These are consistent with an s ± pairing channel, with the pairing order parameter ∆ s± = ∆ 0 cos(k x ) cos(k y ) changing sign between the hole pockets near the Brillouin zone (BZ) center and the electron pockets near the zone corner, which arises within both weak-coupling and strong-coupling approaches 4,[32][33][34][35][36][37][38][39] . Recent experiments find evidences of anisotropic or even nodal superconducting gaps on several iron pnictide compounds in either underdoped or heavily doped regimes.…”

“…For superconducting pairing, several calculations based on five-orbital models have been done. 4,32,37,39,54 To see how accurate is the superconducting phase diagram we obtained in Sec. III for the two-orbital model, we compare our results with the strong-coupling phase diagram of a five-orbital t − J 1 − J 2 model in Ref.…”

“…Additionally, the d xz and d yz orbitals carry most of the spectral weight 52 and the d xy orbital can thus be neglected in the first approximation. It is true that in order to obtain all the Fermi pockets observed in ARPES, one needs to consider all 5 Fe orbitals 32 , and one of us has studied superconductivity in such a 5-orbital t−J 1 −J 2 model 39,54 . However, inclusion of the biquadratic K term would be highly non-trivial for such a model and for reasons of transparency of the analysis, we chose to focus on the two-orbital model first, with the hope that our central results should remain valid upon inclusion of other orbitals.…”

Unified spin model for magnetic excitations in iron chalcogenides

“…Our starting point is the strong-coupling approach, justified in the limit when Coulomb interaction U is considerably larger than the electron hopping t. Although the iron chalcogenides are not charge insulating systems, the strong-coupling approaches have been successfully used to elucidate many aspects of these materials, from the nature of electron nematicity [26,28,39] and effects of orbital selectivity [53][54][55][56], to the origin of the superconducting pairing [57][58][59][60][61]. One of the justifications for using the strong-coupling approach is the large fluctuating iron moment observed in inelastic neutron scattering (M 2 eff ≈ 5μ 2 B per Fe ion [62]), which is difficult to obtain in the weak-coupling scenario from considering only the electrons near the Fermi surface.…”