A multiband Eliashberg‐approach to iron‐based superconductors

Abstract

Since the discovery of the Fe-based superconductors (FeSC) a lot of efforts has been applied to elucidate the microscopic mechanism behind cooper pairing and the symmetry of the order parameter. Examination of effective interactions of electrons with spin-fluctuations in these systems yield that couplings are not weak and the materials have to be considered in a strong coupling scheme. In the present article, we review some aspects of the description of iron-based superconductors in the frame of the Eliashberg approach for multiband systems. The application of the Eliashberg scheme is demonstrated with two compounds Ba0.6K0.4Fe2As2 and Ca0.32Na0.68Fe2As2. In both cases, it was found that these materials are in the intermediate coupling regime. Further, we consider effects of impurities on physical properties of multiband superconductors. It is demonstrated that the impurities affect nontrivially the superconducting properties. In particular, they may trigger a symmetry change of superconducting order parameters. This effect can be used for determination of the superconducting order parameter. Finally, we consider KFe2As2 and show that its thermodynamic properties are consistent with a nodal superconducting order parameter

The Role of Odd‐Frequency Pairing in Multiband Superconductors

“…Still, we here briefly mention that recent advances, coupling ab initio techniques and Eliashberg theory, have suggested the possibility of realizing odd‐ω superconducting gaps in the multiband superconductor MgB 2 , but only in the presence of a magnetic field . We also note that a growing body of evidence suggests that strong‐coupling effects can play an important role in some of the physics of multiband superconductors …”

Constraints on the total coupling strengthto bosons in the iron based superconductors

“…This concerns the characterization of the most strongly overdoped hole superconductors K‐122, Rb ‐122, and Cs ‐122 exhibiting either symmetry with accidental nodes at one of the ‐centered Fermi surface sheets or a protected d ‐wave symmetry . In the presence of increasing disorder and strong enough intraband el–ph and/or coupling to orbital fluctuations there can be a change in the symmetry of the SCOP from a sign ‐reversing to a sign ‐preserving character at a relatively high .…”

“…Our goal is to qualitatively discuss several general aspects pertaining to Eliashberg ‐based approaches for these materials, which has arisen from these projects. In another paper in this volume , we review related work examining how impurity scattering can induce changes in the symmetry of the SOCP in such models. Here, we focus on another issue, namely how measurements of renormalized thermodynamic quantities with respect to their corresponding “bare” values can be used to constrain the total electron–boson (el–b) coupling in exchange boson scenarios.…”

Probing pairing symmetry in multi-band superconductors by quasiparticle interference

“…In general, the single-particle correlation functions, including I ( q , ω ), in multiband systems with strong coupling interaction can be found by using the multiband extension of the Eliashberg theory 20–27 . The theory can be applied to Fe-based superconductors, since the Fermi surface of the moderately doped compounds consists of two or three relatively small and almost circular, hole-like pockets at Γ = (0, 0) and two elliptic electron pockets at M = ( π , 0) and (0, π ) points 28–31 .…”

“…Here, is the quasiparticle energy spectrum. To find the single particle gap function and the renormalization function Z ( ω ), we employ the Eliashberg approach 21,27 . The angle averaged F -function in Eq.…”