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

Abstract

Despite the fact that the Fe-based superconductors (FeSCs) were discovered nearly ten years ago, the community is still devoting a tremendous effort towards elucidating their relevant microscopic pairing mechanism(s) and interactions. At present, there is still no consistent interpretation of their normal state properties, where the strength of the electron-electron interaction and the role of correlation effects are under debate. Here, we examine several common materials and illustrate various problems and concepts that are generic for all FeSCs. Based on empirical observations and qualitative insight from density functional theory, we show that the superconducting and low-energy thermodynamic properties of the FeSCs can be described semi-quantitively within multiband Eliashberg theory. We account for an important high-energy mass renormalization phenomenologically, and in agreement with constraints provided by thermodynamic, optical, and angle-resolved photoemission data. When seen in this way, all FeSCs with T c < 40 K studied so far are found to belong to an intermediate coupling regime. This finding is in contrast to the strong coupling scenarios proposed in the early period of the FeSC history. We also discuss several related issues, including the role of band shifts as measured by the positions of van Hove singularities, and the nature of a recently suggested quantum critical point in the strongly hole-doped systems AFe 2 As 2 (A = K, Rb, Cs). Using high-precision full relativistic GGA-band structure calculations, we arrive at a somewhat milder mass renormalization in comparison with previous studies. From the calculated mass anisotropies of all Fermi surface sheets, only the ε-pocket near the corner of the BZ is compatible with the experimentally observed anisotropy of the upper critical field, pointing to its dominant role in the superconductivity of these three compounds.

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 …”

“…[107] 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. [108][109][110][111][112][113][114][115] In the remainder of this article we provide, in Section 2, a pedagogical overview of the emergence of odd-pairing in a simple two-band model due to interband hybridization, as well as a discussion of when we should expect to find odd-pairing in a generic superconducting system. Following the overview, we focus in Section 3 on specific proposals for realizing odd-pairing in well-known multiband superconductors, and also in materials and systems possessing related active electronic degrees of freedom, such as layer, dot, lead, or valley indices.…”

Mass Enhancements and Band Shifts in Strongly Hole-Overdoped Fe-Based Pnictide Superconductors: KFe2As2 and CsFe2As2

“…However, all these quantities are strongly affected by the presence of non-negligible many-body effects which become evident when compared with traditional density functional theory (DFT) based results, within the framework of the local density approximation (LDA) and the general gradient approximation (GGA) (for details see Ref. [1]) employed here as an effective "bare" description of the electronic structure, when ignoring the weak exchange and correlation effects implemented in present day DFT functionals. 2 There are two observations providing direct evidence for significant many-body effects in the strongly holedoped end members of the so-called 122 compounds AFe 2 As 2 with A = K, Rb, Cs: (i) the mass enhancement of various bands crossing the Fermi energy and (ii) a shifting of these bands resulting in different filling factors.…”

“…Rb-122 Cs-122 both, nodeless [12,13] and d x 2 −y 2 -nodal type [1,6,7,9,14] characters have been proposed.…”

A multiband Eliashberg‐approach to iron‐based superconductors

“…The octet‐gap behavior as reported for the Fermi surface sheet by laser ARPES measurements might be explained , if in the weakly gapped four nodal d‐wave regions a competing nonsuperconducting gap is opened (see Ref. ). Then new nodes at the border line of these competing orderings can occur.…”