The Coulomb problem in iron based superconductors

Abstract

We discuss the role of strong Coulomb interactions in iron-based superconductors (FeSCs). The presumed $s^{\pm }$ character of these superconductors means that the condensate is not symmetry protected against Coulomb repulsion. Remarkably, the transition temperatures and the excitation gap are quite robust across the large family of iron based superconductors, despite drastic changes in Fermi surface geometry. The Coulomb problem is to understand how these superconductors avoid the strong onsite Coulomb interaction at the iron atoms, while maintaining a robust transition temperature. Within the dominant space of $t_{2g}$ orbitals, on-site repulsion in the FeSCs enforces two linearly independent components of the condensate to vanish. This raises the possibility that iron-based superconductors might adapt their condensate to the Coulomb constraints by rotating the pairing state within the large manifold of entangled, extended s-wave gap functions with different orbital and momentum space structure. We examine this "orbital and k-space flexibility" (OKF) mechanism using both Landau theory and microscopic calculations within a multi-orbital t-J model. Based on our results, we conclude that OKF necessitates a large condensate degeneracy. One interesting possibility raised by our results, is that a resolution to the Coulomb problem in FeSC might require a reconsideration of triplet pairing.

8 pages main text, 4 pages appendix, 6 figures. Accepted in Phys. Rev. B

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• Preliminary Tc calculations for iron-based superconductivity in NaFeAs, LiFeAs, FeSe and nanostructured FeSe/SrTiO3 Superconductors
• The antiferromagnetic and phonon-mediated model of the NaFeAs, LiFeAs and FeSe superconductors
• Triplet pairing, orbital selectivity and correlations in Iron-based superconductors
• The interactions between antiferromagnetism, tetrahedral sites and electron-phonon coupling in FeSe1-xTex and FeSe/SrTiO3
• The influence of antiferromagnetism, soft out-of plane phonons and heavy electrons on the superconducting pairing mechanism of Ba1-xKxFe2As2