Near-degeneracy of several pairing channels in multiorbital models for the Fe-pnictides

Abstract

Weak-coupling approaches to the pairing problem in the iron pnictide superconductors have predicted a wide variety of superconducting ground states. We argue here that this is due both to the inadequacy of certain approximations to the effective low-energy band structure, and to the natural near-degeneracy of different pairing channels in superconductors with many distinct Fermi surface sheets. In particular, we review attempts to construct two-orbital effective band models, the argument for their fundamental inconsistency with the symmetry of these materials, and the comparison of the dynamical susceptibilities in two- and five-orbital models. We then present results for the magnetic properties, pairing interactions, and pairing instabilities within a five-orbital Random Phase Approximation model. We discuss the robustness of these results for different dopings, interaction strengths, and variations in band structure. Within the parameter space explored, an anisotropic, sign-changing s-wave state and a d_x2-y2 state are nearly degenerate, due to the near nesting of Fermi surface sheets.

17 pages, 23 figures

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• Origin of Gap Anisotropy in Spin Fluctuation Models of the Fe-pnictides
• Lifting of nodes by disorder in extended-$s$ state superconductors: application to ferropnictides
• Neutron Scattering Resonance and the Fe-pnictide Superconducting Gap
• Superfluid density and penetration depth in Fe-pnictides
• Effects of cobalt doping and three-dimensionality in $BaF{e}_{2}A{s}_2$
• Tight binding model for iron pnictides
• Theory of Raman response in a superconductor with extended s-wave symmetry: Application to Fe-pnictides
• Spectroscopic Imaging Scanning Tunneling Microscopy as a Probe of Orbital Structures and Ordering
• Normal State Spin Dynamics of Five-band Model for Iron-pnictides