Exploring multichannel superconductivity in ThFeAsN

Abstract

We investigate theoretically the superconducting state of the undoped Fe-based superconductor ThFeAsN. Using input from ab initio calculations, we solve the Fermi-surface based, multichannel Eliashberg equations for Cooper-pair formation mediated by spin and charge fluctuations, and by the electron-phonon interaction (EPI). Our results reveal that spin fluctuations alone, when coupling only hole-like with electron-like energy bands, can account for a critical temperature Tc up to ∼ 7.5 K with an s±-wave superconducting gap symmetry, which is a comparatively low Tc with respect to the experimental value T exp c = 30 K. Other combinations of interaction kernels (spin, charge, electronphonon) lead to a suppression of Tc due to phase frustration of the superconducting gap. We qualitatively argue that the missing ingredient to explain the gap magnitude and Tc in this material is the first-order correction to the EPI vertex. In the noninteracting state this correction adopts a form supporting the s± gap symmetry, in contrast to EPI within Migdal's approximation, i.e., EPI without vertex correction, and therefore it enhances tendencies arising from spin fluctuations.

Induced odd-frequency superconducting state in vertex-corrected Eliashberg theory

“…( 9)-( 11) exhibit richer phenomenology and more degrees of freedom than the standard theory, which only takes into account the lowest order Feynman diagram. For example, it has been shown by the current authors that superconductivity can be partially suppressed by vertex corrections [33], and can eventually lead to unconventional BZ symmetries of the pairing function [37,38].…”

“…Our results are enabled by the renormalized vertex function, see Refs. [33,37,38], which is closely associated with Fermi surface nesting conditions. This allows for a self-consistent and strongly anisotropic interaction, supporting both even-and odd-frequency superconducting gaps to coexist.…”