Exploring Multichannel Superconductivity In Thfeasn

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 T-c up to similar to 7.5K with an s(+/-)-wave superconducting gap symmetry, which is a comparatively low T-c with respect to the experimental value T-c(exp) = 30K. Other combinations of interaction kernels (spin, charge, electron-phonon) 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 T-c in this material might be 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.