Pairing Mechanism in Hunds Metal Superconductors and the Universality of the Superconducting Gap to Critical Temperature Ratio*

We propose a simple spin-fluctuation Eliashberg model that contains the physical ingredients of a Hund's metal, the local spin-fluctuations with super-linear correlators, (Ω₀/|Ω|)^γ with γ>1, interacting with electrons. Our model yields a universal superconducting gap to critical temperature ratio 2△_max/T_c≈7.2 that coincides with the experimental observation in a series of iron-based superconductors. We also discuss the implications of our model in the other classes of superconductors, e.g., cuprates and heavy fermion superconductors.