Odd- and Even-Frequency Superconductivities Mediated by Ferromagnetic Magnons

Abstract

On the basis of a generalized Kondo-lattice model (i.e., spin-fermion model), we examine herein superconductivity mediated by magnetic excitations in ferromagnets. By applying the perturbation theory, we derive effective pairing interactions between conduction electrons. Pairing fluctuations are numerically calculated by fully accounting for the momentum and frequency dependences in quasi-one dimension. The results indicate that the normal state is unstable against even- and odd-frequency pairing fluctuations, depending on the temperature and model parameters. In a few cases, we obtain phase diagrams. The two-magnon exchange interaction is found to favor odd-frequency pairing at high temperatures (but below the ferromagnetic transition temperature); however, at low temperatures, triangle processes involving two magnons and one electron induce even- and odd-frequency pairings depending on the signs of the coupling constants of the Kondo interaction. Finally, we discuss the possibility of odd-frequency supe...