Superconductivity via paramagnon and magnon exchange in a 2D near-ferromagnetic full metal and ferromagnetic half-metal

TL;DR

This paper investigates how superconductivity arises in a 2D electron system near ferromagnetic order, revealing the effects of paramagnon and magnon exchange on pairing mechanisms and transition temperatures.

Contribution

It provides a detailed theoretical analysis of superconductivity mediated by spin fluctuations in a 2D system, highlighting differences between paramagnetic and ferromagnetic phases.

Findings

Paramagnon propagator's momentum dependence reduces pairing temperature in the paramagnetic phase.

Ferromagnetic phase exhibits instant polarization of low-energy fermions.

Pairing temperature in ferromagnetic phase is a significant fraction of the Fermi energy.

Abstract

We study superconductivity in paramagnetic and ferromagnetically-ordered phases in a two-dimensional electron system with parabolic fermionic dispersion and short-range repulsive interaction. In the paramagnetic phase, we find that a weak momentum dependence of a paramagnon propagator parametrically reduces the onset temperature for the pairing compared to that in phenomenological theories which assume a strong dispersion of a paramagnon and also changes the topology of the gap function. In the ferromagnetic phase, we show that the order instantly polarizes low-energy fermionic excitations. We derive the fully renormalized pairing interaction between low-energy fermions, mediated by two transverse Goldstone modes and show that it is attractive in a spatially-odd channel. The pairing temperature in the ferromagnetic phase is found to be a fraction of the Fermi energy, significantly larger than in the paramagnetic phase near the transition. Our results are relevant for understanding superconductivity in proximity to itinerant ferromagnetism in multi-valley graphene systems, particularly the ones with full valley and spin polarization.