Magnon-Mediated Superconductivity in Itinerant Ferromagnets

TL;DR

This paper explores magnon-mediated superconductivity in ferromagnetic metals, explaining phenomena in UGe_2, ZrZn_2, and URhGe through spin fluctuations and phase transitions, and predicts unique electronic and thermodynamic behaviors.

Contribution

It introduces a model for magnon-mediated triplet superconductivity in ferromagnets, explaining phase transitions and specific heat behavior in certain uranium-based compounds.

Findings

Magnon fluctuations can induce triplet pairing in ferromagnetic metals.

Superconductivity appears only within the ferromagnetic phase.

The model accounts for the linear low-temperature specific heat in UGe_2.

Abstract

The present paper discusses magnon-mediated superconductivity in ferromagnetic metals. The mechanism explains in a natural way the fact that the superconductivity in UGe_2, ZrZn_2 and URhGe is apparently confined to the ferromagnetic phase.The order parameter is a spin anti-parallel component ofa spin-1 triplet with zero spin projection. The transverse spinfluctuations are pair forming and the longitudinal ones are pair breaking.The competition between magnons and paramagnons explains the existence of two successive quantum phase transitions in UGe_2, from ferromagnetism to ferromagnetic superconductivity, and at higher pressure to paramagnetism. The maximum T_{SC} results from the suppression of the paramagnon contribution. To form a Cooper pair an electron transfers from one Fermi surface to the other. As a result, the onset of superconductivity leads to the appearance of two Fermi surfaces in each of the spin up and spin down momentum distribution functions. This fact explains the linear temperaturedependence at low temperature of the specific heat, and the experimental results for UGe_2.