Magnon Exchange Mechanism of Ferromagnetic Superconductivity

TL;DR

This paper proposes a magnon exchange mechanism to explain ferromagnetic superconductivity, matching experimental data and revealing how superconductivity influences Fermi surfaces and specific heat behavior.

Contribution

It introduces a superconducting solution based on magnon exchange that aligns with experiments on ZrZn2 and URhGe, explaining the coexistence of ferromagnetism and superconductivity.

Findings

Superconductivity causes complex Fermi surfaces with small, simple-connected pieces.

Specific heat varies linearly with temperature at low temperatures, with a reduced coefficient in the superconducting phase.

No quantum transition from ferromagnetism to superconductivity occurs in weak ferromagnets like ZrZn2 and URhGe.

Abstract

The magnon exchange mechanism of ferromagnetic superconductivity (FM-superconductivity) was developed to explain in a natural way the fact that the superconductivity in $UGe_2$, $ZrZn_2$ and $URhGe$ is confined to the ferromagnetic phase.The order parameter is a spin anti-parallel component of a spin-1 triplet with zero spin projection. The transverse spin fluctuations are pair forming and the longitudinal ones are pair breaking. In the present paper, a superconducting solution, based on the magnon exchange mechanism, is obtained which closely matches the experiments with $ZrZn_2$ and $URhGe$. The onset of superconductivity leads to the appearance of complicated Fermi surfaces in the spin up and spin down momentum distribution functions. Each of them consist of two pieces, but they are simple-connected and can be made very small by varying the microscopic parameters. As a result, it is obtained that the specific heat depends on the temperature linearly, at low temperature, and the coefficient $\gamma=\frac {C}{T}$ is smaller in the superconducting phase than in the ferromagnetic one. The absence of a quantum transition from ferromagnetism to ferromagnetic superconductivity in a weak ferromagnets $ZrZn_2$ and $URhGe$ is explained accounting for the contribution of magnon self-interaction to the spin fluctuations' parameters. It is shown that in the presence of an external magnetic field the system undergoes a first order quantum phase transition.