Magnon Exchange Mechanism of Superconductivity: ZrZn_2, URhGe

TL;DR

This paper develops a magnon exchange mechanism model for superconductivity in ferromagnetic materials like ZrZn2 and URhGe, explaining experimental observations and predicting specific heat behavior and phase transitions.

Contribution

It presents a novel superconductivity model based on magnon exchange that aligns with experiments and explains the absence of a quantum transition in certain ferromagnets.

Findings

Superconductivity confined to ferromagnetic phase in ZrZn2 and URhGe.

Specific heat varies linearly with temperature at low temperatures.

External magnetic field induces a first-order quantum phase transition.

Abstract

The magnon exchange mechanism of 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.