Coexistence of ferromagnetism and superconductivity near quantum phase transition: The Heisenberg- to Ising-type crossover

TL;DR

This paper develops a mean-field theory for the coexistence of ferromagnetism and superconductivity near a quantum phase transition, analyzing the effects of spin fluctuation crossover and applying it to ZrZn2 and UGe2.

Contribution

It introduces a microscopic mean-field model incorporating spin fluctuation mechanisms and analyzes the phase crossover from Heisenberg to Ising types near quantum criticality.

Findings

Superconducting transition temperature depends strongly on exchange splitting.

Phase diagram shows crossover from isotropic to uniaxial spin fluctuations.

Application to ZrZn2 explains its coexistence phase and sheds light on UGe2.

Abstract

A microscopic mean-field theory of the phase coexistence between ferromagnetism and superconductivity in the weakly ferromagnetic itinerant electron system is constructed, while incorporating a realistic mechanism for superconducting pairing due to the exchange of critical spin fluctuations. The self-consistent solution of the resulting equations determines the superconducting transition temperature which is shown to depend strongly on the exchange splitting. The effect of phase crossover from isotropic (Heisenberg-like) to uniaxial (Ising-like) spin fluctuations near the quantum phase transition is analysed and the generic phase diagram is obtained. This scenario is then applied to the case of itinerant ferromagnet ZrZn2, which sheds light on the proposed phase diagram of this compound. Possible explanation of superconductivity in UGe2 is also discussed.