Spin conservation and Fermi liquid near a ferromagnetic quantum critical point

TL;DR

This paper develops a comprehensive low-energy theory for itinerant fermions near a ferromagnetic quantum critical point, emphasizing the importance of additional interactions for spin conservation and predicting a spin nematic phase before ferromagnetic order.

Contribution

It introduces a new low-energy model including crucial interactions for SU(2) spin conservation and links Fermi liquid theory with loop expansion near a ferromagnetic quantum critical point.

Findings

Identification of an additional interaction term for spin conservation.

Demonstration of a consistent Fermi liquid description with loop expansion.

Prediction of a Pomeranchuk instability leading to a spin nematic phase.

Abstract

We propose a new low-energy theory for itinerant fermions near a ferromagnetic quantum critical point. We show that the full low-energy model includes, in addition to conventional interaction via spin fluctuations, another type of interaction, whose presence is crucial for the theory to satisfy SU(2) spin conservation. We demonstrate the consistency between a loop-wise expansion and a Fermi liquid description for the full model. We further show that, prior to the ferromagnetic instability, the system develops a Pomeranchuk-type instability into a state with zero magnetization but with p-wave deformations of the Fermi surfaces of spin-up and -down electrons (a spin nematic).