Divergent spin conductivity on the verge of ferromagnetic quantum criticality

TL;DR

This paper investigates how spin conductivity diverges near a ferromagnetic quantum critical point due to critical spin fluctuations, suggesting incipient spin superfluidity.

Contribution

It introduces a spin transport theory capturing divergent fluctuations near quantum criticality, analogous to superconducting paraconductivity, and confirms its consistency with fundamental identities.

Findings

Spin conductivity diverges approaching the critical point.

The theory satisfies Ward identity and predicts vanishing spin stiffness.

Critical enhancement indicates incipient spin superfluidity.

Abstract

We show that the spin conductivity of a metal approaching a ferromagnetic quantum critical point exhibits divergent fluctuation corrections. This effect arises from critical spin fluctuations and constitutes a spin analog of the Aslamazov-Larkin theory of paraconductivity in superconductors. The spin current is derived in linear response within a Gaussian-level treatment of the effective action for a system with easy-plane magnetic anisotropy. We demonstrate the consistency of our spin transport theory by showing that it (i) fulfills the Ward identity and (ii) yields vanishing spin stiffness in the normal state. The critical enhancement of the spin conductivity is interpreted as incipient spin superfluidity in the quantum critical region. This is further supported by an intuitive picture based on the current-loop representation of the easy-plane ferromagnet.