Quantum-critical behavior of itinerant ferromagnets

TL;DR

This paper investigates the stability of quantum critical points in itinerant ferromagnets, revealing that long-range correlations induce non-analytic behavior in magnetic susceptibility, challenging the Hertz-Millis-Moriya theory and suggesting alternative transition types.

Contribution

It demonstrates that in dimensions less than or equal to three, long-range correlations cause non-analyticities that destabilize the traditional quantum critical point theory for itinerant ferromagnets.

Findings

Long-range correlations generate a negative non-analytic correction to magnetic susceptibility.

Hertz-Millis-Moriya theory becomes unstable in D ≤ 3 due to these effects.

Charge susceptibility remains analytic at the critical point.

Abstract

We study the stability of the Quantum Critical Point (QCP) for itinerant ferromagnets commonly described by the Hertz-Millis-Moriya (HMM) theory. We argue that in $D \leq 3$, long-range spatial correlations associated with the Landau damping of the order parameter field generate a universal {\it negative}, non-analytic $|q|^{(D+1)/2}$ contribution to the static magnetic susceptibility $\chi_s (q, 0)$, which makes HMM theory unstable. We argue that the actual transition is either towards incommensurate ordering, or first order. We also show that singular corrections are specific to the spin problem, while charge susceptibility remains analytic at criticality.