On the critical behavior of disordered quantum magnets: The relevance of rare regions

TL;DR

This paper investigates how rare regions caused by quenched disorder influence the critical behavior of itinerant quantum magnets, revealing instability in antiferromagnets and stability in ferromagnets due to long-range interactions.

Contribution

It introduces an effective theory incorporating non-perturbative effects of rare regions and analyzes their impact on critical fixed points in quantum magnets.

Findings

Rare regions destabilize the critical fixed point in antiferromagnets.

In ferromagnets, long-range interactions preserve the critical behavior.

No stable critical fixed point exists for antiferromagnets at one-loop order.

Abstract

The effects of quenched disorder on the critical properties of itinerant quantum antiferromagnets and ferromagnets are considered. Particular attention is paid to locally ordered spatial regions that are formed in the presence of quenched disorder even when the bulk system is still in the paramagnetic phase. These rare regions or local moments are reflected in the existence of spatially inhomogeneous saddle points of the Landau-Ginzburg-Wilson functional. We derive an effective theory that takes into account small fluctuations around all of these saddle points. The resulting free energy functional contains a new term in addition to those obtained within the conventional perturbative approach, and it comprises what would be considered non-perturbative effects within the latter. A renormalization group analysis shows that in the case of antiferromagnets, the previously found critical fixed point is unstable with respect to this new term, and that no stable critical fixed point exists at one-loop order. This is contrasted with the case of itinerant ferromagnets, where we find that the previously found critical behavior is unaffected by the rare regions due to an effective long-ranged interaction between the order parameter fluctuations.