Instability of antiferromagnetic magnons in strong fields

TL;DR

This paper predicts that in strong magnetic fields, antiferromagnetic spin-waves become unstable due to spontaneous two-magnon decays, leading to the disappearance of single-magnon excitations in a square lattice Heisenberg antiferromagnet.

Contribution

It introduces the concept of magnon instability in strong fields and demonstrates this phenomenon through analysis of a square lattice Heisenberg antiferromagnet.

Findings

Single-magnon branch disappears in most of the Brillouin zone.

Instability occurs between threshold field H* and saturation field Hc.

Magnon decay leads to the breakdown of single-particle excitations.

Abstract

We predict that spin-waves in an ordered quantum antiferromagnet (AFM) in a strong magnetic field become unstable with respect to spontaneous two-magnon decays. At zero temperature, the instability occurs between the threshold field $H^*$ and the saturation field $H_c$. As an example, we investigate the high-field dynamics of a Heisenberg antiferromagnet on a square lattice and show that the single-magnon branch of the spectrum disappears in the most part of the Brillouin zone.