| Spin waves in ordered quantum antiferromagnets become unstable in a strong magnetic field with respect to spontaneous two-magnon decays. At zero temperature, the instability occurs between the threshold field H* and the saturation field Hs. 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 most of the Brillouin zone. |
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