Instability of antiferromagnetic magnons in strong fields
  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. 
M. E. Zhitomirsky (1,2) and A. L. Chernyshev (3) ((1) Department of   Physics, University of Toronto, (2) L. D. Landau Insti...