Numerical evidence for unstable magnons at high fields in the Heisenberg   antiferromagnet on the square lattice
  We find evidence for decaying magnons at strong magnetic field in the square lattice spin-1/2 Heisenberg antiferromagnet. The results are obtained using Quantum Monte Carlo simulations combined with a Bayesian inference technique to obtain dynamics and are consistent with predictions from spin wave theory. 
Olav F. Syljuasen