Anomalous Spin Excitation Spectrum of the Heisenberg Model in a Magnetic Field

Making the assumption that high energy fermions exist in the two dimensional spin-1/2 Heisenberg antiferromagnet we present predictions based on the pi-flux ansatz for the dynamic structure factor when the antiferromagnet is subject to a uniform magnetic field. The main result is the presence of gapped excitations in a momentum region near (pi,pi) with energy lower than that at (pi,pi). This is qualitatively different from spin wave theory predictions and may be tested by experiments or by quantum Monte Carlo.