title: Anomalous spin excitation spectrum of the Heisenberg model in a magnetic field.
abstract: 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.
authors: NORDITA, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark. sylju@nordita.dk
authors: Olav F
authors: OF
authors: Syljuåsen
authors: Patrick A
authors: PA
authors: Lee