Boundaries, Cusps and Caustics in the Multimagnon Continua of 1D Quantum   Spin Systems
  The multimagnon continua of 1D quantum spin systems possess several interesting singular features that may soon be accessible experimentally through inelastic neutron scattering. These include cusps and composition discontinuities in the boundary envelopes of two-magnon continuum states and discontinuities in the density of states, "caustics", on and within the continuum, which will appear as discontinuities in scattering intensity. In this note we discuss the general origins of these continuum features, and illustrate our results using the alternating Heisenberg antiferromagnetic chain and two-leg ladder as examples. 
T. Barnes (ORNL Physics Division and Department of Physics and   Astronomy, University of Tennessee)