The dynamical spin structure factor for the anisotropic spin-1/2   Heisenberg chain
  The longitudinal spin structure factor for the XXZ-chain at small wave-vector q is obtained using Bethe Ansatz, field theory methods and the Density Matrix Renormalization Group. It consists of a peak with peculiar, non-Lorentzian shape and a high-frequency tail. We show that the width of the peak is proportional to q^2 for finite magnetic field compared to q^3 for zero field. For the tail we derive an analytic formula without any adjustable parameters and demonstrate that the integrability of the model directly affects the lineshape. 
R. G. Pereira, J. Sirker, J.-S. Caux, R. Hagemans, J. M. Maillet, S.   R. White, I. Affleck