Dynamical Correlations of the Spin-1/2 Heisenberg XXZ Chain in a   Staggered Field
  We consider the easy-plane anisotropic spin-1/2 Heisenberg chain in combined uniform longitudinal and transverse staggered magnetic fields. The low-energy limit of his model is described by the sine-Gordon quantum field theory. Using methods of integrable quantum field theory we determine the various components of the dynamical structure factor. To do so, we derive explicit expressions for all matrix elements of the low-energy projections of the spin operators involving at most two particles. We discuss applications of our results to experiments on one-dimensional quantum magnets. 
Igor Kuzmenko and Fabian H.L. Essler