The 1D spin-1/2 AF-Heisenberg model in a staggered field
  We investigate the scaling properties of the excitation energies and transition amplitudes of the one-dimensional spin-$1\over 2$ antiferromagnetic Heisenberg model exposed to an external perturbation. Two types of perturbations are discussed in detail: a staggered field and a dimerized field. 
A. Fledderjohann, M. Karbach and K.-H. M\"utter