Effect of a Staggered Magnetic Field on the S=1 Haldane Chain with   Single-ion Anisotropy
  We analyze the gaps in the excitation spectrum of a Haldane chain with single-ion anisotropy in a staggered field. We show that the gap along the direction of the field increases at a faster rate than the others, while its spectral weight decreases, being transferred to a two-magnon continuum. 
F. Anfuso, E. Ercolessi, G. Morandi and M. Roncaglia