Effects of staggered magnetic field on entanglement in the anisotropic   XY model
  We investigate effects of staggered magnetic field on thermal entanglement in the anisotropic XY model. The analytic results of entanglement for the two-site cases are obtained. For the general case of even sites, we show that when the anisotropic parameter is zero, the entanglement in the XY model with a staggered magnetic field is the same as that with a uniform magnetic field. 
Zhe Sun, XiaoGuang Wang, and You-Quan Li