Entanglement in spin-1/2 dimerized Heisenberg systems
  We study entanglement in dimerized Heisenberg systems. In particular, we give exact results of ground-state pairwise entanglement for the four-qubit model by identifying a Z_2 symmetry. Although the entanglements cannot identify the critical point of the system, the mean entanglement of nearest-neighbor qubits really does, namely, it reaches a maximum at the critical point. 
Zhe Sun, XiaoGuang Wang, AnZi Hu, and You-Quan Li