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 Z2 symmetry. Although the entanglements cannot identify the critical point of the system, the mean entanglement of the nearest-neighbor qubits really does, namely, it reaches a maximum at the critical point.