Ground-State Phase Diagram of the Spin 1/2 Alternating Heisenberg Chain with Anisotropy on the Antiferromagnetic Bond

The ground state of the alternating spin-1/2 Heisenberg chain is studied with anisotropy on the antiferromagnetic bond. This model reduces to the spin-1 XXZ chain if the isotropic bond is ferromagnetic and infinitely strong. The ground-state phase diagram is determined by the numerical diagonalization of finite-size systems. The results are consistent with the series expansion results by Yamanaka, Hatsugai and Kohmoto, and confirms the qualitative phase diagram predicted by Kohmoto and Tasaki.