Finite-Field Ground State of the S=1 Antiferromagnetic-Ferromagnetic Bond-Alternating Chain

We investigate the finite-field ground state of the S = 1 antiferromagnetic-ferromagnetic bond-alternating chain described by the Hamiltonian H = Sigma(l){(S) over right arrow (2l-1) . (S) over right arrow (2l)+J (S) over right arrow (2l) . (S) over right arrow (2l-1)} + D Sigma(l)(S-l(z))(2)-II Sigma(l) S-l(z), where J <= 0 and -infinity < D < infinity. We find that two kinds of magnetization plateaux at a half of the saturation magnetization, the 1/2-plateaux, appear in the ground- state magnetization curve; one of them is of the Haldane type and the other is of the large-D-type. We determine the 1/2-plateau phase diagram on the D versus J plane, applying the twisted-boundary-condition level spectroscopy methods developed by Kitazawa and Nomura. We also calculate the ground-state magnetization curves and the magnetization phase diagrams by means of the density-matrix renormalization-group method.