HALDANE PHASE AND LARGE-D PHASE OF THE SPIN-1/2 FERROMAGNETIC-ANTIFERROMAGNETIC ALTERNATING HEISENBERG CHAIN WITH ON-SITE ANISOTROPY

We analytically study the boundary between the Haldane phase and the large-D (LD) phase of a ferromagnetic-antiferromagnetic alternating Heisenberg chain with on-site easy-plane anisotropy. We transform this model into a generalized alternating antiferromagnetic Heisenberg model with anisotropy. In the transformed model, the competition between the transverse and longitudinal alternations results in the Haldane-LD transition. Our Haldane-LD boundary calculated through the bosonization method agrees very well with that of Hida [Phys. Rev. B 46 (1992) 8268], determined by the numerical method.