Haldane magnetization plateau and quantum phase transitions in the spin-(12, 1) XXZ chain

The magnetization and quantum phase transitions (QPTs) in a spin-(12, 1) XXZ chain under external magnetic field are investigated by the infinite time-evolving block-decimation method. A Δ−h magnetization phase diagram including three different ground-state phases, i.e., a ferromagnetic phase, a 13 plateau phase, and a spin-fluid phase, is determined. The Kosterlitz-Thouless (KT) point, where the 13 plateau phase vanishes, is determined to be (Δc=−0.53, hc=0.23). Except for the KT transition, all the other QPTs belong to the second-order category. By the logarithm behavior of the bipartite entanglement, the central charge in the whole critical spin-fluid phase is evaluated to be c=1. It is found that the Haldane mechanism (picture) can be used to understand the formation of the 13 plateau state in the present model. Strong enough longitudinal antiferromagnetic correlation is found to be necessary for the appearance of such a Haldane plateau.