Nonlocal Unitary Transformation and Haldane State in S=1⁄2 Antiferromagnetic Ladder Model

Abstract

The Haldane state is investigated in the S=1⁄2 antiferromagnetic (AF) ladder model for open chains with the help of a nonlocal unitary transformation. It is shown that the transformation works well, as in the case of S=1 AF models, so that it transforms the model into a ferromagnetic model with Z₂×Z₂ symmetry, and four degenerate Haldane states are obtained by a variational calculation in a molecular field approximation. The S=1⁄2 edge states are examined in these Haldane states.