NONLOCAL UNITARY TRANSFORMATION AND HALDANE STATE IN S=1/2 ANTIFERROMAGNETIC LADDER MODEL

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 x 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.