Hidden Z2xZ2 symmetry breaking in Haldane-gap antiferromagnets

We show that the Haldane phase of the S=1 antiferromagnetic chain is closely related to the breaking of a hidden Z2×Z2 symmetry. When the chain is in the Haldane phase, this Z2×Z2 symmetry is fully broken, but when the chain is in a massive phase other than the Haldane phase, e.g., the Ising phase or the dimerized phase, this symmetry is broken only partially or not at all. The hidden Z2×Z2 symmetry is revealed by introducing a nonlocal unitary transformation of the chain. This unitary transformation also leads to a simple variational calculation which qualitatively reproduces the phase diagram of the S=1 chain.