Equivalence of the antiferromagnetic Heisenberg ladder to a singleS=1 chain

I introduce two continuous transformations between the S=1 Heisenberg chain and the antiferromagnetic S=1/2 Heisenberg ladder. Both transformations couple diagonally situated next-nearest-neighbor S=1/2's to form each S=1. Using the density matrix renormalization group, I demonstrate that the two systems are in the same phase. Furthermore, I find that the hidden topological long-range order characterizing the S=1 system is even stronger in the isotropic two-chain system. 1996 The American Physical Society.