HIDDEN ORDER AND DIMERIZATION TRANSITION IN S=2 CHAINS

We study ground state properties of the S = 2 quantum antiferromagneti c chain with a bond alternation H = Sigma(j)[1 + delta(-1)(j)]S-j . S- j+1 by a Quantum Monte Carlo calculation. We find that the hidden Z(2) x Z(2) symmetry is broken for 0.3 < /delta/ < 0.5 while it is unbroke n in the other regions. This confirms the successive dimerization tran sitions first predicted by Affleck and Haldane. Our result shows that these transitions can be understood in terms of the hidden Z(2) X Z(2) symmetry breaking, as was discussed using the Valence-Bond-Solid stat es. Furthermore, we find that the behavior of the generalized string c orrelation is qualitatively very similar to that in the Valence-Bond-S olid states, including the location of zeroes as a function of the ang le parameter.