S=3 2 ANTIFERROMAGNETIC SPIN CHAIN WITH BOND ALTERNATION/

The successive phase transitions of the Heisenberg spin chain with bon d alternation, H = Sigma(i=1)(L=1)[1-(-1)(i) delta]S-i . S-i+1, for th e spin S = 3/2 case under the open boundary condition are investigated by the density-matrix renormalization-group method proposed by White. Besides the Heisenberg point, we find that another gapless point exis ts at delta(c) = 0.42 +/- 0.02. This result qualitatively supports the field-theoretical prediction by Affleck and Haldane. However, our est imation of delta(c) deviates from the field-theoretical prediction, de lta(c) = 2/3. We also find a fourfold degeneracy of the ground state i n the region 0 < delta < delta(c) in the thermodynamic limit. Our resu lts show that the degeneracy is described by the edge excitations, whi ch is essentially similar to the one in the S = 1 case. We calculate t he string correlation function and find that its behavior is consisten t with the valence-bond-solid (VBS) picture.