Quantum vs classical magnetization plateaus of S=1/2 frustrated Heisenberg chains

The competition between quantum and classical magnetization plateaus of S = 1/2 frustrated Heisenberg chains with modified exchange couplings is investigated. The conventional S = 1/2 frustrated Heisenberg chain is known to exhibit a 3-fold degenerate ↑↓↑ -type classical plateau at 1/3 of the saturation magnetization accompanied by the spontaneous Z3 translational symmetry breakdown. The stability of this plateau phase against period 3 exchange modulation which favors the ?-? ↑-type quantum plateau state (?-? = singlet dimer) is studied by bosonization, renormalization group and numerical diagonalization methods. The ground state phase diagram and the spin configuration in each phase are numerically determined. The translationally invariant Valence Bond Solid-type model with 4-spin and third neighbor interactions, which has the exact ?-? ↑-type quantum plateau state, is also presented. The phase transition to the classical ↑↓↑ -type ground state is also observed by varying the strength of 4-spin and third neighbor interactions. The relation between these two types of models with quantum plateau states is discussed.Copyright(c)2005 The Physical Society of Japa