Magnetization plateaus as insulator-superfluid transitions in quantum spinsystems

We study the magnetization process in two-dimensional S = 1/2 spin systems, to discuss the appearance of a plateau structure. The following three case s are considered: (1) the Heisenberg antiferromagnet and multiple-spin exch ange model on the triangular lattice, (2) the Shastry-Sutherland-type latti ce [which is a possible model for SrCu2(BO3)(2)], (3) the 1/5-depleted latt ice (for CaV4O9). We find in these systems that magnetization plateaus can appear owing to a transition from superfluid to a Mott insulator of magneti c excitations. The plateau states have charge-density-wave order of the exc itations. The magnetizations of the plateaus depend on components of the ma gnetic excitations, the range of the repulsive interaction, and the geometr y of the lattice.