Superstructures at magnetization plateaus in SrCu2(BO3)2

Magnetization plateaus observed in SrCu2(BO3)(2) are discussed theoretically. We map the two-dimensional Heisenberg model for SrCu2(BO3)(2) to a hard-core boson model and study first the latter in the classical limit and then compare the results with fully quantum ones for finite systems. Plateaus are expected at 1/2, 1/3, 1/4, 1/8,... of the full moment. Translational symmetry of the lattice is spontaneously broken at the plateaus except for the 1/2 plateau. The 1/3 and 1/4 plateaus have magnetic superstructures of stripe orders, while the others have square unit cells.