Magnetization plateaux and bound states of the Shastry–Sutherland model for SrCu2(BO3)2

In this paper, we briefly review our recent studies of the Heisenberg anti-ferromagnet on the Shastry–Sutherland lattice for SrCu2(BO3)2 under magnetic fields. Treating magnetic excitations promoted by the field as Bose particles and using a strongcoupling expansion, we derived an effective Hamiltonian for the magnetic particles. Anisotropic repulsive interactions between particles induce "insulating" states with a stripe spin-density wave structure at magnetization m/msat=1/3 and with a checkerboard structure at 1/2, and thereby form magnetization plateaux. We also demonstrated how the unique geometry of the Shastry–Sutherland lattice affects dynamical properties and makes a novel type of quintuplet (S=2) bound states condense for very small magnetization.