Magnetization plateaus of the Shastry-Sutherland model for SrCu2(BO3)(2): Spin-density wave, supersolid, and bound states

We study the Heisenberg antiferromagnet on the Shastry-Sutherland lattice u nder magnetic fields to clarify the magnetic properties of SrCu2(BO3)(2). T reating magnetic excitations promoted by the field as Bose particles and us ing strong-coupling expansion, we derive an effective Hamiltonian for the e ffective magnetic particles. Anisotropic repulsive interactions between eff ective particles induce "insulating" states with a stripe spin-density wave (SDW) structure at magnetization m/m(sat)= 1/3 and a checkerboard structur e at 1/2, and thereby form magnetization platt aus. Supersolid phases appea r around insulating SDW phases by changing the magnetic field. The nature o f these supersolid phases is discussed in detail. We also demonstrate how t he geometry of the Shastry-Sutherland lattice affects dynamical properties of magnetic excitations significantly and makes quintuplet (S=2) bound stat es condense for very small magnetization.