Exact demonstration of magnetization plateaus and first-order dimer-Neel phase transitions in a modified Shastry-Sutherland model for SrCu2(BO3)(2)

We study a generalized Shastry-Sutherland model for the material SrCu2(BO3) (2). Along a line in the parameter space, we show rigorously that the model has a first-order phase transition between dimerized and Neel-ordered grou nd states. Furthermore, when a magnetic field is applied in the dimerized p hase, magnetization plateaus develop at commensurate values of the magnetiz ation. We also discuss various aspects of the phase diagram and properties of this model away from this exactly soluble line, which include gap-closin g continuous transitions between dimerized and magnetically ordered phases.