SrCu2(BO3)2 – a two-dimensional spin liquid

We study an extended Shastry–Sutherland model for SrCu2(BO3)2 and analyze the low-lying parts of the energy spectrum by means of a perturbative unitary transformation based on flow equations. The derivation of the one-magnon dispersion (elementary triplets) is discussed. Additionally, we give a quantitative description (symmetries and energies) of bound states made from two elementary triplets. Our high-order results allow us to fix the model parameters for SrCu2(BO3)2 precisely: J1 =6.16(10) meV, x :=J2/J1=0.603(3), J=1.3(2) meV. To our knowledge this is the first quantitative treatment of bound states in a true 2D model.