Collective Singlet Excitations and Evolution of Raman Spectral Weights   in the 2D Spin Dimer Compound SrCu2(BO3)2
  We present a Raman light scattering study of the two-dimensional quantum spin system SrCu2(BO3)2 and show that the magnetic excitation spectrum has a rich structure, including several well-defined bound state modes at low temperature, and a scattering continuum and quasielastic light scattering contributions at high temperature. The key to the understanding of the unique features of SrCu2(BO3)2 is the presence of strong interactions between well-localized triplet excitations in the network of orthogonal spin dimers realized in this compound. Based on our analysis of the Heisenberg model relevant for this material, we argue that the collective excitations involving two and three-particle singlet bound states have large binding energies and are observed as well-defined peaks in the Raman spectrum. 
Peter Lemmens, M. Grove, M. Fischer, G. Guntherodt (RWTH-Aachen),   Valeri N. Kotov (University of Florida), H. Kageyama, K....