Collective Singlet Excitations and Evolution of Raman Spectral Weights in the 2D Spin Dimer CompoundSrCu2(BO3)2

Raman light scattering of the two-dimensional quantum spin system ${\mathrm{SrCu}}_2({\mathrm{BO}}_3{)}_2$ shows a rich structure in the magnetic excitation spectrum, 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 ${\mathrm{SrCu}}_2({\mathrm{BO}}_3{)}_2$ is the presence of strong interactions between well-localized triplet excitations in the network of orthogonal spin dimers realized in this compound.