Symmetry and light coupling phononic and collective magnetic excitations in SrCu_2(BO_3)_2

We perform a low temperature Raman scattering study of phononic and collective magnetic excitations in the spin dimer compound SrCu_2(BO_3)_2. Within a 4-spin cluster approximation and by including an effective intra-dimer Dzyaloshinskii-Moriya spin interaction we describe the symmetry selection rules and the behavior in magnetic fields of the spin gap multiplet branches around 24 cm-1. We identify two Raman coupling mechanisms responsible for the distinct resonance behavior of these magnetic modes. Our data also suggest an explanation for the existence of a strongly bound two-triplet state in the singlet sector which has an energy below the spin gap. We find intriguing phononic properties by analyzing the low temperature polarized spectra below 350 cm-1 which show the existence of quasi-degenerate excitations in spite of the fact that they involve different atomic vibrational patterns.