Collective Magnetic Excitations in SrCu2(BO3)2

We showed in this chapter low temperature Raman data on phononic and magnetic excitations in SrCu2(BO3)2. Regarding the former, in the 0 to 350 cm-1 range we find several pairs of quasi-degenerate modes which have different symmetries. Group theoretical analysis suggests that the existence of these modes is related to quite different atomic vibrational pattern, i.e. in-plane and c-axis motions and as a result a quantitative investigation would be very interesting. Collective magnetic excitations were studied in terms of symmetry, resonance and coupling mechanisms in zero and applied magnetic fields. The analysis of the 4-spin cluster allows us to understand the group symmetries of the zero field Brillouin zone center spin gap branches around 24 cm-1 confirming the picture of local elementary one-triplet modes. By considering an additional intra-dimer DM interaction we are also able to understand the observed selection rules and intensity variations of the spin gap branches in external magnetic fields applied parallel or perpendicular to the dimer planes. These selection rules also require that the energy of the S = 0 two-triplet bound state made out of magnons confined within a unit cell is below the gap value suggesting a very high binding energy for this two particle excitation. The 4- spin cluster analysis fails to account for the two-triplet excitations which shows that they have contributions from the different parts of the reciprocal space. The existence of a set of four modes below the onset of two-triplet continuum, at 37.5, 40.8, 44.5 and 50.9 cm-1, in the A_2 symmetry channel, shows that further theoretical analysis is required in order to understand the nature of these composite excitations. Finally, we identified two effective magnetic light scattering Hamiltonians responsible for the coupling to the magnetic modes which allowed us to explain their resonance behavior.