Far-infrared spectroscopy of spin excitations and Dzyaloshinskii-Moriya interactions in the Shastry-Sutherland compound SrCu2(BO3)2 -: art. no. 144417

We have studied spin excitation spectra in the Shastry-Sutherland model compound SrCu2(BO3)(2) in magnetic fields using far-infrared Fourier spectroscopy. The transitions from the ground singlet state to the triplet state at 24 cm(-1) and to several bound triplet states are induced by the electric field component of the far-infrared light. To explain the light absorption in the spin system we invoke a dynamic Dzyaloshinskii-Moriya (DM) mechanism where light couples to a phonon mode, allowing the DM interaction. Two optical phonons couple light to the singlet to triplet transition in SrCu2(BO3)(2). One is a-polarized and creates an intradimer dynamic DM along the c axis. The other is c-polarized and creates an intradimer dynamic DM interaction, it is in the (ab) plane and perpendicular to the dimer axis. Singlet levels at 21.5 and 28.6 cm(-1) anticross with the first triplet as is seen in far-infrared spectra. We used a cluster of two dimers with a periodic boundary condition to perform a model calculation with scaled intra- and interdimer exchange interactions. Two static DM interactions are sufficient to describe the observed triplet state spectra. The static interdimer DM in the c-direction d(1)=0.7 cm(-1) splits the triplet state sublevels in zero field [Cepas , Phys. Rev. Lett. 87, 167205 (2001)]. The static intradimer DM in the (ab) plane (perpendicular to the dimer axis) d(2)=1.8 cm(-1), allowed by the buckling of CuBO3 planes, couples the triplet state to the 28.6 cm(-1) singlet as is seen from the avoided crossing.