Far-infrared spectroscopy of spin excitations and Dzyaloshinskii-Moriya interactions in a Shastry-Sutherland compound SrCu2(BO3)$2
Abstract
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 intra-dimer dynamic DM along the c axis. The other is c-polarized and creates an intra-dimer 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 anti-cross 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 inter-dimer exchange interactions. Two static DM interactions are sufficient to describe the observed triplet state spectra. The static inter-dimer DM in the c-direction d1=0.7 cm-1 splits the triplet state sub-levels in zero field [C\'epas et al., Phys. Rev. Lett. 87, 167205 (2001)]. The static intra-dimer DM in the (ab) plane (perpendicular to the dimer axis) d2=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.
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