Far infrared study of the two dimensional dimer spin system SrCu_2(BO_3)_2
Abstract
Using far infrared spectroscopy in magnetic fields up to 12T we have studied a two dimensional dimer spin gap system SrCu_2(BO_3)_2. We found several infrared active modes in the dimerized state (below 10K) in frequency range from 3 to 100cm^{-1}. The measured splitting from the ground state to the excited triplet M_S=0 sublevel is \Delta_1=24.15cm^{-1} and the other two triplet state sublevels are 1.4cm^{-1} below and above the M_S=0 sublevel. Another multiplet is at \Delta_2=37.6cm^{-1} from the ground state. A strong transition to a singlet state is located at 52.0cm^{-1}. Detailed temperature dependence study of the 24cm^{-1} and 52cm^{-1} transitions shows evidence of a structural phase transition at 8K. All the transitions have Fano lineshape satellites at multiples of +-10cm^{-1} from the main lines. The lowest energy transition of the same kind at 14cm^{-1} may represent interdimer excitation transfer and the spin gap is thus 14cm^{-1} (20K) instead of 24cm^{-1}.Keywords
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