Direct Observation of the Multiple Spin Gap Excitations in Two-Dimensional Dimer System SrCu2(BO3)2

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Abstract
Various spin-gap excitations have been observed in the two-dimensional dimer system SrCu 2 (BO 3 ) 2 by ESR. The zero-field energy gap of the lowest spin-gap excitation shows a splitting into two triplet modes and the energy splitting clearly depends on the magnetic field orientation. A zero-field splitting is also found between the S z =+1 and S z =-1 branches of each triplet. These behaviors are qualitatively explained by considering the anisotropic inter-dimer and intra-dimer exchange couplings. The averaged value of the lowest spin-gap energy is determined to be 722±2 GHz (34.7 K). We have also found a second spin-gap excitation at 1140 GHz (54.7 K), which indicates that the inter-dimer coupling is strong. Besides these modes, a number of gapped ESR absorptions are found and we propose that these excitations are caused by the localized nature of the excited state in the present system.
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