Direct Observation of the Multiple Spin Gap Excitations in Two-Dimensional Dimer System SrCu_<b>2</b>(BO_<b>3</b>)_<b>2</b>

Abstract

Various spin gap excitations have been observed in the two-dimensional dimer\nsystem SrCu_2(BO_3)_2 by means of submillimeter wave ESR. The zero-field energy\ngap of the lowest spin gap excitation shows a splitting into two triplet modes\nand the energy splitting clearly depends on the magnetic field orientation when\na field is rotated in the {\\mib {ac}}-plane. A zero-field splitting is also\nfound between the S(_z)=+1 and S(_z)=-1 branches of each triplet. These\nbehaviors are qualitatively explained by considering the anisotropic exchange\ncoupling of inter-dimer and intra-dimer, respectively. The averaged value of\nthe lowest spin gap energy is determined to be 722 \\pm 2 GHz(34.7 K). We have\nalso found the second spin gap excitation at 1140 GHz(54.7 K), which indicates\nthat the inter-dimer coupling is significantly strong. Besides these modes, a\nnumber of gapped ESR absorption are found and we propose that these multiple\nmagnetic excitations are caused by the localized nature of the excited state in\nthe present system.\n