Spin-triplet excitons in the S = 1/2 gapped antiferromagnet BaCuSi₂O₆: Electron paramagnetic resonance studies

BaCuSi2O6, a S = 1/2 quantum antiferromagnet with a double-layer structure of Cu²⁺ ions in a distorted planar-rectangular coordination and with a dimerized spin singlet ground state, has been studied by means of the electron paramagnetic resonance technique. It is argued that multiple absorptions observed at low temperatures are intimately related to a thermally activated spin-triplet exciton superstructure, which appears to be a characteristic feature of low-dimensional anisotropic spin- 1/2 systems with the dimerized spin-singlet ground state. We showed that analysis of the angular dependence of exciton modes could be used for accurate estimation of anisotropy parameters in BaCuSi₂O₆; the procedure can be applied for a large number of S = 1/2 quantum antiferromagnets. In addition, the temperature dependence of EPR intensity and linewidth in BaCuSi₂O₆ has been studied and discussed.