Spin-triplet excitons and anisotropy effects in the S = 1/2 gapped antiferromagnet BaCuSi₂O₆

BaCuSi₂O₆ can be regarded as an almost ideal realization of an S = 1/2 system of weakly interacting spin dimers with spin-singlet ground state and gapped excitation spectrum. We argue that the fine structure observed in low-temperature EPR spectra of BaCuSi₂O₆ is a fingerprint of triplet excitations (excitons). Analyzing the angular dependence of the exciton modes allows us to precisely calculate the zero-field splitting within the triplet states and, correspondingly, the anisotropy parameter, D = 0.07cm^-1. The proposed procedure can be applied for studying anisotropy effects in a large number of S = 1/2 gapped quantum antiferromagnets with dimerized or alternating spin structure.