Spin-triplet excitons and anisotropy effects in the gapped antiferromagnet BaCuSi2O6

BaCuSi 2 O 6 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 BaCuSi 2 O 6 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 = 0.07 cm - 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.