Spin-Triplet Excitons in the $S=1\mathrm{/}2$ Gapped Antiferromagnet BaCuSi${}_2$O${}_6$: Electron Paramagnetic Resonance Studies

BaCuSi${}_2$O${}_6$, a $S=1\mathrm{/}2$ quantum antiferromagnet with a double-layer structure of Cu${}^{2+}$ ions in a distorted planar-rectangular coordination and with a dimerized spin singlet ground state, is 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. Analysis of the angular dependence of exciton modes in BaCuSi${}_2$O${}_6$ allows us to accurately estimate anisotropy parameters. In addition, the temperature dependence of EPR intensity and linewidth is discussed.