High-field ESR Studies of the Quantum Spin Dimer System Ba₃Cr₂O₈

Results of systematic high-frequency electron spin resonance (ESR) studies of Ba₃Cr₂O₈, a weakly coupled S = 1/2 dimer system, in magnetic fields up to 25 T are reported. Two pairs of ESR gapped modes corresponding to transitions from a spin-singlet ground state to the first excited triplet states with gaps, Δ_AB = 563 GHz and Δ_CD = 399 GHz, are revealed below H_c1 = 12.5 T. The detection of the ground-state excitations by means of ESR clearly indicates the presence of a non-secular term allowing these transitions. A complex structure of the microwave absorption spectrum in magnetic fields above H_c1 is observed, those peculiarities are discussed.