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 $$S=\frac{1}{2}$$ 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, ΔAB=563 GHz and ΔCD=399 GHz, are revealed below Hc1=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 Hc1 is observed, those peculiarities are discussed.