ESR Studies of the Quantum Spin Dimer System Ba₃Cr₂O₈

Ba₃Cr₂O₈ is a system of three-dimensionally coupled dimers, exhibiting a field-induced phase transition from a quantum-disordered to a long-range antiferromagnetically ordered state at H_c1 = 12.5 T. We report tunable-frequency electron spin resonance (ESR) studies of Ba₃Cr₂O₈ single crystals performed in a frequency range between 50 and 700 GHz in fields up to 25 T (applied along the c axis). Two gaps in the magnetic excitation spectrum, Δ1 = 564 GHz and Δ2 = 400 GHz, were observed directly, confirming the spin-singlet ground state of Ba₃Cr₂O₈ below H_c1. The observation of singlet-triplet ESR transitions, which are normally forbidden in spin-1/2 dimer systems by selection rules, indicates the break-down of the axial symmetry in this compound. Details of the ESR excitation spectrum of Ba₃Cr₂O₈ across different regions of its phase diagram will be discussed.