High-field electron spin resonance spectroscopy of singlet-triplet transitions in the spin-dimer systems Sr₃Cr₂O₈ and Ba₃Cr₂O₈

Magnetic excitations in the isostructural spin-dimer systems Sr₃Cr₂O₈ and Ba₃Cr₂O₈ are probed by means of high-field electron spin resonance at subterahertz frequencies. Three types of magnetic modes were observed. One mode is gapless and corresponds to transitions within excited states, while two other sets of modes are gapped and correspond to transitions from the ground to the first excited states. The selection rules of the gapped modes are analyzed in terms of a dynamical Dzyaloshinskii-Moriya interaction, suggesting the presence of phonon-assisted effects in the low-temperature spin dynamics of Sr₃Cr₂O₈ and Ba₃Cr₂O₈.