Magnetic excitations and optical transitions in the multiferroic spin- 1 2 system LiCu₂ O₂

Magnetic excitations in a cycloidal magnet LiCu₂ O₂ are explored using terahertz absorption spectroscopy in magnetic fields up to 12 T. Below the spin ordering temperature, eight optically active transitions are observed in the spin system of LiCu₂ O₂ in the range from 4 to 30 cm-1. In magnetic field the number of modes increases and the electric polarization flop is seen as a change in magnetic field dependence of mode energies. The polarization dependence of two of the modes in zero magnetic field fits the selection rules for the cycloid tilted by θ= (41±1) ° from the bc plane. For the remaining six modes electric and magnetic dipole approximations cannot explain the observed polarization dependence. The electromagnon is not seen in the explored energy range although there is evidence that it could exist below 4 cm-1.