Magnetic excitations and optical transitions in the multiferroic spin-1/2 system LiCu2O2

Abstract

Magnetic excitations in a cycloidal magnet LiCu2O2 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 LiCu2O2 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 theta = (41 +/- 1)degrees 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).