THz magneto-electric excitations in the chiral compound Ba3NbFe3Si2O14

The langasite Ba$_3$NbFe$_3$Si$_2$O$_{14}$ displays a chiral structure and orders magnetically with a Neel temperature $T_N$=27K. We have determined its terahertz (THz) spectrum by means of synchrotron-radiation measurements. Three different types of excitation are present. The first one, at 13cm$^{-1}$, disappears at T$_N$ and is assigned to magnons. The others, at 23cm$^{-1}$ and 29cm$^{-1}$, persist up to four times $T_N$. According to their selection rules, they are interpreted as rotational modes of the lattice whose magneto-electric activity reveals a structural transition into a polar helical state.