Terahertz spectroscopy of central and soft phonon modes in LuFe2O4

The terahertz dielectric response of LuFe2O4 is investigated by terahertz time-domain spectroscopy over a temperature range of 6-290 K. It is revealed that besides the central mode associated with the charge-ordered state, a soft TO1 mode at below similar to 240 K is identified indicating the existence of displacing ferroelectricity, in addition to the charge-ordering-induced ferroelectricity at below 320 K. The anomaly of the soft mode at similar to 180 K reflects the magnetoelectric correlation between the soft TO1 mode and the spin/charge fluctuations revealed recently. Finally, the magnetic property at below similar to 240 K is discussed.