Charge gap and charge-phonon coupling in LuFe₂O₄

Lattice vibrations as well as electronic excitations are used to investigate LuFe₂O₄, which is intensively discussed in the context of electronic multiferroics. All phonon modes and an electronic continuum show drastic changes through a series of magnetic, structural, and charge ordering temperatures. In particular, the 634 cm^-1 A _g mode, corresponding to the relative out-of-plane motion of a combined LuO and Fe₂O₃ layer, is very susceptible to the charge ordering, indicative of antipolar ordering of the Fe₂O ₃ layer. With decreasing temperature below T_CO=320 K, the electronic continuum is gradually suppressed while opening a charge gap of 790 cm^-1. Its temperature dependence suggests that charge ordering is not totally frozen-in even at 5 K, signaling the importance of local charge fluctuations due to concerted charge and spin frustration.