Charge gap and charge-phonon coupling in LuFe2O4

Abstract

Lattice vibrations as well as electronic excitations are used to investigate LuFe${}_{2}$O${}_{4}$, 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${}^{\ensuremath{-}1}$ ${A}_{g}$ mode, corresponding to the relative out-of-plane motion of a combined LuO and Fe${}_{2}$O${}_{3}$ layer, is very susceptible to the charge ordering, indicative of antipolar ordering of the Fe${}_{2}$O${}_{3}$ layer. With decreasing temperature below ${T}_{\text{CO}}=320$ K, the electronic continuum is gradually suppressed while opening a charge gap of 790 cm${}^{\ensuremath{-}1}$. 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.