Charge gap and charge-phonon coupling in LuFe2O4

Abstract

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 ofmagnetic, 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 Fe2O3 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. DOI: 10.1103/PhysRevB.87.144416