Interplay between charge ordering and geometric ferroelectricity in LuFe2O4/LuFeO3 superlattices

Oxide superlattices have drawn great attention owing to the intriguing coupling among elastic, electrical, and magnetic orderings at the interfaces and the emergence of improper ferroelectricity. Here, superlattices composed of hexagonal LuFeO3 (h-LuFeO3) and LuFe2O4 are investigated via density functional theory calculations. h-LuFeO3 is a well-known multiferroic material that is stable only in thin film or doped bulk state, while LuFe2O4 is a charge ordered (CO) material where the existence of ferroelectricity is still a controversy. We have found that the CO-induced polarizations in LuFe2O4 layers coexist with the geometric polarizations in h-LuFeO3 layers in the (LuFe2O4)m/(LuFeO3)(n) superlattices with different periodicities, and the ferroelectric states are generally preferred over the antiferroelectric states for LuFe2O4 in superlattices. The out-of-plane polarizations in h-LuFeO3 and LuFe2O4 layers tend to be aligned in parallel, and the overall polarization increases with the ratio of h-LuFeO3. The influence of layered polarizations on the local electrostatic potential is not significant except the detected small trend caused by the CO-induced polarization within a FeO bilayer. Additionally, the local electronic structures show that the Fermi level position in a certain layer can be tuned by the valences of Fe in this layer and the polarization distributions in neighboring layers. LuFe2O4 layers sandwiched between thick h-LuFeO3 layers are more susceptible. The calculated configurations of the superlattices are supported by atomic-resolution transmission electron microscopy experiments. Our results pave the way for tunable ferroelectricity in superlattice systems and create a playground for manipulating the coupling between various degrees of freedom.

Automated summary

The study investigates oxide superlattices composed of hexagonal LuFeO3 (h-LuFeO3) and LuFe2O4 using density functional theory calculations. The coexistence of CO-induced polarizations and geometric polarizations in different periodicities of superlattices is observed, with LuFe2O4 favoring ferroelectric states over antiferroelectric states. Out-of-plane polarizations tend to align in parallel between h-LuFeO3 and LuFe2O4 layers, and the overall polarization increases with the ratio of h-LuFeO3. The influence of layered polarizations on the local electrostatic potential is minimal, except for small trends caused by CO-induced polarizations within FeO bilayers.