Electron-lattice coupling effects in nonadiabatic polarization switching of charge-order-induced ferroelectrics

We carry out first-principles density-functional-theory calculations to investigate the possibility of nonadiabatic switching (purely electronic hopping without structural change) in charge-order-induced ferroelectrics. In charge-order-induced ferroelectrics, polarization switching can in principle occur through nonadiabatic interionic electron hopping with an ultrafast speed. However, lattice relaxation for a specific charge-ordering state can ``lock'' that state in, making nonadiabatic switching to a different charge-ordering variant energetically prohibitive; whether nonadiabatic switching on electronic time scales can occur thus depends on the strength of lattice-electron coupling. In this work, we propose a simple model predicting the possibility of nonadiabatic switching based on structural analysis. Specifically, we point out that electric field driven nonadiabatic electron hopping is possible in charge-order-induced ferroelectrics whose primary coupling mode is off-centering displacement, rather than polyhedral breathing. We carry out calculations and analysis on three different prototype charge-ordering materials, including the (theoretically designed) (${\mathrm{SrVO}}<sub>3</sub>{)}<sub>1</sub>{({\mathrm{LaVO}}<sub>3</sub>)}<sub>1</sub>$ superlattice, ${\mathrm{LuFe}}<sub>2</sub>{\mathrm{O}}<sub>4</sub>$, and ${\mathrm{Fe}}<sub>3</sub>{\mathrm{O}}<sub>4</sub>$. All the results support our theory and are also in accord with experimental observations. These results show the critical role of electron-lattice coupling in nonadiabatic polarization switching in charge-order-induced ferroelectrics, providing guidance for the design and discovery of ``electronic ferroelectrics'' with ultrafast polarization switching on electronic rather than lattice time scales.