Modeling Small Polaron-Induced Ultrafast Ferroelectric Relaxation in BiFeO₃

In this report, we employed a combination of ultrafast spectroscopic techniques and high-level calculations to uncover the complex interplay between electronic and lattice degrees of freedom in a class of strongly correlated materials. Our observations revealed a transient reduction in the electronic dipole upon optical excitation, which then recovers on 0.5 and 10 ps timescales. Time-dependent density functional theory calculations show that both ligand-to-metal charge transfer and local excitation transitions can be excited. The transient EUV dynamics captured both ultrafast free charge carrier relaxation to excitons and polaron formation. Multireference configuration interaction calculations suggest that only polaron formation is associated with the 0.5 ps electronic dipole relaxation, while the faster electronic relaxation does not contribute to changes in ferroelectric properties. Our results decode the multi-degrees of freedom associated with this ultrafast ferroelectric modulation and pinpoint the critical motion—a local polaron formation—essential for rapid ferroelectric recovery. These findings offer crucial insights into the specific lattice distortions capable of modulating the properties or phase transitions of strongly correlated materials.