Coherent and Dynamic Small Polaron Delocalization in CuFeO$_{2}$

Abstract

Small polarons remain a significant bottleneck in the realization of efficient devices using transition metal oxides. Routes to engineer small polaron coupling to electronic states and lattice modes to control carrier localization remain unclear. Here, we measure the formation of small polarons in CuFeO$_{2}$ using transient extreme ultraviolet reflection spectroscopy and compare it to theoretical predictions in realistically parameterized Holstein models, demonstrating that polaron localization depends on its coupling to the high-frequency versus low-frequency components of the phonon bath. We measure that small polaron formation occurs on a comparable ~100 fs timescale to other Fe(III) compounds. After formation, a dynamic delocalization of the small polaron occurs through a coherent lattice expansion between Fe-O layers and charge-sharing with surrounding Fe(IV) states. Our simulations of polaron formation dynamics reveal that two major factors dictate polaron formation timescales: phonon density and reorganization energy distributions between acoustic and optical modes, matching experimental findings. Our work provides a detailed, real-time observation of how electronic-structural coupling in a polaron-host material can be leveraged to suppress polaronic effects for various applications.