Coherent Charge Hopping Suppresses Photoexcited Polarons in ErFeO3

Abstract

We measure how coherent charge hopping competes with and slows small polaron formation. The coupling of photoexcited charge carriers with phonons in the crystal lattice that results in polaron formation causes carrier localization and limits transport to site-to-site hopping. ErFeO3, a rare-earth orthoferrite, is designed where the large rare earth atom frustrates the FeO6 octahedra, reducing the energetic favorability of polaron formation. We compare transient extreme ultraviolet (XUV) measurements with ab initio theory and transport measurements to quantify the polaron formation rate and mobility. Polaron formation occurs within several picoseconds following multiple coherent charge hopping events between neighboring Fe3+–Fe2+ sites. This timescale is more than an order of magnitude longer compared to previously studied materials. The delay in polaron formation is notable because the photoexcited carriers shed excess thermal energy before forming the polaron. To date, polarons have been measured to form instantaneously with the first electron-phonon scattering event. The measured interplay between optical phonons, electron coupling, and on-site lattice deformation emphasizes the importance of dynamic electron correlations for tuning polaron formation in photocatalysts.