Dynamic Competition between Hubbard and Superexchange Interactions Selectively Localizes Electrons and Holes through Polarons
来源：ACS Publications
Controlling the effects of photoexcited polarons in transition metal oxides can enable the long time-scale charge separation necessary for renewable energy applications and controlling new quantum phases through dynamically tunable electron–phonon coupling. In previously studied transition metal oxides, polaron formation is facilitated by a photoexcited ligand-to-metal charge transfer (LMCT). When the polaron is formed, oxygen atoms move away from iron centers, which increases carrier localization at the metal center and decreases charge hopping. Studies of yttrium iron garnet and erbium iron oxide have suggested that strong electron and spin correlations can modulate photoexcited polaron formation. To understand the interplay between strong spin and electronic correlations in highly polar materials, we studied gadolinium iron oxide (GdFeO3), which selectively forms photoexcited polarons through an Fe–O–Fe superexchange interaction. Excitation-wavelength-dependent transient extreme ultraviolet (XUV) spectroscopy selectively excites LMCT and metal-to-metal charge transfer (MMCT) transitions. The LMCT transition suppresses photoexcited polaron formation due to the balance between superexchange and Hubbard interactions, while MMCT transitions result in photoexcited polaron formation within 250 ± 40 fs. Ab initio theory demonstrates that electron and hole polarons localize on iron centers following MMCT. In addition to understanding how strong electronic and spin correlations can control strong electron–phonon coupling, these experiments separately measure electron and hole polaron interactions on neighboring metal centers for the first time, providing insight into a large range of charge-transfer and Mott–Hubbard insulators.
Here, we employ excitation-wavelength-dependent transient extreme ultraviolet (XUV) reflection spectroscopy to explore the relation between superexchange and Hubbard interactions in influencing strong carrier localization through polarons in single-crystal rare-earth orthoferrite GdFeO3. This intermediate insulator (mixed O 2p/Fe 3d VBM) is a model material system for many perovskite Mott–Hubbard insulators due to its crystal structure. GdFeO3 and other rare-earth orthoferrites have been explored as candidates for photoelectrochemical oxygen reduction and evolution, as well as magneto-optical and spintronic applications due to their demonstrated multiferroicity. Through transient XUV spectroscopy, we find that LMCT from O 2p to Fe 3d orbitals results in the suppression of photoexcited polaron formation, while a metal-to-metal charge transfer (MMCT) induces photoexcited polaron formation. The MMCT enables polarons by superexchange across an Fe–O–Fe bond, resulting in hole and electron polarons localized on neighboring iron centers, and a reduced radiative lifetime (<100 ps for MMCT versus 1.2 ± 0.4 ns for LMCT). Polaron formation is suppressed following a higher energy LMCT due to the balance of on-site Coulomb repulsion (U) from oxygen-dominated valence orbitals and superexchange in the final spin state. While thermalization of these carriers occurs in ∼350 fs, the final spin state from the LMCT prevents the polaron formation. Ab initio density functional theory (DFT) and the Bethe–Salpeter equation (BSE) simulate the photoexcited XUV dynamics. Polaron-induced lattice distortions and charge localization within a defect supercell approach were assessed with different levels of electronic structure theory, ranging from DFT incorporating on-site Hubbard U parameters with variational polaron self-interaction-corrected (pSIC) total-energy functional models to hybrid functionals with different fractions of exact exchange. The results provide new insight into tuning polaronic properties via strong electronic and spin correlations for transition metal oxide materials, particularly identifying superexchange interactions as a new dynamical carrier localization and polaron design parameter.