Kavli Affiliate: Scott K. Cushing
| First 5 Authors: , , , ,
| Summary:
Controlling the effects of photoexcited polarons in transition metal oxides
can enable the long timescale charge separation necessary for renewable energy
applications as well as 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 inter-action. Excitation-wavelength-dependent transient
extreme ultraviolet (XUV) spectroscopy selectively excites LMCT and
metal-to-metal charge transfer transitions (MMCT). 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.
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