Kavli Affiliate: Scott K. Cushing
| First 5 Authors: Ye-Jin Kim, Jocelyn L. Mendes, Young Jai Choi, Scott K. Cushing,
| Summary:
Small polaron formation is dominant across a range of condensed matter
systems. Small polarons are usually studied in terms of ground-state transport
and thermal fluctuations, but small polarons can also be created impulsively by
photoexcitation. The temporal response of the lattice and local electron
correlations can then be separated, such as with transient XUV spectroscopy. To
date, photoexcited small polaron formation has only been measured to be
adiabatic. The reorganization energy of the polar lattice is large enough that
the first electron-optical phonon scattering event creates a small polaron
without significant carrier thermalization. Here, we use transient XUV
spectroscopy to measure antiadiabatic polaron formation by frustrating the
iron-centered octahedra in a rare-earth orthoferrite lattice. The small polaron
is measured to take several picoseconds to form over multiple coherent charge
hopping events between neighboring Fe3+-Fe2+ sites, a timescale that is more
than an order of magnitude longer compared to previous materials. The measured
interplay between optical phonons, electron correlations, and on-site lattice
deformation give a clear picture of how antiadiabatic small polaron transport
would occur in the material. The measurements also confirm the prediction of
the Holstein and Hubbard-Holstein model that the electron hopping integral must
be larger than the reorganization energy to achieve antiadiabaticity. Moreover,
the measurements emphasize the importance of considering dynamical electron
correlations, and not just changes in the lattice geometry, for controlling
small polarons in transport or photoexcited applications.
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