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# Title: Ultrafast Suppression of the Ferroelectric Instability in KTaO$_3$

Abstract: We use an x-ray free-electron laser to study the ultrafast lattice dynamics following above band-gap photoexcitation of the incipient ferroelectric potassium-tantalate, \kto. %
We use ultrafast near-UV (central wavelength 266\,nm and 50 fs pulse duration) laser light to photoexcite charge carriers across the gap and probe the ultrafast lattice dynamics by recording the x-ray diffuse intensity throughout multiple Brillouin zones using pulses from the Linac Coherent Light Source (LCLS) (central wavelength 1.3\,\AA\, and $< 10$~fs pulse duration). We observe changes in the diffuse intensity that we conclude are associated with a hardening of the soft transverse optical and transverse acoustic phonon branches along $\Gamma$ to $X$ and $\Gamma$ to $M$. Using ground- and excited-state interatomic force constants from density functional theory (DFT) and assuming the phonon populations can be described by a time-dependent temperature, we fit the quasi-equilibrium thermal diffuse intensity to the experimental time-dependent intensity. We obtain the instantaneous lattice temperature and density of photoexcited charge carriers as a function of time delay. The DFT calculations demonstrate that photoexcitation transfers charge from oxygen $2p$ derived $\pi$-bonding orbitals to Ta $5d$ derived antibonding orbitals, further suppressing the ferroelectric instability and increasing the stability of the cubic, paraelectric structure.
 Comments: 8 pages, 4 figures Subjects: Materials Science (cond-mat.mtrl-sci) Cite as: arXiv:2201.10769 [cond-mat.mtrl-sci] (or arXiv:2201.10769v1 [cond-mat.mtrl-sci] for this version)

## Submission history

From: Viktor Krapivin [view email]
[v1] Wed, 26 Jan 2022 06:37:05 GMT (3617kb,D)

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