Ultrafast Suppression of the Ferroelectric Instability in KTaO$_3$

TL;DR

This study uses ultrafast x-ray techniques to observe how photoexcitation suppresses ferroelectric instability in KTaO$_3$, revealing rapid lattice dynamics and charge transfer effects that stabilize the cubic phase.

Contribution

It demonstrates the ultrafast suppression of ferroelectric instability in KTaO$_3$ via photoexcitation, combining experimental x-ray measurements with DFT calculations to elucidate charge transfer effects.

Findings

Photoexcitation hardens phonon branches along specific directions.

Lattice temperature and charge density are tracked over time.

Charge transfer from oxygen to tantalum orbitals stabilizes the cubic phase.

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.