Ultrafast Suppression of the Ferroelectric Instability in ${\mathrm{KTaO}}_{3}$

Ultrafast Suppression of the Ferroelectric Instability in ${KTaO}_{3}$

Viktor Krapivin, Mingqiang Gu, D. Hickox-Young, S. W. Teitelbaum, Y. Huang, G. de la Peña, D. Zhu, N. Sirica, M.-C. Lee, R. P. Prasankumar, A. A. Maznev, K. A. Nelson, M. Chollet, James M. Rondinelli, D. A. Reis, and M. Trigo

Phys. Rev. Lett. 129, 127601 – Published 14 September 2022

Abstract

We use an x-ray free-electron laser to study the lattice dynamics following photoexcitation with ultrafast near-UV light (wavelength 266 nm, 50 fs pulse duration) of the incipient ferroelectric potassium tantalate, ${KTaO}_{3}$ . By probing the lattice dynamics corresponding to multiple Brillouin zones through the x-ray diffuse scattering with pulses from the Linac Coherent Light Source (LCLS) (wavelength 1.3 Å and $< 10 fs$ pulse duration), we observe changes in the diffuse intensity associated with a hardening of the transverse acoustic phonon branches along $Γ$ to $X$ and $Γ$ to $M$ . Using force constants from density functional theory, we fit the quasiequilibrium intensity and obtain the instantaneous lattice temperature and density of photoexcited charge carriers. The density functional theory calculations demonstrate that photoexcitation transfers charge from oxygen $2 p$ derived $π$ -bonding orbitals to Ta $5 d$ derived antibonding orbitals, further suppressing the ferroelectric instability and increasing the stability of the cubic, paraelectric structure.

Received 29 January 2022
Revised 23 May 2022
Accepted 19 July 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.127601

Physics Subject Headings (PhySH)

Anharmonic lattice dynamics Ferroelectricity Phase transitions

Perovskites Transition metal oxides

Photoexcitation Ultrafast pump-probe spectroscopy X-ray scattering

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Viktor Krapivin ^1,2,3,*, Mingqiang Gu ⁴, D. Hickox-Young⁴, S. W. Teitelbaum ^1,2, Y. Huang ^1,2,3, G. de la Peña^1,2, D. Zhu⁵, N. Sirica⁶, M.-C. Lee ⁶, R. P. Prasankumar⁶, A. A. Maznev⁷, K. A. Nelson⁷, M. Chollet⁵, James M. Rondinelli ^4,2, D. A. Reis ^1,2,3, and M. Trigo¹

¹Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
²Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
³Department of Applied Physics, Stanford University, Stanford, California 94305, USA
⁴Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA
⁵Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
⁶Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
⁷Department of Chemistry, Massachusetts Institute of Technology, Cambridge, 02139 Massachusetts, USA

^*krapivin@stanford.edu

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Vol. 129, Iss. 12 — 16 September 2022

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Figure 1
(a) Schematic of the grazing incidence scattering geometry. Pump and probe are represented by purple and black arrows, respectively. The scattered x rays are collected with an area detector positioned 110 mm from the sample. The momentum transfer $Q$ is shown schematically. The axis of $ϕ$ rotation is parallel to the sample normal and $ϕ = 0$ corresponds to the (100) direction parallel to the incident x-ray beam. (b) Room temperature diffuse scattering pattern from ${KTaO}_{3}$ for $ϕ = 16.9 8 °$ . The red lines and labels show the boundaries between Brillouin zones and the corresponding reciprocal lattice indices. The scale bar represents linear intensity scale in arbitrary units. (c) Time dependence of the intensity averaged over each box indicated in (b). The labels indicate the reduced wave vector of the center of each box in reciprocal lattice units.
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Figure 2
(a) Experimentally measured and (b) fitted $Δ I (Q, t = 3 ps)$ . $Δ I (Q, t)$ in (b) was calculated for $ρ = 0.08$ electrons per unit cell and $T = 480 K$ (see text for details). The intensity scale is the same as in Fig. 1.
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Figure 3
(a) Fitted carrier density $ρ$ and lattice temperature $T$ for each time delay as described in the text. The error bars are representative errors obtained from the matlab function nlparci [52]. The solid lines show the median value within 30 points. (b) The low frequency part of the calculated equilibrium ( $ρ = 0$ ) transverse phonon dispersion of ${KTaO}_{3}$ is from DFT (blue line). Along the $M$ to $Γ$ direction the transverse branch polarized parallel to $(1 \bar{1} 0)$ and polarized parallel to (001) are plotted. Open circles represent room temperature data from neutron scattering [24]. Red trace shows the transient dispersion of the lowest $TA / TO$ branches at $ρ = 0.07$ per unit cell, obtained from the fit after 1 ps.
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Figure 4
(a) Calculated electron density of states (DOS) and (b) projected crystal orbital Hamiltonian population (PCOHP) for Ta and the apical O atom displacing in the (c) TA mode for ${KTaO}_{3}$ at the $X$ point with displacements indicated with black arrows in its electronic ground state. The gray filled curve corresponds to the total DOS. The horizontal arrow depicts the photoexcitation of electrons from the bonding O $2 p$ states at the valence band (VB) maximum to antibonding Ta $5 d$ states forming the conduction band (CB) as illustrated in (b). The photoexcitation indicated in (a) leads to changes in the (d) bonding interactions at the valence band maximum to the conduction band minimum formed by O $2 p$ orbitals and Ta $5 d$ $t_{2 g}$ orbitals.
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Physical Review Letters

Ultrafast Suppression of the Ferroelectric Instability in KTaO3

Viktor Krapivin, Mingqiang Gu, D. Hickox-Young, S. W. Teitelbaum, Y. Huang, G. de la Peña, D. Zhu, N. Sirica, M.-C. Lee, R. P. Prasankumar, A. A. Maznev, K. A. Nelson, M. Chollet, James M. Rondinelli, D. A. Reis, and M. Trigo

Phys. Rev. Lett. 129, 127601 – Published 14 September 2022

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Ultrafast Suppression of the Ferroelectric Instability in ${KTaO}_{3}$