Inelastic neutron scattering study of the acoustic phonon mode dispersion in KTaO<sub>3</sub>
ORAL
Abstract
KTaO3 (KTO) and SrTiO3 (STO) are well-known paraelectrics, lying very close to a quantum critical point such that slight compositional substitutions can induce ferroelectric or even superconducting ground-states. Upon cooling, these compounds show a strong increase of the dielectric constant which however saturates at a high value at finite temperatures before ferroelectric long-range order is established. The suppression of long-range order is generally attributed to zero-point quantum fluctuations, though these incipient ferrolectric states remain poorly understood. Recent observations of an anomalous softening of the transverse acoustic phonon mode (TA) with a local minimum in the dispersion in STO [1] was attributed to the hybridization between optical and acoustic phonons induced by flexoelectric interactions [2].
Here, we report high resolution inelastic neutron scattering investigations of the TA dispersion of KTO. In contrast to STO, KTO remains cubic down to the lowest measured temperatures and the anomaly predicted due to flexoelectric coupling occurs at a larger momentum transfer, facilitating detailed measurements. Upon cooling to low temperatures, we observe very little, if any, softening of the TA mode near the zone center for q <= 0.04 Å-1, where the maximal softening was observed for STO, but clear softening is observed for larger momentum transfers. The softening of the TA modes in KTO at larger q however occurs along the whole branch along both q=(h,0,0) and q=(h,h,0). While the amplitudes of the softening normalized to the high temperature dispersion do show broad maxima centered around h = 0.1 – 0.15 along both q=(h,0,0) and (h,h,0), we do not observe a sharp anomaly at a particular wavevector or a minimum in the dispersion, but instead observe linear behavior of the dispersion from h > 0.05 up to h=0.3.
[1] Fauqué et al., Phys. Rev. B 106, L140301 (2022)
[2] Guzmán-Verri et al., Phys. Rev. Lett. 131, 048801 (2023)
Here, we report high resolution inelastic neutron scattering investigations of the TA dispersion of KTO. In contrast to STO, KTO remains cubic down to the lowest measured temperatures and the anomaly predicted due to flexoelectric coupling occurs at a larger momentum transfer, facilitating detailed measurements. Upon cooling to low temperatures, we observe very little, if any, softening of the TA mode near the zone center for q <= 0.04 Å-1, where the maximal softening was observed for STO, but clear softening is observed for larger momentum transfers. The softening of the TA modes in KTO at larger q however occurs along the whole branch along both q=(h,0,0) and q=(h,h,0). While the amplitudes of the softening normalized to the high temperature dispersion do show broad maxima centered around h = 0.1 – 0.15 along both q=(h,0,0) and (h,h,0), we do not observe a sharp anomaly at a particular wavevector or a minimum in the dispersion, but instead observe linear behavior of the dispersion from h > 0.05 up to h=0.3.
[1] Fauqué et al., Phys. Rev. B 106, L140301 (2022)
[2] Guzmán-Verri et al., Phys. Rev. Lett. 131, 048801 (2023)
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Presenters
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Stephan Rosenkranz
Argonne National Laboratory
Authors
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Stephan Rosenkranz
Argonne National Laboratory
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Yuliia Tymoshenko
Karlsruhe Institute of Technology
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Tom Laurin Lacmann
Karlsruhe Institute of Technology
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Hengdi Zhao
Argonne National Laboratory
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Paul Steffens
Institute Laue Langevin
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Frank A Weber
Karlsruhe Institute of Technology