Kavli Affiliate: Gregory D. Fuchs
| First 5 Authors: Harikrishnan KP, Varun Harbola, Jaehong Choi, Kevin J. Crust, Yu-Tsun Shao
| Summary:
Modern electromechanical actuators and sensors rely on the piezoelectric
effect that linearly couples strain and electric polarization. However, this
effect is restricted to materials that lack inversion symmetry. In contrast,
the flexoelectric effect couples strain gradients to electric polarization, and
is a universal property in insulating materials of arbitrary symmetry.
Flexoelectricity becomes prominent at the nanoscale from the inverse scaling of
strain gradients with material dimensions. Here, we measure the
strain-gradient-induced structural distortions in strontium titanate using
multislice electron ptychography. This technique enables reliable
picometer-scale measurements of the dominant oxygen-titanium distortions,
correcting for artifacts that limited conventional imaging methods. This
enables us to directly measure the sign of the net ionic contribution to the
flexoelectric polarization. Guided by the experimental measurements,
first-principles calculations show how the sign and magnitude of the bulk
contribution to the flexoelectric coefficient in strontium titanate can be
switched by tuning the strain state. Hybridization between the optical soft
phonon and acoustic phonon modes drives this transition, yielding a large
response and a polarity switch across the resonance. This strain-dependence
might explain the sign discrepancy and orders of magnitude variation in the
values of previously reported flexoelectric coefficients for strontium
titanate. As the strain state of curved membranes can be tuned, our approach
also suggests an approach to engineer nanoscale flexoelectric polarization
using strain as a control parameter.
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