Microscopic origins of flexoelectric effect in oxide membranes

The flexoelectric effect describes the thermodynamic coupling of strain gradients and polarization and is a universal effect in materials of all symmetry. Although flexoelectricity is typically miniscule in bulk, it can become prominent at the nanoscale due to the inverse scaling of strain gradients with material dimensions. In this study, we utilize the enhanced elastic compliance of free-standing oxide membranes^1,2 to stabilize large strain gradients (> 10⁶ m^-1) in nanostructured wrinkles of strontium titanate (SrTiO₃). Using multislice electron ptychography^3,4, a scanning transmission electron microscopy technique, we provide an unambiguous measurement of the sign of the resulting flexoelectric polarization, revealing structural distortions dominated by oxygen displacements and minimal cation-cation displacements. Our theoretical calculations show that the sign of the flexoelectric coefficient in strontium titanate can be switched by tuning the strain state of the membrane between compressive and tensile strain, driven by the coupling of the optical soft phonon mode with the acoustic phonon modes. Our results address the sign discrepancy and order of magnitude variations in the values of previously reported flexoelectric coefficients as different fabrication approaches can stabilize largely different strains.

References

1. V. Harbola et al., Nano Lett., 21, 6, 2470–2475 (2021)

2. G. Dong et al., Science 366, 475-479 (2019)

3. A. M. Maiden et al., J. Opt. Soc. Am. A 29, 1606-1614 (2012)

4. Z. Chen et al., Science 372, 826–831 (2021).