  The piezoelectric response is a measure of the sensitivity of a material's
polarization to stress or its strain to an applied field. Using in-operando
x-ray Bragg coherent diffraction imaging, we observe that topological vortices
are the source of a five-fold enhancement of the piezoelectric response near
the vortex core. The vortices form where several low symmetry ferroelectric
phases and phase boundaries coalesce. Unlike bulk ferroelectric solid solutions
in which a large piezoelectric response is associated with coexisting phases in
the proximity of the triple point, the largest responses for pure BaTiO3 at the
nanoscale are in spatial regions of extremely small spontaneous polarization at
vortex cores. The response decays inversely with polarization away from the
vortex, analogous to the behavior in bulk ceramics as the cation compositions
are varied away from the triple point. We use first-principles-based molecular
dynamics to augment our observations, and our results suggest that nanoscale
piezoelectric materials with large piezoelectric response can be designed
within a parameter space governed by vortex cores. Our findings have
implications for the development of next-generation nanoscale piezoelectric
materials.
