Keynote lecture (remote)
Ferroelectricity at the nanoscale: Complex polarisation textures and emergent functionalities

Ferroelectric materials can host a wide range of novel functional properties as well as unusual structural features, potentially useful for nanoelectronics applications. At domain walls or in regions with high strain gradients, in particular, the complex interaction between polarisation, electrostatics, and strain can lead to localised chiral polarisation textures, electrical conductivity, local mechanical responses, and charge or chemical segregation. My group uses a broad spectrum of primarily scanning probe microscopy techniques, coupled with machine learning analysis to investigate and disentangle the many complex and correlated physical phenomena in these materials.

One promising structure with strongly localised and very high strain gradients are ferroelastic twins, such as a-domains in tetragonal ferroelectric perovskites, and even more so, their intersections. In Pb(Zr0.2Ti0.8)O3 and PbTiO3 thin films, investigated with a combination of PFM, CAFM, CRFM, transmission electron microscopy, and nonlinear optical microscopy, we find a characteristic localised piezoelectric response and evidence of a more complex rotational or closure structure in second harmonic generation polarimetry analysis. The heart of the twin domain crossing presents an extremely high susceptibility to local application of electric bias or pressure, dominating the polarisation switching dynamics in this region. We also observe distinct mechanical properties and enhanced electrical conduction at the heart of the twin crossing, and a conductive superstructure in the twins themselves, reflecting their complex polarisation textures revealed in transmission electron microscopy.