Electrothermal and structural characterisation of ferroelectric domain walls in lithium niobate

Finite element models reproduced the experimental thermal morphology, indicating that domain walls behave as pseudo-planar heat sources distinct from filament-based heating in resistive switching oxides. These results highlight key advantages over filamentary resistive memories, where temperature rises are much larger, risking thermal cross-talk and runaway.<br><br>The second component addresses how sub-surface domain wall structure evolves under bias and governs measured resistivity.

In-situ cross-sectional transmission electron microscopy of LiNbO₃ capacitors revealed previously unobserved domain wall movement under bias. Small negative biases aligned walls with the polar axis, while larger biases induced significant inclination, forming head-to-head or tail-to-tail discontinuities — explaining observations of conductivity 'wake-up' under sub-coercive fields. Temporally-resolved TEM of isolated single domain walls captured continuous movement of charged and neutral wall segments.

A resistor-in-series model applied to extracted wall profiles yielded conductivity values consistent with previous surface studies but now directly linked to sub-surface structure.<br><br>Together, these findings provide the first direct quantification of domain wall Joule heating and first real-time visualisation of sub-surface structural plasticity, advancing fundamental understanding of nanoscale domain wall transport for neuromorphic and memristive applications.<br><br><i>Thesis is embargoed until 31 July 2029</i>.