Electronic properties of nanoscale ferroelectric domain walls

Artificial interfaces between complex oxides (heterointerfaces) have exhibited a vast range of fascinating and exotic phenomena, including metallic conductivity and superconductivity at the interface between insulators, large spin-orbit coupling, complex magnetic states and improper ferroelectricity. At the heart of this functionality are the closely intertwined charge, orbital and spin degrees of freedom that can be drastically modified through heterostructure engineering. Oxide interfaces thus became ideal candidates for the utilisation of their unique properties for the creation of devices such as spin transistors. Similarly, interfaces that occur between pieces of the same material (homointerfaces) have proven to be equally fascinating. These interfaces can be found in oxide ferroelectrics, systems that possess a switchable spontaneous polarisation. When different polarization orientations exist within the same material, boundaries called domain walls form, with dimensions of a few nanometres and properties drastically different from the bulk material, such as altered symmetry, magnetic properties, and conductivity within an otherwise insulating matrix. The unique potential of ferroelectric domain walls became apparent with the discovery of their novel functional properties, particularly their conductivity. Domain microstructures can be manipulated using bias, stress, and temperature, allowing domain walls to be created, moved, and eliminated at will. This capability gave rise to the field of 'domain wall nanoelectronics,' where these boundaries are envisioned as 'ephemeral' electronic components in devices like memristors, transistors, and binary memory bits. In such devices, the domain wall can be moved or annihilated to reconfigure the nanoscale circuit during normal operation. While most studies focused on their control and use in device geometries, research that relates to the fundamental understanding of ferroelectric domain walls is still in its infancy. Their intrinsic electronic properties have remained underexplored, and little is known about their behaviour as a function of temperature and doping. Ferroelectric domain walls could exhibit metal-to-insulator transitions akin to other complex oxides including the rare-earth nickelates and superconducting cuprates. In such systems, the complex phase diagrams spanning magnetic and superconducting phases can be explored by chemical doping, revealing unconventional signatures in electronic transport. Additionally, further reduction of the dimensionality and the proximity of the nanoscale conducting channels to a ferroelectric polarisation can further break inversion symmetry and lead to complex transport behaviour. Domain walls in ferroic materials therefore hold great potential for the exploration of such novel phenomena, as fascinating physics can occur in these two-dimensional mobile systems that would open a whole new family of possible device geometries in the field of oxide electronics. This project aims to fill the gap in the fundamental understanding of ferroelectric domain walls by determining their intrinsic electronic properties as a function of doping and temperature, through a combination of electronic transport and spectroscopic measurements. This goal will be accomplished by developing new methods that will go beyond the state of the art and will allow the detailed characterisation and fine-tuning of the properties of domain walls. With this project, we will be able to determine whether domain walls behave as two-dimensional electron gases and search for novel phenomena that arise due to the two-dimensional nature of the system, including the integer and fractional quantum Hall effect, phenomena dominated by the spin-orbit interaction, and superconductivity. This proposal will facilitate the discovery and fundamental characterisation of new two-dimensional conducting systems and allow them to be optimised for future exploitation.