Submitted Abstract
Ferroelastic materials typically present regions (domains) displaying equivalent, but differently oriented, elastic deformations. In ferroelectrics domains are characterized by differently-oriented electric dipoles. Structural domains may appear as the result of kinetic processes (e.g., in a temperature-driven transition) or to accommodate global constraints (e.g., elastic or electric). The boundaries between such domains are the domain walls (DWs), sometimes described as natural interfaces or topological defects. The DWs are very especial objects, as in them the material locally adopts a high-energy configuration that would not occur otherwise. Thus, by construction, DWs can be expected to present properties that differ from those of the domains. Indeed, the field is currently living a great revival fueled by spectacular findings showing that some DWs display electric, conductive, magnetic, chemical and optical behaviors that do not occur in the bulk of the material, which confers them specific and potentially useful functionalities. Additionally, in the all-important family of functional perovskite oxides, DWs tend to be very thin (a few nanometers wide) and controllable (they can be written in high densities, injected, moved erased), which turns them into the ultimate nano-objects. The current expectation is that the walls can themselves act as the active part of novel nano-devices, and thus the motto “the wall is the device”. The present challenge is to understand the physical origin of the incredible domain wall properties observed experimentally, and to find ways to engineer the DW behavior to match specific technological needs. For both purposes, first-principles simulation methods – which provide us with a predictive quantum-mechanical treatment of the walls at an atomistic level (where the new physical effects originate) and can be used in multi-scale schemes to access larger length scales (to realistically mimic the domain wall as a dynamic, deformable two-dimensional object) – are expected to play a very important role.NEWALLS will tackle the challenge of developing such first-principles multi-scale simulation methods, and will apply them to better understand how elastic constraints and the presence of free carriers determine the DW stable structure and behavior. For that purpose, we will build on novel lattice-dynamical simulation techniques recently introduced by NEWALLS principal investigator (PI) and which have already delivered incredible discoveries, e.g., the occurrence of a ferroelectric phase transition at the ferroelectric domain walls of prototype compound PbTiO3. The extension of such a scheme to incorporate electronic effects (recently achieved), and the development of a multi-scale method fully based on first-principles effective models (within NEWALLS), will render a unique tool that will allow us to investigate the hottest open issues (intrinsic DW conductivity, conditions for the formation of charged walls) and explore exciting new ideas (topological charge localization in selected boundaries?). Moreover, NEWALLS will also pursue one of the most exciting possibilities in the field today, namely, that DWs may act as chemical nano-reactors to synthesize new two-dimensional crystals. This incredible concept stems from recent outstanding results [Farokhipoor… Íñiguez… Noheda, Nature 515, 379 (2014)] obtained by a group of researchers including NEWALLS PI, who showed that the stress fields occurring in some (very common) ferroelastic walls may lead to a long-range-ordered chemical substitution and the occurrence of never-seen coordination environments. NEWALLS will run systematic computational investigations to verify whether this concept can help us achieve some major breakthroughs (e.g., stable ferroelectric walls in a paraelectric matrix, stable magnetic walls in a paramagnetic matrix) that constitute the Holy Grail of envisioned domain-wall applications.