Ferroelectric Domain Walls
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Ferroelectrics are materials with a spontaneous electric polarization that can be reoriented by an external electric field, a property that lends itself to key applications in electronics. This polarization typically points in different directions for different regions of the ferroelectric. In analogy with ferromagnetism, each region with a uniform polarization is called a domain, and the thin boundary between adjacent domains is a domain wall. Domain walls dramatically affect the properties of ferroelectrics [1–10]. Experimentally, it is difficult to characterize domain walls with atomic resolution, since in many cases this involves measuring atom positions with bettern than 0.05 A accuracy. Exam- ˚ ples of recent work in this regard are the first high-resolution transmission electron microscopy results on single crystals of BiFeO3  and the high spatial resolution study of ErMnO3 using X-ray photoemission electron microscopy . In this context, atomistic simulation calculations have emerged as a powerful complementary tool to experimental studies in unravelling the properties of these walls and how they affect the material where they are present. Moreover, ferroelectric domain walls are one of the simplest two-dimensional defects in crystals, so they represent a good test case in the quest of computational methods to tackle defects in solids, a necesary step towards the goal of modeling realistic materials. A substantial amount of recent work in this field is related to the discovery of electronic conductivity in the domain walls of insulating BiFeO3 , a phenomenon that can turn these walls into nanoscale functional elements [14–20]. A possible mechanism for this conduction is related to the presence of charged domain walls; in principle, these are electrostatically unfavourable, but they seem to form is some cases [21–23]. Other possible mechanisms involve point deffects, that are relevant in the study of domain walls because, among other things, they can pin the walls in place [24,25]. This is important in connection to the dynamics of the walls, another topic of intense research [26–28], which in turn relates to the polarization switching mechanisms [29–37]. Other relevant recent research activities have to do with the interplay between ferroelectric and magnetic domain walls in hexagonal manganites [38–44], the control and manipulation of domain wall motion [45, 46], the relation of domain walls with flexoelectricity [47, 48], the possibility of using domain walls as a reactive area for structures not achievable by conventional means  or to manipulate materials such as graphene , the role of domain walls in organometal halide perovskites used in solar cells , and the presence of ferroelectric domain walls in non-ferroelectric materials . Many of the recent references that we have cited so far report experimental results together with theoretical or computational analysis to help to their understanding. Typical computational methods used in this context include those based on thermodynamic Landau-GinzburgDevonshire theory [16, 18, 21, 22, 29, 31, 32, 36, 39, 41–43, 48, 52], effective Hamiltonians [11, 26], molecular dynamics with classical potentials [35, 37], and density-functional theory [13, 22, 25, 36, 40–42, 49–51]. Apart from helping to understand the results of particular experiments, computational methods have also provided general insights into the structure, energetics, and behaviour of domain walls, starting with pioneering work on simple perovskites [53–58] and reaching recently more complicated configurations [59–64]. These methods have recently been used to make predictions such as that domain-wall motion can give rise to negative capacitance .
Our workshop will bring together researchers in fields related to ferroelectric domain walls. This includes scientists using models which are solved with the help of computers—from models based on Landau-Ginzburg-Devonshire theory to models based on density-functional theory. It also includes experimentalists, since we believe that their view is important regarding how calculations can best contribute to the field. The workshop will maximize scientific interaction between attendees, with the following goals in mind: • To refine the attendees’ picture of the state of the art regarding ferroelectric domain walls properties and applications, focusing on what are the open problems, and what are the opportunities for development of new computational methodology. • To allow the interaction between computational researchers with different methodology backgrounds, in order to foster opportunities for designing hybrid approaches. • To allow the interation between computational researchers and experimentalists, so that open problems can be discussed and new approaches can be envisioned.
Oswaldo Dieguez (Tel Aviv University) - Organiser
Ilya Grinberg (Department of Chemistry, Bar Ilan University) - Organiser
Yael Yogev (Tel Aviv University) - Organiser
Jorge Iniguez (Luxembourg Institute of Sciene and Technology) - Organiser