The Topology of Conductive Ferroelectric Domain Walls

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Domain walls (DWs) in ferroelectrics have become a topic of major interest over the last 10 years because of their exceptional dielectric, optical, magnetic, electronic and mechanical properties. The DWs represent nanometric interfaces that extend across the full bulk system and display an ultra-high electronic conductivity, reaching several 10 µA for a single DW in bulk single crystalline LiNbO3 (LNO). These remarkable properties propel ferroelectric DWs as one of the most promising functional nanostructure for modern-type and reconfigurable applications in nanoelectronic devices. According to recent studies, ferroelectric DWs could contain novel topological structures in their dielectric polarization that are much more complex than the Ising-type configuration, which is the traditionally expected DW type in uniaxial ferroelectrics. The local sym¬metry breaking at the DWs is particularly important as it can promote exotic polar topological structures, similar to those observed in magnetic systems. Exploring the detailed ferroic structure of ferroelectric DWs is a prerequisite for the understanding and control of DW properties. The goal of this joint research project is to elucidate the local symmetry and topology at such DW regions and to investigate and quantify their interrelated physical and optical properties when being rendered highly conductive.The two teams allied within this joint German-French project have shown that DWs can be elegantly tuned for transporting high electronic currents along the two-dimensional DW. In LNO, the free charge carrier density within such a wall can be steered by simply varying the DW’s inclination with respect to the polar axes. We then expect this DW to convert from its pure Ising-type configuration into a Bloch- or Néel-type state, depending on both the material under investigation, a possible sample doping, or an electrical bias field applied across the crystal. In addition, we have developed sophisticated local probe and nonlinear optical techniques that are able to quantify and three-dimensionally map the presence of such non-Ising configurations. We accordingly intend to engineer chiral DWs in the LNO single crystals family, both with and without Mg doping, and monitor their behavior in real time and real space using, for instance, second-harmonic generation polarimetry. This project is expected to deliver groundbreaking insight on the origin and build-up of such non-Ising, often chiral polarization structures at DWs, as is necessary for the profound understanding and tuning of future optoelectronic nano-devices based on ferroelectric DWs.