The Topology of Conductive Ferroelectric Domain Walls

近十年来，铁电体中的磁畴壁因其优异的介电、光学、磁学、电学和机械性能而成为人们研究的热点。DW代表在整个块体系统中延伸的纳米界面，并显示出超高的电子导电性，在块状单晶LiNbO_3(LNO)中，单个DW的电子导电性可达到数个10微安。这些显着的性质推动铁电纳米结构成为最有前途的功能纳米结构之一，在现代类型和可重构的纳米电子器件中应用。根据最近的研究，铁电离散波结构在其介电极化中可能包含新的拓扑结构，这些拓扑结构比传统上预期的单轴铁电材料中的伊辛型组态要复杂得多。DWS的局域对称性破缺特别重要，因为它可以促进奇异的极地拓扑结构，类似于在磁系统中观察到的结构。探索铁电离散波结构的详细结构是理解和控制离散波特性的前提。这个联合研究项目的目标是阐明这些DW区域的局域对称性和拓扑结构，并研究和量化它们在被呈现为高导电性时的相互关联的物理和光学性质。在这个德法联合项目中的两个团队已经证明，DW可以被优雅地调谐来沿着二维DW传输强电子电流。在LNO中，可以通过简单地改变DW相对于极轴的倾角来控制这种壁内的自由电荷载流子密度。然后，我们预计这种DW将从其纯伊辛型组态转变为Bloch或Néel型态，这取决于所研究的材料、可能的样品掺杂或施加在晶体上的偏置电场。此外，我们还开发了复杂的局部探测和非线性光学技术，能够量化和三维绘制这种非伊辛构型的存在。因此，我们打算在LNO单晶家族中设计手性DW，包括掺镁和不掺镁，并使用例如二次谐波偏振技术实时和真实空间监测它们的行为。该项目有望对这种非伊辛的、通常是手性极化结构的起源和形成提供开创性的见解，这对于深刻理解和调整未来基于铁电离散波结构的光电纳米器件是必要的。

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.