Domain Walls in LNT Mixed Crystals

该提案是“极性氧化物中的周期性低维缺陷结构”研究小组的一部分，该小组致力于利用铌酸锂-钽酸锂（LiNb1-xTaxO3， LNT）模型系统研究缺陷结构、电子和离子输运以及机电性能的相关性。目前的建议特别关注这些极性LNT（混合）晶体中的畴壁，由于能量最小化，它们存在于每个这样的铁系统中。热力学平衡自然取决于多种参数，包括尺寸、维度、化学成分等。LNT体系的特定目标化学掺杂（即MgO， Hf, Fe）将改变这种平衡，并导致各种各样的畴和畴壁分布，我们将在这里探讨。值得注意的是，我们期望在这些LNT系统中发现具有前所未有和新颖性质的畴壁，如可调谐畴壁电导率，可变光学折射率或铁电拓扑。首先，我们将通过应用宏观迟滞测量来探索畴和畴壁的介电和电子特性，从而推导出它们的积分响应，例如它们的矫顽力场、自发极化和各种LNT混合晶体的漏电流。然而，在量化它们在超高和超低温下的交流和直流输运特性时，一个明确的焦点在于畴壁电子特性。在可变磁场中的霍尔输运测量将允许指定载流子类型和密度，以及它们在单一畴壁内的迁移率。此外，我们将在这些畴壁上应用一种新的交流输运技术，通过高谐波分析直接访问这些二维输运通道中的缺陷特征。其次，一套专用的非侵入性光学方法将补充上述LNT混合晶体的电子研究。应用微拉曼光谱，二次谐波显微镜/偏振法和荧光显微镜分析域和畴壁，可以量化它们的化学，结构和介电性能。特别是在畴壁处，我们预计局部电场和局部张量性质会发生巨大变化，这可以被视为局部非平凡拓扑存在的直接暗示。

The proposal is part of the research group "Periodic low-dimensional defect structures in polar oxides", which is dedicated to the correlation of defect structure, electron and ion transport, and electromechanical properties using the model system lithium niobate-lithium tantalate (LiNb1-xTaxO3, LNT). The present proposal specifically focuses on domain walls in these polar LNT (mixed) crystals, that are present in every such ferroic system due to energy minimization. The thermodynamic equilibrium naturally depends on a manifold of parameters, including size, dimensionality, chemical composition, and others more. The specifically targeted chemical doping of the LNT systems (i.e with MgO, Hf, Fe) will vary this balance, and lead to a broad variety of domain and domain wall distributions that we will explore here. Notably, we expect to discover domain walls in these LNT systems with unprecedented and novel properties, as are tunable domain wall conductivities, variable optical refractive indices, or ferroelectric topologies. Firstly, we will explore the dielectric and electronic properties of domains and domain walls by applying macroscopic hysteresis measurements that allow deducing their integral responses, such as their coercive fields, the spontaneous polarization, and the leakage currents across the various LNT mixed crystals. A clear focus, however, lies on the domain wall electronic properties, in quantifying their AC and DC transport characteristics at both ultra-high and ultra-low temperatures. Hall-transport measurements in a variable magnetic field will allow to specify charge carrier type and densities, as well as their mobilities within a single domain wall. Complementary, we will apply a novel AC transport technique to these domain walls that provides direct access to the defect characteristics in these 2-dimensional transport channels through higher harmonic analysis. Secondly, a set of dedicated, non-invasive optical methods will complement the above electronic investigations of the LNT mixed crystals. Applying µ-Raman-spectroscopy, Second-Harmonic-Microscopy/Polarimetry and Fluorescence microscopy analysis to domains and domain walls, allows quantifying their chemical, structural and dielectric properties. Specifically at domain walls, we expect the local electric fields, and hence the local tensor properties to vary dramatically, which can be taken as a direct hint towards the presence of locally non-trivial topologies.