DFG-RSF: Contribution of topological states to thermoelectric properties of Weyl semimetals

热电能量转换器在低势能余热利用技术中具有十分诱人的应用前景。然而，目前热电材料的效率相对较低，阻碍了热电器件的大规模扩展。因此，进一步提高热电材料的效率、稳定性和与环境的相容性要求是十分必要的。最近，一类新的物质——拓扑绝缘体（TI）的发现，给电输运和热输运的解耦策略带来了另一个自由度。在这种情况下，由于拓扑表面态的高迁移率，在一些实验可及的条件下，电输运由表面主导，而热输运由体主导。在不断增长的具有拓扑非平凡电子结构的材料家族中，最近增加了更奇特的性质：Weyl和密切相关的Dirac半金属。Weyl半金属具有表面态，其拓扑上的非平凡性质表现在其费米表面的形状上，具有开弧（费米弧）的形式，而不是像任何常规二维（2D）金属那样的封闭曲线。最近加入Weyl半金属族的是SrSi2。SrSi2化合物的热电性能实验数据表明，在室温下，其ZT可调谐到0.4左右，与良好的中温热电材料的值相当。独特的电子结构、良好的热电性能和宽温度范围内相稳定性的结合，使SrSi2成为研究体积和表面拓扑状态（来自费米弧）对热电性能影响的有前途的材料。研究SrSi2作为Weyl半金属类候选材料将包括材料合成技术和掺杂程序的发展。采用气体输运和Bridgman技术制备SrSi2单晶。利用角分辨光电子能谱（ARPES）研究材料的电子结构，以确定材料属于Weyl类半金属，并揭示电子结构的拓扑特征与热电性能之间的关系。测量了SrSi2的体样和薄膜样的电子结构和热电性能。在对结果进行理论分析的基础上，确定拓扑结构对热电性能的贡献。为使热电性能最大化而调整拓扑非平凡电子结构参数的理论模型将被开发。

Thermoelectric energy converters have very attractive features for application in the low-potential waste heat utilization technologies. However, a comparatively low efficiency of the state of the art thermoelectric materials hinders a wide scale expansion of thermoelectric devices. Therefore further improvement of the efficiency of thermoelectric materials, their stability and compatibility with environment requirements are necessary. Recently, the discovery of a new class of matter, the topological insulators (TI), brought another degree of freedom to the strategy of decoupling of electrical transport from thermal transport. In this case, due to high mobility of topological surface states, under some, experimentally accessible conditions, the electrical transport is dominated by surfaces, while thermal transport is dominated by bulk. Even more exotic properties have the most recent additions to the growing family of materials with topologically nontrivial electronic structure: The Weyl and closely related Dirac semimetals. A Weyl semimetal possess surface states, whose topologically nontrivial nature is manifest in the shape of their Fermi surface, having the form of an open arc (Fermi arc), rather than a closed curve, as in any regular two dimensional (2D) metal. The most recent candidate to the Weyl semimetals family is SrSi2. Experimental data on thermoelectric properties of SrSi2 compounds demonstrated that it can be tuned to have ZT at room temperature around 0.4, which is comparable to values of good mid-temperature thermoelectric materials. The combination of the unique electronic structure, promising thermoelectric properties and phase stability in a broad temperature range makes SrSi2 to a promising material to study the effect of topological states, both bulk and surface (from Fermi arcs), on the thermoelectric properties. The investigation of SrSi2 as the candidate material to the Weyl semimetal class will include development of the material synthesis technique and doping procedures. Single crystals of SrSi2 will be prepared by gas transport and Bridgman techniques. Electronic structure will be investigated by means of the Angle Resolved Photo electron Spectroscopy (ARPES) in order to confirm that the material belongs to the class of Weyl semimetals and to reveal the relation between the topological features of electronic structure and thermoelectric performance. Electronic structure and thermoelectric properties of bulk and thin film samples of the SrSi2 will be measured. Basing on the theoretical analysis of the results, the topological contributions to the thermoelectric properties will be determined. Theoretical model for tuning of the topologically non-trivial electronic structure parameters in order to maximize thermoelectric performance will be developed.