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非常吸引人的应用特点。然而，现有技术的热电材料的相对低的效率阻碍了热电装置的大规模扩展。因此，进一步提高热电材料的效率、其稳定性和与环境的兼容性要求是必要的。最近，一类新物质拓扑绝缘体（TI）的发现，为电输运与热输运解耦的策略带来了另一个自由度。在这种情况下，由于拓扑表面状态的高迁移率，在一些实验可达到的条件下，电输运由表面主导，而热输运由体主导。更奇特的性质最近加入了具有拓扑非平凡电子结构的材料家族：外尔半金属和密切相关的狄拉克半金属。外尔半金属具有表面态，其拓扑非平凡性质表现在它们的费米表面的形状上，具有开放弧（费米弧）的形式，而不是任何规则的二维（2D）金属中的闭合曲线。外尔半金属家族的最新候选者是SrSi 2。关于SrSi 2化合物的热电性能的实验数据表明，它可以被调谐到在室温下具有约0.4的ZT，这与良好的中温热电材料的值相当。SrSi 2具有独特的电子结构、良好的热电性能和较宽温度范围内的相稳定性，是研究体相和表面相（费米弧）拓扑状态对热电性能影响的理想材料。SrSi 2作为外尔半金属类候选材料的研究将包括材料合成技术和掺杂程序的发展。采用气体输运法和Bridgman法制备SrSi_2单晶。电子结构将通过角分辨光电子能谱（ARPES）进行研究，以确认该材料属于Weyl半金属类，并揭示电子结构的拓扑特征与热电性能之间的关系。电子结构和热电性能的块状和薄膜样品的SrSi 2将被测量。基于对结果的理论分析，确定了拓扑结构对热电性能的贡献。理论模型的拓扑非平凡的电子结构参数的调整，以最大限度地提高热电性能将被开发。

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.