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

点击翻译按钮获取中文摘要

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.