to the emerging class of materials with non-trivial electronic band structure like topological insulators, Dirac or Weyl materials, featuring different topologically protected electronic states. In Weyl materials, these topologically protected electronic states are highly stable quasiparticles in the bulk, known as Weyl fermions, that were first predicted by Hermann Weyl in 1929. [1] These Weyl fermions carry a non-zero chiral charge and always appear in pairs. In a common Weyl material, this chiral charge is ±1; hence, the total chiral charge is zero. In contrast, the fermionic materials host Weyl fermions that are degenerated and carry a higher chiral charge. Remarkably, recently long Fermi arcs were observed in the fermionic material CoSi by Rao et al. [2] Also, Sanchez et al. [3] discovered helicoid-arc quantum states in CoSi.A necessary condition for the existence of Weyl fermions is a broken space or time inversion symmetry. The chiral charge of the material strongly depends on the space group of the crystal structure. CoSi has a cubic B20 crystal structure (space group 198 P2 1 3) with broken space inversion symmetry. This space group was predicted to host sixfold degenerated Weyl fermions. [4] In CoSi, the Weyl fermions carry a chiral charge of up to ±4. In detail, spin 1 excitations with a chiral charge of ±2 and spin 3/2 excitations with a chiral charge of ±4 (Rarita-Schwinger-Weyl fermions) were predicted. [5,6] Electrical transport experiments are a powerful tool to probe the energy spectrum of a material and thereby the specific nature of Weyl fermions. Topological states are massless due to the linear dispersion relation, and thus have an extremely high mobility. While often superimposed by the normal band transport, contributions of the Weyl fermions increase the total electrical conductivity. Consequently, the emerging class of materials with non-trivial electronic band structure has a large overlap with other classes of functional materials like the class of thermoelectric materials. [7] This also holds for CoSi. [5,8,9] Interestingly, related silicides like MnSi, [10] Fe 0.75 Co 0.25 Si [11] and germanides like Fe 1 −x Co x Ge [12,13] also feature skyrmion states.In general, Weyl materials show characteristic quantum effects in the transport, resulting for example in quantum Materials with topological electronic states have emerged as one of the most exciting discoveries of condensed quantum matter, hosting quasiparticles with extremely low effective mass and high mobility. Weyl materials contain such topological states in the bulk and additionally have a non-trivial chiral charge. However, despite known quantum effects caused by these chiral states, the interplay between chiral states, and a charge density wave phase, an ordering of the electrons to a correlated phase is not experimentally explored. Indications for the formation of a charge density wave phase in the Weyl material cobalt monosilicide CoSi are observed. Furthermore, the typical signatures of the charge density wave phase together...

The temperature and thickness dependent thermoelectric properties of Bi 87 Sb 13 nano-films with a thickness from 84 nm to 282 nm have been studied in a temperature range from 110 K up to 450 K. The films have been prepared by thermal evaporation of the raw material from an Al 2 O 3 coated tungsten boat under vacuum conditions of at least 10 −6 mbar. The measurements have been performed using a novel measurement platform, which allows the nearly simultaneous characterization of the thermal conductivity, electrical conductivity, the Seebeck coefficient and the Hall coefficient. All properties are measured in the in-plane direction at the same sample within one measurement run, avoiding many sources of uncertainties and allowing the calculation of the direction dependent, in-plane thermoelectric figure of Merit ZT with high precision. The maximum ZT value of 0.28 has been obtained for the thickest sample at a temperature of 265 K. All comparative measurements have been performed after an initial thermal annealing step, as the heat treatment shows a strong impact on the thermoelectric performance of the films.

3D TIs feature a bulk band gap of a conventional semiconductor and topological surface states on all crystal facets. [3] Electrons on these topological surface states are robust with respect to localization. [4] Even with strong disorder on the atomic scale, these electrons do not backscatter between states of opposite momentum and opposite spin. [5] This confers the high mobility of the electrons occupying these surface states. Such electrons also penetrate energetic barriers caused by materials imperfections and atomic steps at the surfaces. [4] These unique electronic properties propel visions of potential applications in quantum computing and spintronics. [6] Tetradymite-type bismuth telluride, Bi 2 Te 3 , is a famous representative of 3D TIs. However, its defect chemistry is rather complex. For instance, slight changes in the material stoichiometry defined by anti-site defects [7] and disorders [8] determine the dominant carrier transport mechanism. The n-type semiconducting behavior can be attributed to naturally occurring Te-vacancies that donate two electrons each. Additionally, anti-site defects of Te-atoms on Bi-lattice sites lead to intrinsic n-type doping of these materials. [9] According to a density functional theory 3D topological insulators (TI) host surface carriers with extremely high mobility. However, their transport properties are typically dominated by bulk carriers that outnumber the surface carriers by orders of magnitude. A strategy is herein presented to overcome the problem of bulk carrier domination by using 3D TI nanoparticles, which are compacted by hot pressing to macroscopic nanograined bulk samples. Bi 2 Te 3 nanoparticles well known for their excellent thermoelectric and 3D TI properties serve as the model system.As key enabler for this approach, a specific synthesis is applied that creates nanoparticles with a low level of impurities and surface contamination. The compacted nanograined bulk contains a high number of interfaces and grain boundaries. Here it is shown that these samples exhibit metallic-like electrical transport properties and a distinct weak antilocalization. A downward trend in the electrical resistivity at temperatures below 5 K is attributed to an increase in the coherence length by applying the Hikami-Larkin-Nagaoka model. THz time-domain spectroscopy reveals a dominance of the surface transport at low frequencies with a mobility of above 10 3 cm 2 V −1 s −1 even at room temperature. These findings clearly demonstrate that nanograined bulk Bi 2 Te 3 features surface carrier properties that are of importance for technical applications.