Signatures of a Charge Density Wave Phase and the Chiral Anomaly in the Fermionic Material Cobalt Monosilicide CoSi

Abstract

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...

Crystal Structure Analysis and Magneto‐Transport Investigation of Co1−xFexSi (with x = 0% to x = 20%)

“…In combination with ρ(T), the peak was previously interpreted as the suppression of a CDW phase. [17] For the 6% Fe composition, we observed a similar feature ρ(T) or more visible in the first derivative dρ/dT between 50 and 75 K (compare Section S2, Supporting Information). For the 2% Fe composition, we observed the same feature at 20 K, but only as a small change in the slope of ρ(T).…”

“…C pos is shown in Figure 8a,c,e, while C WAL is shown in Figure 8b,d,f for E ∥ B , E ⊥ B , ip and E ⊥ B , oop, respectively. The fitting parameters from our previous study on the 4% Fe composition [ 17 ] are added to this figure. We will start the discussion with the C pos part.…”

“…[ 15,16 ] In a preceded study on Fe‐doped CoSi with 4% of Co replaced by Fe, we observed features of the chiral anomaly and WAL in combination with potential signatures of a CDW phase in the electrical transport. [ 17 ]…”

“…[15,16] In a preceded study on Fe-doped CoSi with 4% of Co replaced by Fe, we observed features of the chiral anomaly and WAL in combination with potential signatures of a CDW phase in the electrical transport. [17] Within this work, we exemplary evaluate crystal structure of Co 1−x Fe x Si with from 0% to 20% Fe (0%, 4%, 10%, 20%). Additionally, we systematically characterized and analyzed the electrical transport properties of Co 1−x Fe x Si compositions with 0% Fe, 2% Fe, 6% Fe, 10% Fe, and 20% Fe.…”

Effect of Deformation on Topological Properties of Cobalt Monosilicide

“…They were successfully interpreted by the coexistence of two close Fermi surfaces in agreement with DFT results for the band structure. The influence of chiral fermions and charge density waves on magnetic field dependent electrical transport was studied in [23]. The experimental and theoretical investigation of optical conductivity of CoSi revealed various exotic multifold quasiparticles [15].…”