Band structure and unconventional electronic topology of CoSi

Abstract

Semimetals with certain crystal symmetries may possess unusual electronic structure topology, distinct from that of the conventional Weyl and Dirac semimetals. Characteristic property of these materials is the existence of band-touching points with multiple (higher than two-fold) degeneracy and nonzero Chern number. CoSi is a representative of this group of materials exhibiting the so-called 'new fermions'. We report on an ab initio calculation of the electronic structure of CoSi using density functional methods, taking into account the spin-orbit interactions. The linearized [Formula: see text] Hamiltonian, describing the anisotropic electronic structure of CoSi near the Γ point is derived. The topological features of band-touching nodes with four- and six-fold degeneracy located at the Γ and R points in the first Brillouin zone are analysed using the linearized Hamiltonians and first principle calculations. In particular, we show, using the non-Abelian Berry curvature, that these band-touching points carry topological charges of [Formula: see text], which change signs at certain values of parameters of the Hamiltonians. We describe the resulting Fermi arc surface states and their spin texture. We also discuss the influence of many body [Formula: see text] corrections on the electronic band structure and the topological properties of CoSi.

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

“…The transport signature of the chiral anomaly is a positive correction to the magneto-conductance for parallel components of the electrical and magnetic field (I∥B) since the chiral symmetry of the Weyl fermions is broken. [5,6] it was shown that the nodal point with topological charge 4 at Gamma point of the Brillouin zone is situated by about 20 meV above Fermi energy, while another Weyl node with charge 1 is located by about 30 meV below Fermi energy. [14,15] In order to evidence features of topological states in transport experiments, it is necessary that the Fermi energy is close to the crossing of the linear states in the band structure.…”

“…[1] These Weyl fermions carry a non-zero chiral charge and always appear in pairs. [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. In contrast, the fermionic materials host Weyl fermions that are degenerated and carry a higher chiral charge.…”

“…In contrast, the fermionic materials host Weyl fermions that are degenerated and carry a higher chiral charge. [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. 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. [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. [4] In CoSi, the Weyl fermions carry a chiral charge of up to ±4.…”

Unconventional superconductors with spin-orbit interaction

“…Similar to Weyl semimetals, there are type II Dirac semimetals with tilted Dirac cones; the following materials are predicted to be this type of Dirac semimetals, PdTe 2 [73], PtSe 2 [73], VAl 3 [74], and KMgBi [75]. Furthermore, ab initio calculations have predicted that θ-TaN [76], ZrTe [77], and RERh 6 Ge 4 (RE=Y,La,Lu) [78] have triple degenerate gapless points (triple point fermion), and CoSi [79] has a six-fold degenerate point. In addition, it is predicted that Ba 3 SnO has multiple Dirac points with different energies near the Fermi level [80].…”