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 k ⋅ p 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 ± 4 , 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 G 0 W 0 corrections on the electronic band structure and the topological properties of CoSi.

Monosilicides of transition metals crystallizing in a B20 (FeSi-type) structure (space group P2 ${}_1$ 3, #198) possess a wide range of specific properties. Among them are semiconductors, metals, and paramagnetic, diamagnetic, and ferromagnetic compounds. Some of them were studied as promising thermoelectric materials. Recently, B20 monosilicides have attracted attention as a new class of topological semimetals with topological charge greater than unity. In the present work, we analyze the electronic structures of B20-type monosilicides of the fourth, fifth, and sixth periods of the Periodic Table in order to reveal their common features and peculiarities. To make this analysis more consistent, we performed a density-functional study of the electronic structures of the monosilicides in a unified manner. We reviewed the results of previous calculations and the available experimental data, comparing them with our results. The band structures of ReSi and TcSi not found in the literature were calculated and analyzed as well. The topological properties of these materials and of some isostructural germanides and stannides were investigated. Analysis reveals the current understanding of electronic structures and properties of this compound group.

This paper reports on a theoretical and experimental investigation of the behavior of the electrical and thermal conductivities of a nanostructured material based on Bi _x Sb_{2 − x} Te₃ solid solutions. The effect of boundary scattering has been taken into account by introducing the scattering mechanism with a constant mean free path equal to the nanoparticle size. A comparison with the results of the measurements has demonstrated that one can describe satisfactorily the experimental dependences of the electrical and thermal conductivities on the nanoparticle size by using only the parameters of the initial solid solution and its pure constituents. The estimates have revealed that the lattice thermal conductivity of nanostructured materials can be reduced by 20–30% as compared to the initial solid solution with nanoparticle sizes of the order of 20 nm, which should produce a favorable effect on the magnitude of the thermoelectric figure-of-merit.