Band structure and unconventional electronic topology of CoSi

Abstract: Crystalline semimetals with certain space group symmetries may possess unusual electronic structure topology, distinct from 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 topological charge. 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. We demonstrate the existence of band-touching nodes with four- and six-fold degeneracy, located at the $\Gamma$ and $R$ points in the first Brillouin zone and near the Fermi energy. We show that these band-touching points carry topological charges of $\pm 4$ and describe the resulting Fermi arc surface states, connecting the projections of these nodes onto the surface Brillouin zone. We also discuss the influence of many body $G_0W_0$ corrections on the electronic band structure and the topological properties of CoSi.