Landau levels of the topological semimetal CoSi near the $\Gamma$-point and their contribution to the orbital magnetic susceptibility

The band structure of the topological semimetal CoSi in a magnetic field is studied. Using tight-binding model based on first-principle calculations, electronic spectrum was obtained over the entire Brillouin zone. Its features near the topological node at the $\Gamma$-point are studied in the $\mathbf{k}\cdot\mathbf{p}$ approximation using parameters obtained from ab initio calculations. Since the topological charge of the node is greater than unity, several chiral levels appear in the magnetic field, the number of which is consistent with the magnitude of the charge. The contribution of the spectrum region near the node at the $\Gamma$-point to the orbital magnetic susceptibility of CoSi is calculated and its differences from the case of an ordinary Weyl node are analyzed.