Orbital magnetic susceptibility of multifold fermions

Abstract

Topological semimetals are intensively studied in recent years. Besides the well known Weyl and Dirac semimetals, some materials possess nodes with linear crossing of multiple bands. Low energy excitations around these nodes are called multifold fermions and can be described by $\mathbf{k}\cdot\mathbf{p}$ Hamiltonian with pseudospin greater than 1/2. In the present work we investigate the contribution of these states into orbital magnetic susceptibility $\chi$. We have found that, similarly to Weyl semimetals, the dependence of susceptibility on chemical potential $\mu$ shows an extremum when $\mu$ is close to the band crossing energy. In the case of half-integer pseudospin, this extremum is a minimum and the susceptibility is negative (diamagnetic). While in the case of integer pseudospin, the susceptibility is large and positive (paramagnetic) due to the contribution of dispersionless band, corresponding to zero pseudospin projection. This leads also to nonmonotonic temperature dependence of $\chi$. As an example, we considered the case of cobalt monosilicide, where the states near the $\Gamma$ point correspond to pseudospin 1 without spin-orbital interaction, and to a combination of Weyl node and pseudospin-3/2 states taking into account spin-orbit coupling.