Current-Induced Spin Accumulation and Magnetoresistance in Chiral Semimetals

Abstract

Weyl fermions possess the property of spin-momentum locking: the expectation value of the spin is parallel or antiparallel to the momentum at any given point in the Brillouin zone in the vicinity of a Weyl node. This is a direct consequence of the fact that Weyl nodes are monopoles of the Berry curvature, and in this sense an expression of the nontrivial Weyl electronic structure topology. Thanks to this property, an isolated Weyl node produces a large spin accumulation in response to a charge current, $\hbar/2$ per electron, similar to surface states of time-reversal invariant topological insulators. However, in bulk Weyl semimetals, the nodes must occur in pairs of opposite chirality and, when the nodes are at the same energy, the effect cancels out. Here we show that this cancellation is avoided in chiral semimetals, in which Weyl nodes of opposite chirality occur at different energies due to broken mirror symmetry. We find that the spin accumulation is maximized when the Fermi energy coincides with one of the nodes in a pair and reaches the same value as for an isolated node in this case. Moreover, we demonstrate the existence of a distinct magnetoresistance mechanism, closely related to this current-induced spin accumulation.