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.

• The paper develops a theory quantifying spin accumulation in chiral semimetals where Weyl nodes are energetically separated.
• It employs diagrammatic perturbation theory to derive coupled spin-charge diffusion equations that explain complex transport phenomena.
• Numerical results reveal a nonmonotonic Fermi energy dependency and a spin-valve-like magnetoresistance with promising spintronic applications.

Current-Induced Spin Accumulation and Magnetoresistance in Chiral Semimetals

The paper "Current-Induced Spin Accumulation and Magnetoresistance in Chiral Semimetals" investigates the theoretical framework underlying spin-momentum locking and its resultant effects in Weyl semimetals. The focus centers on the role of broken inversion and mirror symmetries in chiral semimetals wherein Weyl nodes of opposite chirality occur at different energies, in contrast to traditional bulk Weyl semimetals where these nodes cancel out any net spin accumulation.

Key Findings and Contributions

The paper develops a generalized theory to quantify spin accumulation in response to an electrical current in chiral topological semimetals. It presents a minimal theoretical model considering a pair of linearly dispersing Weyl nodes separated in both momentum and energy. By incorporating impurity scattering using a diagrammatic perturbation theory, the researchers derived coupled spin-charge diffusion equations that capture the interaction between spin and charge transport.

The key finding is that spin accumulation is maximized when the Fermi energy aligns with one of the nodes, achieving an effective spin polarization equivalent to an isolated node. This is a significant departure from previous understandings where such effects were considered entirely negligible in bulk semimetals.

Numerical Insights and Implications

Noteworthy numerical insights include the demonstration that the spin accumulation exhibits a nonmonotonic dependency on the Fermi energy. The maximum value, equivalent to ℏ/2 per electron, aligns with conditions where the Fermi energy matches one of the Weyl nodes, while both at charge neutrality and far from the nodes, the effect dissipates. Furthermore, a novel magnetoresistance mechanism emerges, contingent on nonequilibrium spin density induced by spin-polarized current injection. This results in an additional contribution to voltage affected by the magnetization's orientation, leading to a distinct spin-valve-like effect.

The implications of these findings extend both practically and theoretically. Practically, these results offer potential advancements in spintronic applications, wherein control over spin currents is vital. Chiral semimetals such as CoSi and RhSi present promising platforms for experimental observation of these theoretical predictions due to their complex electronic structures characterized by non-trivial topology.

Future Directions

This research opens several avenues for further exploration. Experimentally, verifying the theory in chiral semimetal samples is crucial, potentially accounting for the more elaborate electronic nature of real materials as opposed to the idealized model presented. Theoretically, extending studies to include interactions between different transport phenomena, as well as disorder effects beyond the self-consistent Born approximation, could enrich the comprehension of spin-related phenomena in topologically nontrivial systems.

In conclusion, the paper provides robust theoretical insights into spin phenomena in chiral semimetals, laying groundwork for experimental validations and future studies in topological materials and spintronics.