Unconventional Spin Hall Effect and Axial Current Generation in a Dirac Semimetal

TL;DR

This paper explores an unconventional spin Hall effect and axial current generation in a Dirac semimetal, revealing boundary-localized axial currents and spin accumulation despite zero conventional spin current.

Contribution

It derives diffusion equations for Dirac semimetals showing boundary spin and axial currents, highlighting novel transport phenomena in topological materials.

Findings

Spin Hall effect with spin accumulation occurs without conventional spin current.

Axial current flows near the boundary, perpendicular to electric current.

Boundary-localized axial currents are demonstrated in Dirac semimetals.

Abstract

We investigate electrical transport in a three-dimensional massless Dirac fermion model that describes a Dirac semimetal state realized in topological materials. We derive a set of interdependent diffusion equations with eight local degrees of freedom, including the electric charge density and the spin density, that respond to an external electric field. By solving the diffusion equations for a system with a boundary, we demonstrate that a spin Hall effect with spin accumulation occurs even though the conventional spin current operator is zero. The Noether current associated with chiral symmetry, known as the axial current, is also discussed. We demonstrate that the axial current flows near the boundary and that it is perpendicular to the electric current.