Dynamics of domain-wall Dirac fermions on a topological insulator: A chiral fermion beam splitter

The intersection of two ferromagnetic domain walls placed on the surface of topological insulators provides a one-way beam splitter for domain-wall Dirac fermions. Based on an analytic expression for a static two-soliton magnetic texture we perform a systematic numerical study of the propagation of Dirac wave packets along such intersections. A single-cone staggered-grid finite difference lattice scheme is employed in the numerical analysis. It is shown that the angle of intersection plays a decisive role in determining the splitting ratio of the fermion beam. For a nonrectangular intersection, the width and, to a lesser extent, the type of domain walls, e. g., Bloch or Neel, determine the properties of the splitter. As the ratio between domain-wall width and transverse localization length of the Dirac fermion is increased its propagation behavior changes from quantum-mechanical (wavelike) to classical ballistic (particlelike). An electric gate placed near the intersection offers a dynamic external control knob for adjusting the splitting ratio.