Anisotropic surface transport in topological insulators in proximity to a helical spin density wave

TL;DR

This paper investigates how a helical spin density wave influences surface transport in topological insulators, revealing anisotropic Dirac cone deformation and the emergence of semi-Dirac points with highly directional electron velocities.

Contribution

It introduces the concept of a helical spin density wave acting as a one-dimensional magnetic potential, causing anisotropic surface states and new semi-Dirac points in topological insulators.

Findings

Surface Dirac cone becomes highly anisotropic.

New semi-Dirac points appear at Brillouin zone boundaries.

Electron group velocity is highly directional at semi-Dirac points.

Abstract

We study the effects of spatially localized breakdown of time reversal symmetry on the surface of a topological insulator (TI) due to proximity to a helical spin density wave (HSDW). The HSDW acts like an externally applied one-dimensional periodic(magnetic) potential for the spins on the surface of the TI, rendering the Dirac cone on the TI surface highly anisotropic. The decrease of group velocity along the direction $\hat{x}$ of the applied spin potential is twice as much as that perpendicular to $\hat{x}$. At the Brillouin zone boundaries (BZB) it also gives rise to new semi-Dirac points which have linear dispersion along $\hat{x}$ but quadratic dispersion perpendicular to $\hat{x}$. The group velocity of electrons at these new semi-Dirac points is also shown to be highly anisotropic. Experiments using TI systems on multiferroic substrates should realize our predictions. We further discuss the effects of other forms of spin density wave on the surface transport property of topological insulator.