Planar Hall effect from the surface of topological insulators

TLDR

Topological insulators host spin‑nondegenerate massless Dirac surface states, enabling studies of their unique Dirac physics. The effect is observed in dual‑gated bulk‑insulating Bi₂₋ₓSbₓTe₃ thin films, where an in‑plane magnetic field breaks time‑reversal symmetry and anisotropically lifts back‑scattering protection of surface Dirac fermions. We discovered a novel planar Hall effect from the TI surface, characterized by a gate‑voltage‑dependent two‑peak anisotropy near the Dirac point, offering a new tool to probe and manipulate topological surface protection.

Abstract

A prominent feature of topological insulators (TIs) is the surface states comprising of spin-nondegenerate massless Dirac fermions. Recent technical advances have made it possible to address the surface transport properties of TI thin films while tuning the Fermi levels of both top and bottom surfaces across the Dirac point by electrostatic gating. This opened the window for studying the spin-nondegenerate Dirac physics peculiar to TIs. Here we report our discovery of a novel planar Hall effect (PHE) from the TI surface, which results from a hitherto-unknown resistivity anisotropy induced by an in-plane magnetic field. This effect is observed in dual-gated devices of bulk-insulating Bi$_{2-x}$Sb$_{x}$Te$_{3}$ thin films, in which both top and bottom surfaces are gated. The origin of PHE is the peculiar time-reversal-breaking effect of an in-plane magnetic field, which anisotropically lifts the protection of surface Dirac fermions from back-scattering. The key signature of the field-induced anisotropy is a strong dependence on the gate voltage with a characteristic two-peak structure near the Dirac point which is explained theoretically using a self-consistent T-matrix approximation. The observed PHE provides a new tool to analyze and manipulate the topological protection of the TI surface in future experiments.