# Planar Hall effect from the surface of topological insulators

## TL;DR

This paper reports the discovery of a novel planar Hall effect on topological insulator surfaces caused by in-plane magnetic fields, revealing a new way to analyze and control surface state protection.

## Contribution

It introduces the first observation of a surface-induced planar Hall effect in topological insulators, linked to magnetic field-induced anisotropy and surface state back-scattering.

## Key findings

- PHE observed in dual-gated Bi2-xSbxTe3 thin films.
- PHE depends strongly on gate voltage with a two-peak structure near the Dirac point.
- Theoretically explained using a self-consistent T-matrix approximation.

## 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.