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 by tuning the Fermi levels of both top and bottom surfaces. 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 Bi2−xSbxTe3 thin films, where the field-induced anisotropy presents a strong dependence on the gate voltage with a characteristic two-peak structure near the Dirac point. 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 backscattering. The observed PHE provides a useful tool to analyze and manipulate the topological protection of the TI surface.
Highlights
A prominent feature of topological insulators (TIs) is the surface states comprising of spinnondegenerate massless Dirac fermions
When the field is applied along the surface of a TI, time-reversal symmetry (TRS) is broken, but such a parallel magnetic field will not affect helical surface states besides a shift of the Dirac dispersion in the momentum space; no gap will open in the Dirac dispersion for highsymmetry orientations of the surface and the magnetic field
We found that the scattering of Dirac fermions in this situation becomes anisotropic because the spin-momentum locking causes a difference in the scattering amplitudes for particles with the spin parallel and perpendicular to the magnetic field direction
Summary
A prominent feature of topological insulators (TIs) is the surface states comprising of spinnondegenerate massless Dirac fermions. We found that the scattering of Dirac fermions in this situation becomes anisotropic because the spin-momentum locking causes a difference in the scattering amplitudes for particles with the spin parallel and perpendicular to the magnetic field direction. This leads to a magnetic fieldinduced anisotropy in the resistivity measured along and perpendicular to the field, which results in a novel planar Hall effect (PHE). In other words, this intriguing effect is a manifestation of the momentum-selective lifting of the topological protection due to TRS breaking
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