Nonmagnetic band gap at the Dirac point of the magnetic topological insulator (Bi$_{1-x}$Mn$_x)_2$Se$_3$

TL;DR

This study reveals that the observed band gap at the Dirac point in magnetic topological insulator (Bi,Mn)2Se3 is not due to magnetic order but likely caused by resonant scattering, challenging existing theories of topological protection.

Contribution

It identifies a nonmagnetic mechanism, resonant scattering, responsible for the Dirac point gap in magnetic topological insulators, contrary to previous assumptions about magnetic effects.

Findings

The surface band gap is not due to ferromagnetic order.

Mn doping does not alter the topological nature of the bulk bands.

Resonant scattering causes the Dirac point gap, supported by in-gap states observations.

Abstract

Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi$_{1-x}$Mn$_{x}$)$_{2}$Se$_{3}$ is a prototypical magnetic topological insulator with a pronounced surface band gap of $\sim100$ meV. We show that this gap is neither due to ferromagnetic order in the bulk or at the surface nor to the local magnetic moment of the Mn, making the system unsuitable for realizing the novel phases. We further show that Mn doping does not affect the inverted bulk band gap and the system remains topologically nontrivial. We suggest that strong resonant scattering processes cause the gap at the Dirac point and support this by the observation of in-gap states using resonant photoemission. Our findings establish a novel mechanism for gap opening in topological surface states which challenges the currently known conditions for topological protection.