Large magnetic gap at the Dirac point in a Mn-induced Bi$_2$Te$_3$ heterostructure

TL;DR

This study reveals a large magnetic gap at the Dirac point in Mn-doped Bi$_2$Te$_3$, exceeding theoretical predictions, due to a self-organized heterostructure that enhances magnetic properties crucial for room-temperature quantum anomalous Hall effect.

Contribution

It demonstrates a significant magnetic gap in Mn-doped Bi$_2$Te$_3$ caused by a unique heterostructure, advancing understanding of magnetic topological insulators for practical applications.

Findings

Magnetic gap of ~90 meV observed below $T_C$

Heterostructure formation enhances the magnetic gap

Bi$_2$Se$_3$ does not form a similar magnetic gap

Abstract

Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE) which provides quantized edge states for lossless charge transport applications. The edge states are hosted by a magnetic energy gap at the Dirac point but all attempts to observe it directly have been unsuccessful. The gap size is considered crucial to overcoming the present limitations of the QAHE, which so far occurs only at temperatures one to two orders of magnitude below its principle limit set by the ferromagnetic Curie temperature $T_C$. Here, we use low temperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi$_2$Te$_3$ films, which is present only below $T_C$. Surprisingly, the gap turns out to be $\sim$90 meV wide, which not only exceeds $k_BT$ at room temperature but is also 5 times larger than predicted by density functional theory. By an exhaustive multiscale structure characterization we show that this enhancement is due to a remarkable structure modification induced by Mn doping. Instead of a disordered impurity system, it forms an alternating sequence of septuple and quintuple layer blocks, where Mn is predominantly incorporated in the septuple layers. This self-organized heterostructure substantially enhances the wave-function overlap and the size of the magnetic gap at the Dirac point, as recently predicted. Mn-doped Bi$_2$Se$_3$ forms a similar heterostructure, however, only a large, nonmagnetic gap is formed. We explain both differences based on the higher spin-orbit interaction in Bi$_2$Te$_3$ with the most important consequence of a magnetic anisotropy perpendicular to the films, whereas for Bi$_2$Se$_3$ the spin-orbit interaction it is too weak to overcome the dipole-dipole interaction. Our findings provide crucial insights for pushing the lossless transport properties of topological insulators towards room-temperature applications.