Large magnetic gap at the Dirac point in a Mn-induced Bi$_2$Te$_3$ heterostructure (1810.06238v1)

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.

• The paper reports a magnetic gap of approximately 90 meV at the Dirac point in Mn-doped Bi2Te3 using low-temperature ARPES.
• It demonstrates that Mn induces structural reorganization into septuple layers, enhancing the gap beyond DFT predictions.
• The study proposes that enhancing the magnetic gap is key to achieving QAHE at higher temperatures in topological insulators.

Large Magnetic Gap at the Dirac Point in Mn-induced Bi$_2$Te$_3$ Heterostructure

This research paper presents an in-depth investigation into the large magnetic gap observed at the Dirac point within Mn-doped Bi$_2$Te$_3$ films. This paper is crucial to advancing the understanding of magnetically doped topological insulators, particularly in the context of the Quantum Anomalous Hall Effect (QAHE), which has significant implications for lossless charge transport applications.

Key Findings and Methodology

The authors employed low-temperature angle-resolved photoemission spectroscopy (ARPES) to reveal a substantial magnetic gap of approximately 90 meV in Mn-doped Bi$_2$Te$_3$, which is significantly larger than room temperature thermal excitations, $k_BT$, and five times the size predicted by density functional theory (DFT) calculations. The formation of this gap is attributed to an induced structural modification where Mn induces the self-organization of a heterostructure containing septuple and quintuple layers, with Mn predominantly located in the centers of the septuple layers.

Contrastingly, a similar Mn doping in Bi$_2$Se$_3$ did not result in a magnetic gap, although a large nonmagnetic gap was observed. This discrepancy has been accounted for by the greater spin-orbit interaction in Bi$_2$Te$_3$ leading to a stronger magnetic anisotropy perpendicular to the films. Conversely, in Bi$_2$Se$_3$, the spin-orbit interaction was insufficient to surmount dipole-dipole interactions, therefore impeding the magnetic anisotropy necessary for the gap opening at the Dirac point.

Implications

The findings hold significant implications for the technological application of topological insulators, potentially extending lossless edge state conduction to room temperature. This is particularly notable as the current operational temperature for the QAHE is restricted to significantly lower temperatures, typically well below 2 K, which is notably lower than the ferromagnetic Curie temperature of these systems.

The paper highlights that instead of pursuing higher Curie temperatures, it may be more advantageous to focus on enhancing the magnetic gap itself as a route to elevate operational temperatures for QAHE applications. The structuring of Mn layers within septuple layers could provide a pathway towards these goals.

Future Directions

Future research could extend these findings by exploring varying compositions and structural configurations within magnetic topological insulators to optimize the magnetic gap and anisotropy. Additionally, further theoretical support through advanced computational modeling might provide deeper insights into the mechanisms driving these structural and magnetic phenomena.

The potential for realizing new topological phases such as axion insulators and the chiral Majorana mode amplifies the significance of these observations, projecting Mn-doped Bi$_2$Te$_3$ as a promising candidate for advanced quantum device applications. This exploration opens up new avenues for practical implementations in quantum computing and spintronics, ultimately revolutionizing the understanding of quantum materials.