Researchers from Monash University have discovered that a structure made up of two 2D ferromagnetic insulators and an incredibly thin topological insulator transforms into a large-bandgap quantum anomalous Hall insulator.
Such a heterostructure offers a path to topological photovoltaics or possibly practical ultra-low energy future electronics.
Topological Insulator: The Filling in the Sandwich
The “bread” of the sandwich in the new heterostructure created by the researchers is ferromagnetic, and the “filling” is a topological insulator or a material with nontrivial topology.
Quantum anomalous Hall (QAH) insulators and rare quantum phases like the QAH effect, where current flows without dissipation along quantized edge states, are produced when magnetism and nontrivial band topology are combined.
For lossless transport applications, a viable route to attaining the QAH effect at higher temperatures (approaching or exceeding room temperature) involves inducing magnetic order in topological insulators by contact to a magnetic substance.
A sandwich architecture with two single layers of the 2D ferromagnetic insulator MnBi2Te4 on either side of an incredibly thin layer of Bi2Te3 in the middle is one intriguing design (a topological insulator). According to predictions, this arrangement will produce a stable QAH insulator phase with a bandgap significantly higher than the thermal energy present at ambient temperature (25 meV).
In a recent study, lead by Monash University, it was shown how to build a MnBi2Te4/Bi2Te3/MnBi2Te4 heterostructure using molecular beam epitaxy. Angle resolved photoelectron spectroscopy was used to examine the electronic structure of the structure.
“We observed strong, hexagonally-warped massive Dirac fermions and a bandgap of 75 meV,” says lead author Monash Ph.D. candidate Qile Li.
By detecting the bandgap disappearing above the Curie temperature, as well as the exchange-Rashba effect and broken time-reversal symmetry, and in great agreement with simulations from density functional theory, the magnetic origin of the gap was established.
To minimise the interface potential when inducing magnetic order via proximity, we needed to find a 2D ferromagnet that possessed similar chemical and structural properties to the 3D topological insulator. This way, instead of an abrupt interface potential, there is a magnetic extension of the topological surface state into the magnetic layer. This strong interaction results in a significant exchange splitting in the topological surface state of the thin film and opens a large gap.
Qile Li
“These findings provide insights into magnetic proximity effects in topological insulators, which will move lossless transport in topological insulators towards higher temperature,” says Monash group leader and lead author Dr. Mark Edmonds.
How It Works
Through magnetic closeness, the 2D MnBi2Te4 ferromagnets cause magnetic order (i.e., an exchange interaction with the 2D Dirac electrons) in the incredibly thin topological insulator Bi2Te3. Due to the heterostructure’s transformation into a quantum anomalous Hall (QAH) insulator and the resulting wide magnetic gap, the material becomes metallic (i.e., electrically conducting) along its one-dimensional borders but electrically insulating inside.
Its near-zero resistance along the 1D edges is what makes the QAH insulator such a potential route for developing next-generation, low-energy electronics. The QAH effect has so far been achieved using a variety of techniques, such as dilutions of magnetic dopants in ultrathin films of 3D topological insulators.
However, it can be difficult to introduce magnetic dopants into the crystal lattice and this causes magnetic disorder, which significantly lowers the temperature at which the QAH effect can be seen and restricts potential applications.
A more favorable method is to deposit two ferromagnetic materials on the top and bottom surfaces of a 3D topological insulator, as opposed to adding 3D transition metals to the crystal lattice.
Due to the disruption of time-reversal symmetry, a bandgap is created in the topological insulator’s surface state, resulting in the formation of a QAH insulator.
Making the Right Kind of Sandwich
However, because of the unfavorable impact of the abrupt interface potential that results from the lattice mismatch between the magnetic materials and topological insulator, it is difficult to induce enough magnetic order to generate a significant gap by magnetic proximity effects.
“To minimise the interface potential when inducing magnetic order via proximity, we needed to find a 2D ferromagnet that possessed similar chemical and structural properties to the 3D topological insulator,” says Qile Li, who is also a PhD student with the Australian Research Council Centre for Excellence in Future Low-Energy Electronic Technologies (FLEET).
“This way, instead of an abrupt interface potential, there is a magnetic extension of the topological surface state into the magnetic layer. This strong interaction results in a significant exchange splitting in the topological surface state of the thin film and opens a large gap,” says Li.
The intrinsic magnetic topological insulator MnBi2Te4 is a ferromagnetic insulator with a Curie temperature of 20 K, making a single septuple layer of this material especially attractive.
“More importantly, this setup is structurally very similar to the well-known 3D topological insulator Bi2Te3, with a lattice mismatch of only 1%,” says Dr. Mark Edmonds, who is an associate investigator in FLEET.
The research team traveled to the Advanced Light Source part of the Lawrence Berkeley National Laboratory in Berkeley, USA, where they grew the ferromagnet/topological/ferromagnet heterostructures and investigated their electronic bandstructure in collaboration with beamline staff scientist Dr. Sung-Kwan Mo.
“Although we cannot directly observe the QAH effect using angle-resolved photoemission spectroscopy (ARPES), we could use this technique to probe the size of the bandgap opening, and then confirm it is magnetic in origin,” says Dr. Edmonds.
“By using angle-resolved photoemission we could also probe the hexagonal warping in the surface state. It turns out, the strength of the warping in the Dirac fermions in our heterostructure is almost twice as large as in Bi2Te3,” says Dr. Edmonds
The research team was also able to confirm the electronic structure, gap size, and temperature at which this MnBi2Te4/Bi2Te3/MnBi2Te4 heterostructure is likely to support the QHE effect by combining experimental ARPES observations with magnetic measurements to determine the Curie temperature (performed by FLEET associate investigator Dr. David Cortie at the University of Wollongong) and first-principles density functional theory calculations performed by the group of Dr. Shengyuan Yang (Singapore University of Technology and Design).
The Australian Synchrotron provided travel funding to Berkeley, while the Centres of Excellence and DECRA Fellowship programs of the Australian Research Council provided funding for the project.
