A Monash University-guided research group has discovered that a large-bandgap quantum anomalous Hall insulator can be created by sandwiching an ultrathin topological insulator between two 2D ferromagnetic insulators.
This type of heterostructure opens the door to ultra-low-energy future electronics and even topological photovoltaics.
Topological Insulator: The Filling in the Sandwich
A ferromagnetic material serves as the “bread” of the sandwich in the investigators’ novel heterostructure, while a topological insulator (i.e., a material with nontrivial topology) serves as the “filling.”
Quantum anomalous Hall (QAH) insulators, and also exotic quantum phases like the QAH effect, in which current flows without dissipation along quantized edge states, are created by combining magnetism and nontrivial band topology.
To achieve the QAH effect at elevated temperatures (exceeding or approaching room temperature) for lossless transport applications, triggering magnetic order in topological insulators via vicinity to a magnetic material is a promising pathway.
A sandwich structure with two single layers of MnBi2Te4 (2D ferromagnetic insulator) on either side of ultra-thin Bi2Te3 (topological insulator) in the middle is one hopeful architecture. This structure should produce a strong QAH insulator phase with a bandgap far above the thermal energy available at room temperature (25 meV).
The current Monash-led research used molecular beam epitaxy to grow a MnBi2Te4/Bi2Te3/MnBi2Te4 heterostructure and angle-resolved photoelectron spectroscopy to probe the structure’s electronic structure.
We observed strong, hexagonally-warped massive Dirac fermions and a bandgap of 75 meV.
Qile Li, Study Lead Author and PhD Candidate, Monash University
The magnetic origin of the gap was affirmed by the bandgap disappearing above the Curie temperature, and also broken time-reversal symmetry and the exchange-Rashba effect, all of which were in good accordance with density functional theory calculations.
Dr. Mark Edmonds, study lead author and Monash group leader adds, “These findings provide insights into magnetic proximity effects in topological insulators, which will move lossless transport in topological insulators towards higher temperature.”
How it Works
Magnetic proximity is used by the 2D MnBi2Te4 ferromagnets to stimulate magnetic order (an exchange interaction with the 2D Dirac electrons) in the ultra-thin topological insulator Bi2Te3.
The heterostructure then becomes a quantum anomalous Hall (QAH) insulator, causing the material to become metallic (electrically conducting) along its one-dimensional edges while still being electrically insulating in its interior. The QAH insulator’s nearly-zero resistance along the 1D edges is what makes it such a favorable path to next-generation, low-energy electronics.
Until now, numerous methods for achieving the QAH effect were used, including incorporating dilute amounts of magnetic dopants into ultrathin films of 3D topological insulators. However, incorporating magnetic dopants into the crystal lattice is difficult and causes magnetic disorder, which lowers the temperature at which the QAH effect can be noticed and restricts future applications.
Instead of integrating 3D transition metals into the crystal lattice, placing two ferromagnetic materials on the top and bottom surfaces of a 3D topological insulator is a more advantageous strategy. This knocks time-reversal symmetry in the topological insulator with magnetic order, resulting in the opening of a bandgap in the topological insulator’s surface state and the formation of a QAH insulator.
Making the Right Kind of Sandwich
However, due to the unwanted influence of the abrupt interface potential that occurs due to lattice mismatch between the magnetic materials and the topological insulator, inducing adequate magnetic order to open a large gap via magnetic proximity effects is difficult.
To minimize 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.
Qile Li, Study Lead Author and PhD Candidate, Monash University
Qile Li is also a Ph.D. student with the Australian Research Council Centre for Excellence in Future Low-Energy Electronic Technologies (FLEET).
Qile Li remarks, “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.”
As it is a ferromagnetic insulator with a Curie temperature of 20 K, a single-septuple layer of the intrinsic magnetic topological insulator MnBi2Te4 is especially promising.
Dr. Mark Edmonds, an associate investigator in FLEET comments, “More importantly, this setup is structurally very similar to the well-known 3D topological insulator Bi2Te3, with a lattice mismatch of only 1%.”
In association with beamline staff scientist Dr. Sung-Kwan Mo, the research group traveled to the Advanced Light Source at the Lawrence Berkeley National Laboratory in Berkeley, California, where they grew the ferromagnet/topological/ferromagnet heterostructures and examined their electronic bandstructure.
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.
Dr. Mark Edmonds, Associate Investigator, ARC Centre of Excellence in Future Low-Energy Electronics Technologies
Dr. Edmonds adds, “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.”
The study integrated experimental ARPES observations with magnetic measurements to evaluate the Curie temperature (conducted by FLEET associate investigator Dr. David Cortie at the University of Wollongong) and first-principles density functional theory calculations executed by Dr. Shengyuan Yang's group (Singapore University of Technology and Design).
The investigators determined that the electronic structure, gap size and temperature at which this MnBi2Te4/Bi2Te3/MnBi2Te4 heterostructure is likely to favor the QHE effect.
The research was published in the Advanced Materials journal.
The growth of this heterostructure was found at Monash University’s Edmonds Electronic Structure Laboratory. The heterostructure films were then grown and characterized at the Advanced Light Source (Lawrence Berkeley National Laboratory) in California using ARPES measurements.
The research was financially supported by the Australian Research Council’s Centres of Excellence and DECRA Fellowship programs. Travel to Berkeley was funded by the Australian Synchrotron.
Li, Q., et al. (2022) Large Magnetic Gap in a Designer Ferromagnet–Topological Insulator–Ferromagnet Heterostructure. Advanced Materials. doi.org/10.1002/adma.202107520.