News: Microelectronics
22 September 2021
Intrinsic magnetic topological insulator MnBi2Te4 found to have large bandgap in QAH state
A team led by researchers at Monash University has discovered an intrinsic magnetic topological insulator MnBi2Te4 with a large bandgap, making it a promising material platform for fabricating ultralow-energy electronics and observing exotic topological phenomena (Chi Xuan Trang et al, ‘Crossover from 2D Ferromagnetic Insulator to Wide Band Gap Quantum Anomalous Hall Insulator in Ultrathin MnBi2Te4’, ACS Nano 15 (2021), issue 8 (August), p13444).
Hosting both magnetism and topology, ultrathin (only several nanometers in thickness) MnBi2Te4 was found to have a large bandgap in a quantum anomalous Hall (QAH) insulating state, where the material is metallic (i.e. electrically conducting) along its one-dimensional edges, while electrically insulating in its interior. The almost zero resistance along the 1D edges of a QAH insulator make it promising for lossless transport applications and ultralow-energy devices.
Previously, the path towards realising the QAH effect was to introduce dilute amounts of magnetic dopants into ultrathin films of 3D topological insulators. However, dilute magnetic doping results in a random distribution of magnetic impurities, causing non-uniform doping and magnetization. This greatly suppresses the temperature at which the QAH effect can be observed and limits possible future applications.
A simpler option is to use materials that host this electronic state of matter as an intrinsic property. Recently, classes of atomically thin crystals have emerged, similar to graphene, that are intrinsic magnetic topological insulators (i.e. possess both magnetism and topological protection). These materials have the advantage of having less disorder and larger magnetic bandgaps, allowing robust magnetic topological phases operating at higher temperature (i.e. closer to the ultimate aim of room-temperature operation).
“At FLEET’s labs at Monash University, we grew ultrathin films of an intrinsic magnetic topological insulator MnBi2Te4 and investigated their electronic band structure,” says lead author Dr Chi Xuan Trang, a research fellow at the Australian Research Council (ARC) Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET).
Magnetism introduced in topological insulator materials breaks time-reversal symmetry in the material, resulting in opening a gap in the surface state of the topological insulator.
“Although we cannot directly observe the QAH effect using angle-resolved photoemission spectroscopy (ARPES), we can use this technique to probe the size of a bandgap opening on the surface of MnBi2Te4 and how it evolves with temperature,” says Trang.
In an intrinsic magnetic topological insulator, such as MnBi2Te4, there is a critical magnetic ordering temperature where the material is predicted to undergo a topological phase transition from QAH insulator to a paramagnetic topological insulator.
Picture: Phase transition from QAH insulator phase (left) to paramagnetic gapless topological insulator phase (right), when above the magnetic ordering temperature.
“By using angle-resolved photoemission at different temperatures, we could measure the bandgap in MnBi2Te4 opening and closing to confirm the topological phase transition and magnetic nature of the bandgap,” says Qile Li a FLEET PhD student and co-lead author on the study.
“The bandgaps of ultrathin-film MBT can also change as a function of thickness, and we observed that a single-layer MnBi2Te4 is a wide-bandgap 2D ferromagnetic insulator,” adds Qile Li. “A single layer of MBT as a 2D ferromagnet could also be used in proximity magnetization when combined in a heterostructure with a topological insulator.”
“By combining our experimental observations with first-principles density functional theory (DFT) calculations, we can confirm the electronic structure and the gap size of layer-dependent MnBi2Te4,” says FLEET associate investigator and group leader Dr Mark Edmonds.
MnBi2Te4 has potential in a number of classical computing applications, such as in lossless transport and ultralow-energy devices, say the researchers. Furthermore, it could be coupled with a superconductor to give rise to chiral Majorana edge states, which are important for topological quantum computing device schemes.
The recipe of the ultrathin MnBi2Te4 film in this study was initially found in Edmonds Electronic Structure laboratory at Monash University. Afterwards, the ultrathin films were grown and characterized using ARPES measurements at the Advanced Light Source (Lawrence Berkeley National Laboratory) in California.
The study was funded by the Australian Research Council’s Centres of Excellence and DECRA Fellowship programs, while travel to Berkeley was funded by the Australian Synchrotron. The computational work was performed on resources of the Monash Computing Cluster, the NationalComputing Infrastructure and the Pawsey Supercomputing Facility.
