Electrons on the edge: The story of an intrinsic magnetic topological insulator
Intrinsic magnetic topological insulator with large band gap promising
for ultra-low-energy electronics

Date:
September 21, 2021
Source:
ARC Centre of Excellence in Future Low-Energy Electronics
Technologies
Summary:
An intrinsic magnetic topological insulator MnBi2Te4 has been
discovered with a large band gap, making it a promising material
platform for fabricating ultra-low-energy electronics and observing
exotic quantum phenomena.



FULL STORY ==========================================================================
An intrinsic magnetic topological insulator MnBi2Te4has been discovered
with a large band gap, making it a promising material platform for
fabricating ultra- low-energy electronics and observing exotic topological phenomena.


========================================================================== Hosting both magnetism and topology, ultra-thin (only several nanometers
in thickness) MnBi2Te4was found to have a large band-gap in a Quantum
Anomalous Hall (QAH) insulating state, where the material is metallic
(ie, 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 ultra-low energy devices.

HISTORY OF QAH: HOW TO ACHIEVE THE DESIRED EFFECT Previously, the path
towards realising the QAH effect was to introduce dilute amounts of
magnetic dopants into ultra-thin films of 3D topological insulators.

However, dilute magnetic doping results in a random-distribution of
magnetic impurities, causing non-uniform doping and magnetisation. 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
the famous graphene, that are intrinsic magnetic topological insulators
(ie, possess both magnetism and topological protection).

These materials have the advantage of having less disorder and larger
magnetic band-gaps, allowing robust magnetic topological phases operating
at higher temperature (ie, closer to the ultimate aim of room-temperature operation).

"At FLEET's labs at Monash University, we grew ultra-thin films of an
intrinsic magnetic topological insulator MnBi2Te4and investigated their electronic band structure," explains lead author Dr Chi Xuan Trang.

MIND THE GAP: HOW TO OBSERVE THE BAND-GAP IN A MAGNETIC TOPOLOGICAL
INSULATOR 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 band-gap opening on the surface of MnBi2Te4 and how it evolves
with temperature." says Dr Trang, who is a Research Fellow at FLEET.

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.

"By using angle-resolved photoemission at different temperatures, we
could measure the band gap 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. A single layer of MBT as a 2D
ferromagnet could also be used in proximity magnetisation when combined
in a heterostructure with a topological insulator." says Qile Li.

"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 AI
and group leader Dr. Mark Edmonds.

APPLICATIONS OF THE INTRINSIC MAGNETIC TOPOLOGICAL INSULATOR MNBI2TE4
MnBi2Te4 has potential in a number of classical computing applications,
such as in lossless transport and ultra-low energy devices. 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.

========================================================================== Story Source: Materials provided by ARC_Centre_of_Excellence_in_Future_Low-Energy_Electronics
Technologies. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Chi Xuan Trang, Qile Li, Yuefeng Yin, Jinwoong Hwang, Golrokh
Akhgar,
Iolanda Di Bernardo, Antonija Grubisić-Čabo, Anton
Tadich, Michael S. Fuhrer, Sung-Kwan Mo, Nikhil V. Medhekar,
Mark T. Edmonds.

Crossover from 2D Ferromagnetic Insulator to Wide Band Gap Quantum
Anomalous Hall Insulator in Ultrathin MnBi2Te4. ACS Nano, 2021;
15 (8): 13444 DOI: 10.1021/acsnano.1c03936 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210921172647.htm

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