Making a `sandwich' out of magnets and topological insulators, potential
for lossless electronics

Date:
April 4, 2022
Source:
ARC Centre of Excellence in Future Low-Energy Electronics
Technologies
Summary:
A research team has discovered that sandwiching a topological
insulator between two 2D ferromagnetic insulators provides a
quantum avenue towards ultra-low energy future electronics, or
topological photovoltaics.



FULL STORY ==========================================================================
A Monash University-led research team has discovered that a structure comprising an ultra-thin topological insulator sandwiched between two
2D ferromagnetic insulators becomes a large-bandgap quantum anomalous
Hall insulator.


==========================================================================
Such a heterostructure provides an avenue towards viable ultra-low energy future electronics, or even topological photovoltaics.

Topological Insulator: The Filling in the Sandwich In the researchers'
new heterostructure, a ferromagnetic material forms the 'bread' of
the sandwich, while a topological insulator (ie, a material displaying nontrivial topology) takes the place of the 'filling'.

Combining magnetism and nontrivial band topology gives rise to quantum anomalous Hall (QAH) insulators, as well as exotic quantum phases such
as the QAH effect where current flows without dissipation along quantized
edge states.

Inducing magnetic order in topological insulators via proximity to
a magnetic material offers a promising pathway towards achieving QAH
effect at higher temperatures (approaching or exceeding room temperature)
for lossless transport applications.



==========================================================================
One promising architecture involves a sandwich structure comprising two
single layers of MnBi2Te4 (a 2D ferromagnetic insulator) either side of ultra-thin Bi2Te3 in the middle (a topological insulator). This structure
has been predicted to yield a robust QAH insulator phase with a bandgap
well above the thermal energy available at room temperature (25 meV).

The new Monash-led study demonstrated the growth of a MnBi2Te4 / Bi2Te3
/ MnBi2Te4 heterostructure via molecular beam epitaxy, and probed the structure's electronic structure using angle resolved photoelectron spectroscopy.

"We observed strong, hexagonally-warped massive Dirac fermions and a
bandgap of 75 meV," says lead author Monash PhD candidate Qile Li.

The magnetic origin of the gap was confirmed by the observing the bandgap vanishing above the Curie temperature, as well as broken time-reversal
symmetry and the exchange-Rashba effect, in excellent agreement with
density functional theory calculations.

"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 The 2D MnBi2Te4 ferromagnets induce magnetic order (ie,
an exchange interaction with the 2D Dirac electrons) in the ultra-thin topological insulator Bi2Te3 via magnetic proximity.

This creates a large magnetic gap, with the heterostructure becoming a
quantum anomalous Hall (QAH) insulator, such that the material becomes
metallic (ie, electrically conducting) along its one-dimensional edges,
whilst remaining electrically insulating in its interior. The almost-zero resistance along the 1D edges of the QAH insulator are what make it such
a promising pathway towards next-generation, low-energy electronics.

To date, several strategies have been used to realise the QAH effect, such
as introducing dilute amounts of magnetic dopants into ultrathin films
of 3D topological insulators. However, introducing magnetic dopants into
the crystal lattice can be challenging and results in magnetic disorder,
which greatly suppresses the temperature at which the QAH effect can be observed and limits future applications.

Rather than incorporating 3dtransition metals into the crystal lattice,
a more advantageous strategy is to place two ferromagnetic materials on
the top and bottom surfaces of a 3D topological insulator. This breaks time-reversal symmetry in the topological insulator with magnetic order,
and thereby opens a bandgap in the surface state of the topological
insulator and gives rise to a QAH insulator.

Making the Right Kind of Sandwich Yet, inducing sufficient magnetic order
to open a sizable gap via magnetic proximity effects is challenging due
to the undesired influence of the abrupt interface potential that arises
due to lattice mismatch between the magnetic materials and topological insulator.

"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.

A single-septuple layer of the intrinsic magnetic topological insulator MnBi2Te4 is particularly promising, as it is a ferromagnetic insulator
with a Curie temperature of 20 K.

"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 travelled 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 the 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 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.


========================================================================== 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. Qile Li, Chi Xuan Trang, Weikang Wu, Jinwoong Hwang, David Cortie,
Nikhil
Medhekar, Sung‐Kwan Mo, Shengyuan A. Yang, Mark
T. Edmonds. Large Magnetic Gap in a Designer Ferromagnet-Topological
Insulator-Ferromagnet Heterostructure. Advanced Materials, 2022;
2107520 DOI: 10.1002/ adma.202107520 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220404105745.htm

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