With a little twist and the turn of a voltage knob, Cornell researchers have
shown that a single material system can toggle between two of the wildest
states in condensed matter physics: The quantum anomalous Hall insulator and
the two-dimensional topological insulator.
By doing so, they realized an elusive model that was first proposed more
than a decade ago, but which scientists have never been able to demonstrate
because a suitable material didn't seem to exist. Now that the researchers
have created the right platform, their breakthrough could lead to advances
in quantum devices.
The team's paper, "Quantum Anomalous Hall Effect from Intertwined Moiré
Bands," published Dec. 22 in Nature. The co-lead authors are former
postdoctoral researchers Tingxin Li and Shengwei Jiang, doctoral student
Bowen Shen and Massachusetts Institute of Technology researcher Yang Zhang.
The project is the latest discovery from the shared lab of Kin Fai Mak,
associate professor of physics in the College of Arts and Sciences, and Jie
Shan, professor of applied and engineering physics in the College of
Engineering, the paper's co-senior authors. Both researchers are members of
the Kavli Institute at Cornell for Nanoscale Science; they came to Cornell
through the provost's Nanoscale Science and Microsystems Engineering (NEXT
Nano) initiative.
Their lab specializes in exploring the electronic properties of 2D quantum
materials, often by stacking ultrathin monolayers of semiconductors so their
slightly mismatched overlap creates a moiré lattice pattern. There,
electrons can be deposited and interact with each other to exhibit a range
of quantum behavior.
For the new project, the researchers paired molybdenum ditelluride
(MoTe2) with tungsten diselenide (WSe2), twisting
them at a 180-degree angle for a configuration that is known as an AB stack.
After applying a voltage, they observed what's known as a quantum anomalous
Hall effect. This has its roots in a phenomenon called the Hall effect,
first observed in the late 19th century, in which electrical current is
flowed through a sample and then bent by a magnetic field that is applied at
a perpendicular angle.
The quantum Hall effect, discovered in 1980, is the supersized version, in
which a far greater magnetic field is applied, triggering even stranger
phenomena: The interior of the bulk sample becomes an insulator, while an
electrical current moves in a single direction along the outer edge, with
resistances quantized to a value defined by the fundamental constants in the
universe, regardless of the details of the material.
The quantum anomalous Hall insulator, first discovered in 2013, achieves the
same effect but without the intervention of any magnetic field, the
electrons speeding along the edge as if on a highway, without dissipating
energy, somewhat like a superconductor.
"For a long time people thought that a magnetic field is needed for the
quantum Hall effect, but you actually don't need one," Mak said. "So what
replaces the role of a magnetic field? It turns out that it is magnetism.
You have to make the material magnetic."
The MoTe2/WSe2 stack now joins the ranks of only
handful of materials that are known to be quantum anomalous Hall insulators.
But that is only half of its appeal.
The researchers found that by simply tweaking the voltage, they could turn
their semiconductor stack into a 2D topological insulator, which is a cousin
of sorts to the quantum anomalous Hall insulator, except that it exists in
duplicate. In one "copy," the electron highway flows clockwise around the
edge, and in the other, it flows counterclockwise.
The two states of matter have never before been demonstrated in the same
system.
After consulting with collaborators led by co-author Liang Fu at MIT, the
Cornell team learned its experiment had realized a toy model for graphene
first proposed by physics professors Charles Kane and Eugene Mele at the
University of Pennsylvania in 2005. The Kane-Mele model was the first
theoretical model for 2D topological insulators.
"That was a surprise to us," Mak said. "We just made this material and did
the measurements. We saw the quantum anomalous Hall effect and the 2D
topological insulator and said 'Oh, wow. That's great.' Then we talked to
our theory friend, Liang Fu, at MIT. They did the calculations and figured
out the material actually realized a long sought-after model in condensed
matter. We never expected this."
Like graphene moiré materials, MoTe2/WSe2 can
switch between a range of quantum states, including a transition from a
metal to a Mott insulator, a discovery the team reported in Nature in
September.
Now Mak and Shan's lab is investigating the full potential of the material
by coupling it with superconductors and using it to build quantum anomalous
Hall interferometers, both of which in turn could generate qubits, the basic
element for quantum computing. Mak is also hopeful they may find a way to
significantly raise the temperature at which the quantum anomalous Hall
effect occurs—which is at about 2 kelvin—resulting in a high-temperature
dissipationless conductor.
Co-authors include doctoral students Lizhong Li and Zui Tao; and researchers
from MIT and the National Institute for Materials Science in Tsukuba, Japan.
Reference:
Tingxin Li et al, Quantum anomalous Hall effect from intertwined moiré
bands, Nature (2021).
DOI: 10.1038/s41586-021-04171-1
Tingxin Li et al, Continuous Mott transition in semiconductor moiré
superlattices, Nature (2021).
DOI: 10.1038/s41586-021-03853-0
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Physics