In a study that may spark interest in the mysterious class of materials
known as quasicrystals, researchers at MIT and its associates have found a
flexible and somewhat easy method for producing new, atomically thin
versions that can be tweaked for significant events. They explain doing just
that to induce superconductivity and other properties in materials in work
published in
Nature.
The work presents a new framework for investigating unusual phenomena that
can be challenging to understand but have the potential to provide
significant applications and novel physics, in addition to providing further
information on quasicrystals. For instance, a deeper comprehension of
superconductivity—the property of materials through which electrons flow
without resistance—could lead to the development of far more effective
electrical devices.
The study unites quasicrystals with twistronics, two previously unrelated
areas. Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics
at MIT and the paper's corresponding author, was the pioneer of the latter
only five years ago.
"The surprising connections that the field of twistronics keeps making to
other areas of physics and chemistry, like the fascinating and exotic world
of quasiperiodic crystals, is really amazing," says Jarillo-Herrero, who is
also connected to the MIT Research Laboratory for Electronics and the
Materials Research Laboratory at MIT.
Execute the twist
Atomically thin layers of materials are layered on top of one another in
twistronics. By slightly angling one or more of the layers, a moiré
superlattice—a distinctive pattern—is produced. Additionally, a moiré
pattern affects how electrons behave.
Co-first author Sergio C. de la Barrera, one of four, says of the latest
research, "It changes the spectrum of energy levels available to the
electrons and can provide the conditions for interesting phenomena to
arise." De la Barrera, an assistant professor at the University of Toronto,
carried out the research while working as a postdoctoral fellow at
MIT.
By varying the quantity of electrons introduced to the system, a moiré
system may also be tuned for various behaviors. Consequently, during the
past five years, the discipline of twistronics has flourished as researchers
from all around the world have used it to create novel atomically thin
quantum materials. To name only MIT, some examples are:
creating three distinct, practical electrical devices out of magic-angle
twisted bilayer graphene, a moiré material. Daniel Rodan-Legrain, a co-first
author of the present paper and a postdoctoral associate in physics at MIT,
was one of the scientists engaged in that study, which was published in
2021. Jarillo-Herrero was in charge of them.)
introducing ferroelectricity, a novel feature, into the well-known
semiconductor family. (The scientists under Jarillo-Herrero's leadership
participated in that effort, which was reported in 2021).
forecasting novel and unusual magnetic events and providing a "recipe" to
achieve them. (The scientists that worked on that project, which was
published in 2023, were Nisarga Paul, an MIT graduate student studying
physics, and Liang Fu, a professor of physics at MIT. The present paper's
co-authors are Paul and Fu.)
New quasicrystals in the making
In the present work, the scientists were experimenting with a three-sheet
graphene moiré system. A single sheet of carbon atoms organized in hexagons
to resemble a honeycomb pattern makes up graphene. Here, the group stacked
three graphene sheets on top of each other, twisting two of the sheets to
slightly different angles.
They were taken aback when the system produced a quasicrystal, a peculiar
kind of material only found in the 1980s. As the name suggests,
quasicrystals fall between an amorphous substance, like glass, "where the
atoms are all jumbled, or randomly arranged," and a crystal, like a diamond,
which has a regular repeating pattern. Quasicrystals, to put it succinctly,
"have really strange patterns," according to de la Barrera (see some
examples
here).
Quasicrystals, however, are comparatively less understood than crystals and
amorphous materials. That's partly because they're challenging to produce.
"It just means we haven't paid as much attention to them, particularly to
their electronic properties," says de la Barrera. "That doesn't mean they're
not interesting." That could change with the new platform, which is really
straightforward.
Finding out more
The initial researchers contacted Professor Ron Lifshitz of Tel Aviv
University since he is a specialist in quasicrystals, while they were not.
During his undergraduate studies in Tel Aviv, Aviram Uri, an MIT Pappalardo
and VATAT Postdoctoral Fellow and one of the co-first authors of the
publication, was a student of Lifshitz and was aware of his work on
quasicrystals. The team, led by Lifshitz, who authored the Nature
publication, was able to gain a deeper understanding of the object they were
observing—a moiré quasicrystal.
Subsequently, the scientists adjusted a moiré quasicrystal to achieve
superconductivity, which is the ability to transport electricity without any
resistance below a specific low temperature. This is significant because,
although the phenomena is still mostly unknown, superconducting devices have
the potential to transmit current through electronic devices far more
effectively than is now feasible. A novel approach to studying the moiré
quasicrystal system has been introduced.
Another event that "tells us that the electrons are interacting with one
another very strongly" was evidence of symmetry breakdown, which the
researchers also discovered. Furthermore, we want our electrons to interact
with one another since there is where unusual physics occurs as physicists
and quantum material scientists," de la Barrera adds.
Uri believes that ultimately, "through discussions across continents we
were able to decipher this thing, and now we believe we have a good handle
on what's going on," but he emphasises that "we don't yet fully understand
the system." Quite a few mysteries remain to be solved."
"Solving the puzzle of what it was we had actually created," according to
de la Barrera, was the most enjoyable aspect of the investigation. "We were
looking at something very different and new, so it was a very pleasant
surprise when we realized we were actually expecting [something
else]."
"For me, it's the same response," replies Uri.
