Physicists coax superconductivity and more from quasicrystals

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.