Researchers at the University of Toronto and the Massachusetts Institute of Technology have discovered a way to create new atomically thin versions of quasicrystals - an enigmatic class of materials - that exhibit superconductivity.
The work by Sergio C. de la Barrera, an assistant professor in U of T's department of physics, and his MIT colleagues promises to jumpstart interest in quasicrystals by creating a new platform for further research. That, in turn, could lead to new physics insights and important applications such as more efficient electronic devices.
Recently published in Nature, the research and brings together two previously unconnected fields: "quasicrystals" and "twistronics."
"It's really extraordinary that the field of twistronics keeps making unexpected connections to other areas of physics and chemistry - in this case the beautiful and exotic world of quasiperiodic crystals," says Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT who pioneered the field of twistronics only five years ago.
Twistronics involves placing atomically thin layers of materials on top of one another. Rotating, or twisting, one or more of the layers at a slight angle creates a unique pattern called a moiré superlattice. And a moiré pattern, in turn, has an impact on the behaviour of electrons.
"It changes the spectrum of energy levels available to the electrons and can provide the conditions for interesting phenomena to arise," says de la Barrera, one of four co-first authors of the recent paper who conducted the work while a postdoctoral associate at MIT.
A moiré system can also be tailored for different behaviors by changing the number of electrons added to the system. As a result, the field of twistronics has exploded over the last five years as researchers around the world have applied it to creating new atomically thin quantum materials.
In the current work, the researchers were tinkering with a moiré system made of three sheets of graphene. Graphene is composed of a single layer of carbon atoms arranged in hexagons resembling a honeycomb structure. In this case, the team layered three sheets of graphene, but twisted two of the sheets at slightly different angles.
To their surprise, the system created a quasicrystal, an unusual class of material discovered in the 1980s. As the name implies, quasicrystals are somewhere between a crystal such as a diamond, which has a regular repeating structure, and an amorphous material like glass, "where the atoms are all jumbled, or randomly arranged," says de la Barrera.
In a nutshell, quasicrystals "have really strange patterns," de la Barrera says.
Compared to crystals and amorphous materials, however, relatively little is known about quasicrystals. That's in part because they're hard to make. "That doesn't mean they're not interesting; it just means that we haven't paid as much attention to them, particularly to their electronic properties," says de la Barrera, adding that the relatively simple quasicrystal created by the study's authors could be used by other researchers as a platform to advance the field.
Because the original researchers weren't experts in quasicrystals, they reached out to Professor Ron Lifshitz of Tel Aviv University, a co-author who helped the team to better understand what they were looking at, which they call a moiré quasicrystal.
The physicists then tuned a moiré quasicrystal to make it superconducting, or transmit current with no resistance at all below a certain low temperature. That's important because superconducting devices could transfer current through electronic devices much more efficiently than is possible today, but the phenomenon is still not fully understood in all cases.
The team also found evidence of symmetry breaking - a phenomenon that "tells us that the electrons are interacting with one another very strongly," de la Barrera says. "And as physicists and quantum material scientists, we want our electrons interacting with each other because that's where the exotic physics happens."
In the end, "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," says Aviram Uri, a co-first author of the paper and an MIT Pappalardo and VATAT postdoctoral fellow, although he notes that "we don't yet fully understand the system. There are still quite a few mysteries."
The best part of the research was "solving the puzzle of what it was we had actually created," de la Barrera says. "We were expecting [something else], so it was a very pleasant surprise when we realized we were actually looking at something very new and different."
With files from Elizabeth A. Thomson, MIT
