Engineers at Caltech observed an unusual phenomenon in twisted trilayer graphene.

So-called “magic-angle twisted graphene” provides the ability to turn superconductivity off and on with a literal flip of a switch. This has allowed engineers at Caltech to observe an unusual phenomenon that may shed new light on superconductivity in general.

The research was recently published in the journal Nature. It was led by Stevan Nadj-Perge, assistant professor of applied physics and materials science.

Magic-angle twisted graphene, first discovered in 2018, is made from two or three sheets of graphene layered atop one another, with each sheet twisted at precisely 1.05 degrees in relation to the one below it. (Graphene is a form of carbon consisting of a single layer of atoms in a honeycomb-like lattice pattern.) The resulting bilayer or trilayer has unusual electronic properties: for instance, it can be made into an insulator or a superconductor depending on how many electrons are added.

Superconductors are materials that exhibit a peculiar electronic state in which electrons can flow freely through the materials without resistance—meaning that electricity flows through them without losing any energy to heat. Such hyper-efficient transmission of electricity has endless potential applications in the fields of computing, electronics, and elsewhere.

However, the catch with superconducting is that in most materials, it takes place at extremely low temperatures. In fact, these temperature thresholds are usually only a few degrees above absolute zero (-273.15 degrees Celsius). At such temperatures, electrons forms pairs that behave in a fundamentally different way compared to individual electrons and condense into a quantum mechanical state that allows for electron pairs to flow without being scattered.

Although superconductivity was first discovered more than a century ago, scientists still do not fully understand the precise mechanisms behind electron-pair formation for some materials. In conventional superconductors, such as metal aluminum, it is well understood that the attraction between electrons that leads to the formation of electron pairs is due to the interaction of the electrons with the material’s crystal lattice. The behavior of these materials is described using the Bardeen–Cooper–Schrieffer (BCS) theory, named after John Bardeen, Leon Cooper, and John Robert Schrieffer, who shared the Nobel Prize in Physics in 1972 for the theory’s development.

While studying magic-angle twisted trilayers of graphene, Nadj-Perge and his colleagues discovered that superconductivity in this material exhibits several very unusual properties that cannot be described using BSC theory, making it likely also an unconventional superconductor.

They measured the evolution of the so-called superconducting gap as the electrons are removed from the trilayer with the flip of a switch to turn an electric field on or off. The superconducting gap is a property that describes how difficult it is to add or remove individual electrons into a superconductor. Because electrons in a superconductor want to be paired, a certain amount of energy is required to break those pairs. However, the amount of energy can be different for pairs moving in different directions relative to the crystal lattice. As a result, the “gap” has a specific shape that is determined by the likelihood that pairs will be broken by a particular amount of energy.

“While superconductors have been around for a long time, a remarkably new feature in twisted graphene bilayers and trilayers is that superconductivity in these materials can be turned on simply through the application of a voltage on a nearby electrode,” says Nadj-Perge, corresponding author of the Nature paper. “An electric field effectively adds or removes extra electrons. It works in a very similar way as the current is controlled in conventional transistors, and this allowed us to explore superconductivity in ways that one cannot do in other materials.”

The scientists established that in twisted trilayers, two superconductivity regimes with differently shaped superconducting gap profiles are present. While one of the regimes can perhaps be explained with a theory that is to some extent similar to BCS, the presence of two regimes shows that within the superconducting phase an additional transition is likely to take place. This observation, alongside measurements taken at various temperatures and magnetic fields, points to the unconventional nature of superconductivity in the trilayers.

The new insights by Nadj-Perge’s group give essential clues for the future theories of superconductivity in twisted graphene multilayers. Nadj-Perge notes that it appears that more layers make superconductivity more robust while remaining highly tunable, a property that opens up various possibilities to use twisted trilayers for superconducting devices that may someday be used in quantum science and perhaps quantum information processing.

“Besides its fundamental implications to our understanding of superconductivity, it is remarkable that adding an extra graphene layer made studying superconducting properties easier. Ultimately this is what enabled our findings,” Nadj-Perge says.

Reference: “Evidence for unconventional superconductivity in twisted trilayer graphene” by Hyunjin Kim, Youngjoon Choi, Cyprian Lewandowski, Alex Thomson, Yiran Zhang, Robert Polski, Kenji Watanabe, Takashi Taniguchi, Jason Alicea and Stevan Nadj-Perge, 15 June 2022, Nature.
DOI: 10.1038/s41586-022-04715-z

Co-authors include Jason Alicea, William K. Davis Professor of Theoretical Physics; Caltech graduate students Hyunjin Kim and Youngjoon Choi, leading authors on the paper; graduate students Robert Polski and Yiran Zhang; Cyprian Lewandowski, Moore Postdoctoral Scholar in Theoretical Physics; and Alex Thomson, Sherman Fairchild Postdoctoral Scholar in Theoretical Physics, who is now an assistant professor at UC Davis; and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan.

This research was funded by the National Science Foundation, the Office of Naval Research, the United States Department of Energy, the Kavli Nanoscience Institute, the Institute for Quantum Information and Matter at Caltech, the Walter Burke Institute for Theoretical Physics at Caltech, and the Kwanjeong Educational Foundation.


This underwater camera operates wirelessly without batteries

New ultralow-power imaging method employs underwater backscatter imaging.

View more: This underwater camera operates wirelessly without batteries

How Long Older Adults Will Live Comes Down to 17 Often Surprising Factors

Summary: Researchers have designed a new model of life expectancy that’s based less on disease diagnosis, and more on other factors including cholesterol levels and lifestyle. Source: Duke University A new model to predict the life expectancy of older people relies less on their specific disease diagnoses and more on ...

View more: How Long Older Adults Will Live Comes Down to 17 Often Surprising Factors

Researchers Share Guidelines for Inclusive Language in Autism Research

Summary: In order to make autism research less harmful to the autism community, researchers propose new data-driven guidelines for discussing ASD in their work. Source: Cell Press In the decades since the “clinical” definitions of autism in the 1980s, many have been advocating to understand autism as a normal part ...

View more: Researchers Share Guidelines for Inclusive Language in Autism Research

Postbiotics Could Deliver Immunity Boost in Your Morning Coffee

Summary: Postbiotics with a morning coffee can help give your immune system a boost, a new study reveals. Source: Griffith University Your morning coffee could now give an added immunity boost, thanks to postbiotics and Griffith University and CSIRO researchers. Coffee Roasters Australia founder Alana Beattie is behind the innovation ...

View more: Postbiotics Could Deliver Immunity Boost in Your Morning Coffee

Research explores how biased perceptions may drive erosion of democratic values in US

Credit: Unsplash/CC0 Public Domain A new report published in the journal Scientific Reports sheds light on the current state of democracy in the United States. The research by a University of Illinois Chicago social psychologist and colleagues shows that both Democrats and Republicans personally value core democratic principles, such ...

View more: Research explores how biased perceptions may drive erosion of democratic values in US

Super-Earth Found Near the Habitable Zone of Red Dwarf

The green region represents the habitable zone where liquid water can exist on the planetary surface. The planetary orbit is shown as a blue line. Ross 508 b skims the inner edge of the habitable zone (solid line), possibly crossing into the habitable zone for part of the orbit ...

View more: Super-Earth Found Near the Habitable Zone of Red Dwarf

What's the future of in-person work?

Credit: CC0 Public Domain While office buildings traditionally have been at the center of work for many people, factors including the COVID-19 pandemic have forced employers to reconsider the look of their workspaces. Three professors from the University of Cincinnati’s Carl H. Lindner College of Business discuss the state ...

View more: What's the future of in-person work?

Mapping Honduras' water supply

Graphical abstract. Credit: Science of The Total Environment (2022). DOI: 10.1016/j.scitotenv.2022.156941 In Tegucigalpa and surrounding areas, Hondurans often wait weeks for tap water to flow. A new study, designed and co-authored by Ricardo Sánchez-Murillo, associate professor of earth and environmental sciences at The University of Texas at Arlington, could ...

View more: Mapping Honduras' water supply

Hidden microbiome fortifies animals, plants too

Baltic Sea pipeline leak damages marine life and climate

Wave sensors deployed to improve hurricane forecasts

Study offers new perspective on 2008 housing crash

Scientists find link between fast-melting Arctic ice and ocean acidification

Team develops method for neural net computing in water

Room-temperature superconducting claim is retracted

The share of female managers in finance has increased but their earnings are still lagging far behind male managers

How Hurricane Ian compares to Florida's most destructive storms

Scientists use machine learning to help fight antibiotic resistance in farmed chickens

When air and road travel dropped during COVID, so did air pollution levels

What Is the Effect of Hierarchy on Moral Behavior?


Top Car News Car News