Finely tuning adjacent graphene layers to induce superconductivity

Twisting graphene layers article - image 1
The researchers wrote: “Applying pressure to twisted bilayer graphene transforms the material from a metal to a superconductor.” (Image: news.columbia.edu)

Researchers have developed a new way to finely tune adjacent graphene layers to induce superconductivity. The graphene layers are lacy, honeycomb-like sheets of carbon atoms.

The researchers say that their study provides new insights into the physics that underlies graphene’s intriguing characteristics.

Scientists and a growing number of business people have been heralding graphene as a wonder material. It is the strongest, thinnest, and lightest material on Earth. It is also an exceptional conductor of electricity and heat. Graphene is currently being applied to a wide range of industries, from medicine to energy to electronics.

The researchers, from Columbia University, the University of California Santa Barbara, the National Institute for Materials Science (Japan), and the National High Magnetic Field Laboratory in Tallahassee, wrote about their work in the journal Science (citation below). The authors were Matthew Yankowitz, Shaowen Chen, Hryhoriy Polshyn, Yuxuan Zhang, K. Watanabe, T. Taniguchi, David Graf, Andrea F. Young, and Cory R. Dean.

Twisted graphene layers

Lead investigator, Cory Dean, Assistant Professor of physics at Columbia University, said:

“Our work demonstrates new ways to induce superconductivity in twisted bilayer graphene, in particular, achieved by applying pressure.”

“It also provides critical first confirmation of last year’s MIT results—that bilayer graphene can exhibit electronic properties when twisted at an angle—and furthers our understanding of the system, which is extremely important for this new field of research.”

Twisting graphene layers to the ‘magic angle’

In March last year, MIT researchers reported a ground-breaking discovery. They found that two graphene layers could conduct electricity with no resistance when they were at the ‘magic angle.’ In other words, when the twist angle between them was 1.1° (degrees).

The only problem was that hitting that ‘magic angle’ has proven extremely difficult.

Prof. Dean said:

“The layers must be twisted to within roughly a tenth of a degree around 1.1, which is experimentally challenging. We found that very small errors in alignment could give entirely different results.”

So, Prof. Dean and fellow scientists set out to determine whether bigger rotations of graphene layers might improve magic-angle conditions.

First author, Matthew Yankowitz, a Postdoctoral Research Scientist at Columbia University’s Physics Department, said:

“Rather than trying to precisely control the angle, we asked whether we could instead vary the spacing between the layers, In this way, any twist angle could, in principle, be turned into a magic angle.”

From superconductor to insulator

They examined a sample with a 1.3° twist angle, which was only marginally bigger than the magic angle. However, it was also far enough away from 1.1° to prevent superconductivity.

By applying pressure, they turned the material from a metal into either a superconductor or an insulator. Electricity flows through a superconductor without resistance but cannot flow through an insulator. Whether it was an insulator or superconductor depended on how many electrons there were.

Prof. Dean said:

“Remarkably, by applying pressure of over 10,000 atmospheres, we observe the emergence of the insulating and superconducting phases. Additionally, the superconductivity develops at the highest temperature observed in graphene so far, just over 3 degrees above absolute zero.”

Working with Maglab team

To reach the required high pressures to induce superconductivity, the researchers worked closely with the Maglab team. The Maglab, in Tallahassee, is the name for the National High Magnetic Field user facility.

Prof. Dean said:

“This effort was a huge technical challenge. After fabricating one of the most unique devices we’ve ever worked with, we then had to combine cryogenic temperatures, high magnetic fields, and high pressure- all while measuring electrical response.”

“Putting this all together was a daunting task and our ability to make it work is really a tribute to the fantastic expertise at the Maglab.”

It may be possible to enhance the superconductivity’s critical temperature further at even higher pressures, the researchers believe.

According to a Columbia University press release:

“The ultimate goal is to one day develop a superconductor that can perform under room temperature conditions, and although this may prove challenging in graphene, it could serve as a roadmap for achieving this goal in other materials.”

Squeezing and twisting graphene layers

Andrea Young, an Assistant Professor of physics at UC Santa Barbara, said their work clearly demonstrated that squeezing the graphene layers had the same effect as twisting them. It offered an alternative paradigm for manipulating graphene’s electronic properties.

Prof. Young said:

“Our findings significantly relax the constraints that make it challenging to study the system and gives us new knobs to control it.”

Prof. Dean and Prof. Young are currently twisting and squeezing a variety of atomically-thin materials. They hope to find superconductivity in other 2-dimensional systems.

Citation

Tuning superconductivity in twisted bilayer graphene,” Matthew Yankowitz, Shaowen Chen, Hryhoriy Polshyn, Yuxuan Zhang, K. Watanabe, T. Taniguchi, David Graf, Andrea F. Young, and Cory R. Dean. Science, 24 Jan 2019: eaav1910. DOI: 10.1126/science.aav1910.


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