Study: Superconductivity turns on and off in “magic-angle” graphene | MIT news

By carefully twisting and stacking, MIT physicists have revealed a new and unusual property in “magic-angle” graphene: superconductivity that can be turned on and off with an electrical pulse, like a switch.

The discovery could lead to ultrafast, energy-efficient superconducting transistors for neuromorphic devices – electronic devices designed to mimic the rapid firing/off of neurons in the brain. a person.

Magic-angle graphene refers to a special collection of graphene – an atomically thin material made of carbon atoms linked together in a hexagonal structure like chicken wire. When one sheet of graphene is placed on top of a second sheet in a precisely “magical” manner, the twisted structure creates a slightly “moiré” pattern, or superlattice, capable of supporting many surprising electronic structures. .

In 2018, Pablo Jarillo-Herrero and his team at MIT were the first to demonstrate magically twisted graphene. They showed that the new bilayer structure could act as an insulator, similar to wood, when they applied a continuous electric field. When they raised the field, the shielding machine suddenly became a giant machine, allowing electrons to flow, without friction.

The discovery was a watershed in the field of “twisttronics,” which examines how certain electronic materials are formed when two-dimensional materials are twisted and turned. Researchers including Jarillo-Herrero have gone on to reveal surprising properties in the magical graphene, including different ways to switch properties between different electronic states. Until now, such “switches” have worked like dimmers, in that the researchers have to continuously apply electricity or gravity to turn on the superconductivity, and maintain it.

Now Jarillo-Herrero and his team have shown that superconductivity in magic-angle graphene can be turned on, and continuously, with a short pulse instead of a continuous electric field. The key, they found, was a combination of twisting and stacking.

In a paper published today in Natural nanotechnologyThe group reports that, by stacking magic-angle graphene between two offset layers of boron nitride – a protective material on both sides – the unique compatibility of the sandwich structure has enabled researchers to change the superconductivity of graphene and turn it off with short circuit voltage.

“For most materials, if you remove the electric field, zzzzip, the electric field is gone,” says Jarillo-Herrero, who is the Cecil and Ida Green Professor of Physics at MIT. “This is the first time that a superconducting material has been made that can be switched on and off suddenly. This could pave the way for a new generation of flexible, graphene-based electronics.”

His co-authors at MIT are lead author Dahlia Klein PhD ’21, graduate student Li-Qiao Xia, and former postdoc David MacNeill, along with Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan.

Flipping the switch

In 2019, a team at Stanford University discovered that magic graphene can be forced into a ferromagnetic state. Ferromagnets are materials that retain their magnetic properties, even when no external magnetic field is applied.

The researchers found that magic-angle graphene can exhibit ferromagnetic properties that can be switched on and off. This happened when sheets of graphene were placed between two sheets of boron nitride so that the crystal structure of graphene matched one of the boron nitride layers. The arrangement was similar to a cheese sandwich where the top slice of bread and cheeses are aligned, but the bottom slice of bread is rotated irregularly in relation to the top slice. The result impressed the MIT team.

Jarillo-Herrero says: “We were trying to get a strong magnet by connecting both slices. Instead, we got something completely different.

In their current study, the team made a sandwich of materials with angles and carefully folded. The “cheese” of the sandwich consisted of magic graphene – two sheets of graphene, the top rotated slightly at a “magic” angle of 1.1 degrees with respect to the bottom plate. On top of this structure, they placed a layer of boron nitride, which is perfectly aligned with the top layer of graphene. Finally, they placed a second layer of boron nitride under the entire structure and reduced it by 30 degrees with respect to the top layer of boron nitride.

Next, the team measured the electrical resistance of the graphene layers while applying a gate voltage. They found, like others, that twisted bilayer graphene changed electronic states, switching between insulating, conducting, and superconducting states. with some known power.

What the team did not expect was that each electronic state continued instead of disappearing immediately when the voltage was removed – a property known as bistability. They found that, with a certain voltage, the graphene layers became a superconductor, and remained high, as the researchers removed this voltage.

This bistable effect suggests that superconductivity can be turned on and off with short electric pulses instead of a continuous electric field, similar to turning on a light bulb. It is not clear what allows this variable superconductivity, although the researchers suspect that it is related to the special order of graphene twisted in two layers of boron nitride, which facilitates the ferroelectric-like response of the system. (Ferroelectric materials exhibit bistability in their electrical properties.)

“By paying attention to stacking, you can add another tuning knob to the growing complexity of the magic, superconducting materials,” says Klein.

For now, the team sees the new superconducting material as another tool for researchers to consider as they develop materials for faster, smaller, more energy-efficient electronics.

Jarillo-Herrero says: “People are trying to make electronic devices that do math in a way that is influenced by the brain. “In the brain, we have neurons that, beyond a certain limit, fire. Similarly, we have now found a way for magic-angle graphene to suddenly change superconductivity, beyond a certain threshold. This is an important property to implement neuromorphic computing. ”

This research was supported in part by the US Air Force Office of Scientific Research, the US Army Research Office, and the Gordon and Betty Moore Foundation.

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