Superconductivity switches on and off in ‘magic angle’ graphene

With some careful twisting and stacking, MIT physicists have revealed a new and exotic property in “magic angle” graphene: superconductivity that can be turned on and off with an electrical pulse, much like a switch.

This discovery could lead to ultra-fast, energy-efficient superconducting transistors for neuromorphic devices – electronic components designed to function similar to the rapid on/off of neurons in the human brain.

Magic angle graphene refers to a very special stack of graphene, an atom-thin material, made up of carbon atoms bonded in a hexagonal pattern resembling chicken wire. When a sheet of graphene is stacked on top of a second sheet at a precise “magic” angle, the twisted structure creates a slightly shifted “moiré” pattern, or superlattice, capable of supporting a multitude of surprising electronic behaviors.

In 2018, Pablo Jarillo-Herrero and his group at MIT were the first to demonstrate magic-angle twisted bilayer graphene. They showed that the new bilayer structure could behave like an insulator, much like wood, when they applied a certain DC electric field. When they increased the field, the insulator suddenly transformed into a superconductor, allowing electrons to flow without friction.

This discovery gave rise to “twistronics”, a field that explores how certain electronic properties emerge from the twisting and layering of two-dimensional materials. Researchers including Jarillo-Herrero have continued to reveal surprising properties of magic-angle graphene, including various ways to switch the material between different electronic states. Until now, these “switches” have acted more like dimmers, in that researchers must continually apply an electric or magnetic field to activate superconductivity, and maintain it.

Now, Jarillo-Herrero and his team have shown that superconductivity in magic-angle graphene can be activated and maintained with just a short pulse rather than a continuous electric field. The key, they found, was a combination of twisting and stacking.

In an article published today in Nature’s nanotechnologythe team reports that by stacking graphene at a magic angle between two staggered layers of boron nitride – a two-dimensional insulating material – the unique alignment of the sandwich structure allowed the researchers to switch the superconductivity of graphene on and off with a short electrical pulse.

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

His co-authors from MIT are lead author Dahlia Klein, Li-Qiao Xia, and David MacNeill, and Kenji Watanabe and Takashi Taniguchi from the National Institute of Materials Science in Japan.

Flip the switch

In 2019, a team from Stanford University found that magic-angle graphene could be forced into a ferromagnetic state. Ferromagnets are materials that retain their magnetic properties even in the absence of an externally applied magnetic field.

The researchers found that magic-angle graphene could exhibit ferromagnetic properties in a way that could be turned on and off. This happened when graphene sheets were layered between two boron nitride sheets so that the crystal structure of graphene was aligned with one of the boron nitride layers.

The arrangement resembled a cheese sandwich in which the top slice of bread and the orientations of the cheese are aligned, but the bottom slice of bread is rotated at a random angle to the top slice. The result intrigued the MIT group.

“We were trying to get a stronger magnet by aligning the two slices,” says Jarillo-Herrero. “Instead, we found something completely different.”

In their current study, the team made a sandwich of carefully angled and stacked materials. The “cheese” of the sandwich was made of magic angle graphene – two sheets of graphene, with the top rotating slightly at the “magic” angle of 1.1 degrees to the bottom sheet. Above this structure, they placed a layer of boron nitride, exactly aligned with the upper graphene sheet. Finally, they placed a second layer of boron nitride under the entire structure and offset it 30 degrees from the top layer of boron nitride.

The team then measured the electrical resistance of the graphene layers when they applied a gate voltage. They found, along with others, that twisted bilayer graphene switched electronic states, changing between insulating, conducting, and superconducting states. at certain known voltages.

What the group didn’t expect was that each electronic state would persist instead of immediately disappearing once the voltage was removed, a property known as bistability. They found that at a particular voltage, the graphene layers transformed into a superconductor and remained superconductive, even when the researchers removed that voltage.

This bistable effect suggests that superconductivity can be turned on and off with short electrical pulses rather than a continuous electrical field, similar to the flip of a switch. It’s unclear what enables this switchable superconductivity, though the researchers suspect it has something to do with the special alignment of the twisted graphene on the two layers of boron nitride, which enables a ferroelectric-like response from the system. . (Ferroelectric materials exhibit bistability in their electrical properties.)

“By paying attention to stacking, you could add another tuning knob to the growing complexity of magic-angle superconducting devices,” Klein says.

For now, the team sees the new superconducting switch as another tool researchers can consider when developing materials for faster, smaller, and more energy-efficient electronics.

“People are trying to build electronic devices that perform calculations in a brain-inspired way,” says Jarillo-Herrero. “In the brain, we have neurons that, beyond a certain threshold, fire. Similarly, we have now found a way for magic-angle graphene to abruptly switch superconductivity, beyond a certain threshold. This is a key property in achieving neuromorphic computing.”

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation, and education.