A quick electrical pulse completely flips the electronic properties of a material, opening the way to ultrafast, brain-inspired superconducting electronics.
Physicists have discovered a new way to switch superconductivity on and off in magic-angle graphene. The discovery could lead to ultrafast, energy-efficient superconducting transistors for use in “neuromorphic” electronics, which work in a manner similar to the rapid on/off firing of neurons in the human brain. The discovery could lead to ultrafast, energy-efficient superconducting transistors for use in neuromorphic devices — electronics designed in a way that resembles the rapid on/off firing of neurons in the human brain.
Magic-angle graphene refers to a very specific stack of graphene — an atomically thin material made of carbon atoms linked together in a hexagonal pattern that resembles barbed wire. When one sheet of graphene is stacked on top of another at a precise “magic” angle, the twisted structure creates a slightly shifted “ripple” pattern, or superlattice, capable of supporting many surprising electronic behaviors.
In 2018, Pablo Jarillo-Herrero and his team at MIT were the first to demonstrate magic-angle twisted bilayer graphene. They showed that when they applied a certain continuous electric field, the new bilayer structure could act as an insulator, like wood. When they boosted the magnetic field, the insulator suddenly became a superconductor, allowing electrons to flow without friction.
This discovery gave rise to “twisted electronics,” a field that explores how certain electronic properties arise from the twisting and layering of two-dimensional materials. Researchers including Jarillo-Herrero continue to reveal surprising properties of magic-angle graphene, including various ways to switch the material between different electronic states. So far, this “switch” has acted more like a dimmer, since researchers have to continuously apply an electric or magnetic field to turn on superconductivity and keep it on.
Now, Jarillo-Herrero and his team have shown that superconductivity in magic-angle graphene can be turned on and kept on by short pulses rather than a continuous electric field. The key, they found, was a combination of twisting and stacking.
In a paper published today in Nature Nanotechnology, the team reports that by stacking magic-angle graphite between two offset layers of boron nitride, a two-dimensional insulating material, Graphene, the unique arrangement of the sandwich structure, allowed the researchers to switch graphene’s superconductivity on and off with short electrical pulses.
“For the vast majority of materials, if you remove the electric field, zzzzip, the electric state disappears,” said Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT. “This is the first time a superconducting material has been created that can switch electrically on and off suddenly. This could pave the way for a new generation of twisted, graphene-based superconducting electronics.”
His co-authors at MIT are lead authors Dahlia Klein, Li-Qiao Xia and David MacNeill, and Kenji Watanabe and Takashi Taniguchi of Japan’s National Institute for Materials Science.
flip switch
In 2019, a team at Stanford University found that magic-angle graphene can be forced into a ferromagnetic state. Ferromagnets are materials that retain their magnetic properties even in the absence of an applied magnetic field.
The researchers found that magic-angle graphene can exhibit ferromagnetic properties in a way that it can be switched on and off. This occurs when graphene sheets are stacked between two sheets of boron nitride so that the graphene’s crystal structure aligns with one of the boron nitride layers. This arrangement is similar to a cheese sandwich, where the top slice of the bread is aligned with the direction of the cheese, but the bottom slice of the bread is rotated at a random angle relative to the top slice. The results intrigued the MIT group.
“We tried to get a stronger magnet by aligning the two slices,” Jarillo-Herrero said. “Instead, we found something completely different.”
In their current study, the team created a sandwich consisting of carefully tilted and stacked materials. The “cheese” of the sandwich consists of magic-angle graphene — two graphene sheets with the top sheet slightly rotated at a “magic angle” of 1.1 degrees relative to the bottom sheet. On top of this structure, they placed a layer of boron nitride, precisely aligned with the top graphene sheet. Finally, they placed a second layer of boron nitride under the entire structure, offset by 30 degrees relative to the top layer of boron nitride.
The team then measured the resistance of the graphene layer when a gate voltage was applied. Like others, they found that twisted bilayer graphene switches electronic states, changing between insulating, conducting and superconducting states at certain known voltages.
What the team didn’t anticipate was that once the voltage was removed, each electronic state persisted rather than disappearing immediately — a property known as bistability. They found that at a certain voltage, the graphene layer becomes a superconductor and remains superconducting even when the researchers remove the voltage.
This bistability effect suggests that superconductivity can be switched on and off with short electrical pulses rather than a continuous electric field, similar to flicking a light switch. It’s not clear what makes this switchable superconductivity possible, although the researchers suspect it has something to do with the peculiar arrangement of the twisted graphene with the two boron nitride layers, which allows the system to produce a ferroelectric-like response . (Ferroelectric materials exhibit bistability in their electrical properties.)
Superconductivity switched on and off in ‘magic-angle’ graphene
“By focusing on stacking, you can add another tuning knob to the growing complexity of magic-angle superconducting devices,” Klein said.
For now, the team sees the new superconducting switch as another tool researchers could consider when developing materials for faster, smaller, more energy-efficient electronics.
“People are trying to make electronic devices that do computation in a way inspired by the brain,” Jarillo-Herrero said. “In the brain, our neurons fire when a certain threshold is crossed. Likewise, we’ve now found A way to make magic-angle graphene switch superconductivity abruptly above a certain threshold. This is a key property to enable neuromorphic computing”