MIT physicists have discovered a brand-new method to change superconductivity on and off in magic-angle graphene. The graphene layers are sandwiched in between boron nitride layers (in blue and purple). The angle and alignment of each layer allows the researchers to turn superconductivity on and off in graphene with a short electric pulse.
Applying a fast electrical pulse completely turns the products electronic properties, opening a route to ultrafast, brain-inspired, superconducting electronics.
MIT physicists have actually revealed a unique and new property in “magic-angle” graphene: superconductivity that can be turned on and off with an electric pulse, similar to a light switch. To achieve this, they utilized some precise twisting and stacking of layers of graphene and boron nitride.
The discovery could lead to ultrafast, energy-efficient superconducting transistors for neuromorphic devices– electronic devices created to operate in a manner similar to the fast on/off firing of nerve cells in the human brain.
Magic-angle graphene describes a very particular stacking of graphene– an atom-thin product made from carbon atoms that are linked in a hexagonal pattern looking like chicken wire. When one sheet of graphene is stacked atop a 2nd sheet at an accurate “magic” angle, the twisted structure produces a slightly balanced out “moiré” pattern, or superlattice, that is able to support a host of unexpected electronic behaviors.
In 2018, Pablo Jarillo-Herrero and his group at MIT were the first to show magic-angle twisted bilayer graphene. They showed that the brand-new bilayer structure might behave as an insulator, much like wood, when they used a specific constant electric field. When they upped the field, the insulator suddenly changed into a superconductor, permitting electrons to stream, friction-free.
That discovery was a watershed in the field of “twistronics,” which explores how specific electronic properties emerge from the layering and twisting of two-dimensional materials. Scientists consisting of Jarillo-Herrero have actually continued to reveal unexpected homes in magic-angle graphene, consisting of different ways to change the material in between different electronic states. Far, such “switches” have acted more like dimmers, in that researchers must continually use a magnetic or electric field to turn on superconductivity, and keep it on.
Now Jarillo-Herrero and his group have actually revealed that superconductivity in magic-angle graphene can be switched on, and kept on, with just a short pulse rather than a constant electric field. The secret, they discovered, was a combination of stacking and twisting.
In a paper published on January 30 in the journal Nature Nanotechnology, the team reports that, by stacking magic-angle graphene between 2 offset layers of boron nitride– a two-dimensional insulating material– the distinct alignment of the sandwich structure made it possible for the scientists to turn graphenes superconductivity on and off with a short electric pulse.
” For the large majority of products, if you get rid of the electrical field, zzzzip, the electrical state 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 electrically turned on and off, quickly. This could lead the way for a brand-new generation of twisted, graphene-based superconducting electronic devices.”
His MIT co-authors are lead author Dahlia Klein PhD 21, college student Li-Qiao Xia, and previous postdoc David MacNeill, together with Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan.
Turning the switch
In 2019, a group at Stanford University discovered that magic-angle graphene might be pushed into a ferromagnetic state. Ferromagnets are products that retain their magnetic residential or commercial properties, even in the lack of an externally applied magnetic field.
The scientists discovered that magic-angle graphene could exhibit ferromagnetic properties in a method that might be tuned on and off. This occurred when the graphene sheets were layered between two sheets of boron nitride such that the crystal structure of the graphene was lined up to among the boron nitride layers. The arrangement resembled a cheese sandwich in which the top piece of bread and the cheese orientations are lined up, however the bottom piece of bread is rotated at a random angle with regard to the leading slice. The outcome captivated the MIT group.
” We were attempting to get a more powerful magnet by lining up both slices,” Jarillo-Herrero says. “Instead, we found something entirely different.”
The “cheese” of the sandwich consisted of magic-angle graphene– two graphene sheets, the leading turned a little at the “magic” angle of 1.1 degrees with respect to the bottom sheet. Above this structure, they positioned a layer of boron nitride, exactly aligned with the top graphene sheet.
The group then determined the electrical resistance of the graphene layers as they used a gate voltage. They discovered, as others have, that the twisted bilayer graphene switched electronic states, changing between insulating, performing, and superconducting states at specific known voltages.
Once the voltage was removed– a residential or commercial property known as bistability, what the group did not expect was that each electronic state persisted rather than instantly vanishing. They discovered that, at a particular voltage, the graphene layers developed into a superconductor, and remained superconducting, even as the scientists removed this voltage.
This bistable impact suggests that superconductivity can be turned on and off with brief electric pulses rather than a continuous electric field, similar to flicking a light switch. It isnt clear what enables this switchable superconductivity, though the scientists believe it has something to do with the unique alignment of the twisted graphene to both boron nitride layers, which makes it possible for a ferroelectric-like response of the system. (Ferroelectric products show bistability in their electric homes.).
” By taking note of the stacking, you might include another tuning knob to the growing intricacy of magic-angle, superconducting devices,” Klein says.
In the meantime, the team sees the new superconducting switch as another tool researchers can think about as they establish materials for faster, smaller sized, more energy-efficient electronics.
” People are trying to construct electronic devices that do computations in a way thats inspired by the brain,” Jarillo-Herrero states. We now have discovered a way for magic-angle graphene to switch superconductivity abruptly, beyond a particular limit.
Recommendation: “Electrical changing of a bistable moiré superconductor” by Dahlia R. Klein, Li-Qiao Xia, David MacNeill, Kenji Watanabe, Takashi Taniguchi and Pablo Jarillo-Herrero, 30 January 2023, Nature Nanotechnology.DOI: 10.1038/ s41565-022-01314-x.
This research was supported in part by the U.S. Air Force Office of Scientific Research, the U.S. Army Research Office, and the Gordon and Betty Moore Foundation.
The graphene layers are sandwiched in between boron nitride layers (in purple and blue). The angle and alignment of each layer enables the scientists to turn superconductivity on and off in graphene with a short electric pulse. The “cheese” of the sandwich consisted of magic-angle graphene– two graphene sheets, the leading turned somewhat at the “magic” angle of 1.1 degrees with regard to the bottom sheet. Above this structure, they placed a layer of boron nitride, exactly aligned with the top graphene sheet. It isnt clear what allows this switchable superconductivity, though the scientists suspect it has something to do with the special alignment of the twisted graphene to both boron nitride layers, which makes it possible for a ferroelectric-like reaction of the system.
