Rapid electrical pulses completely flip the electronic properties of materials, paving the way for ultrafast, brain-inspired superconducting electronics.
With some careful twisting and stacking, MIT physicists have revealed new and exotic properties of “magic angle” graphene. This is a superconductivity that can be switched on and off with electrical pulses, like an optical switch.
This discovery may lead to ultrafast and energy efficient superconducting transistors for neuromorphic devices. This is an electronic device designed to work in a manner similar to the rapid on/off firing of neurons in the human brain.
Magic angle graphene refers to a very special stack of graphene. It is an atom-thin material made up of carbon atoms connected in a hexagonal pattern resembling wire mesh. When one graphene sheet is stacked on top of his second sheet at a precise ‘magic’ angle, it creates a ‘moire’ pattern of slightly offset twisted structures, a superlattice, and a multitude of can support the surprising electronic behavior of
In 2018, Pablo Jarillo-Herrero and his group at MIT demonstrated for the first time bilayer graphene twisted at a magical angle. They showed that the new bilayer structure can behave as an insulator, like wood, when a constant, continuous electric field is applied. When they raised the electric field, the insulator suddenly turned into a superconductor, allowing electrons to flow without friction.
This discovery was a watershed in the field of ‘twistronics’, investigating how specific electronic properties arise from the twisting and layering of two-dimensional materials. Researchers, including Jarillo-Herrero, continue to uncover surprising properties of magic-angle graphene, such as the different ways in which the material can be switched between different electronic states. So far, such ‘switches’ have acted like dimmers, requiring researchers to continuously apply an electric or magnetic field to turn on and maintain superconductivity.
Jarillo-Herrero and his team now show that superconductivity in magic-angle graphene can be switched on and off with just a short pulse rather than a continuous electric field. The key they found was a combination of twisting and stacking.
In a paper out today on nanotechnology in naturethe team found that by stacking magic angle graphene between two offset layers of boron nitride (a two-dimensional insulating material), the unique arrangement of the sandwich structure allows researchers to turn on/on graphene superconductivity with short electrical pulses. I am happy to report that it can now be turned off.
“For most materials, if you remove the electric field zzzzip, the electric state disappears,” says Jarillo-Herrero, 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 switched on and off abruptly. This could pave the way for a new generation of twisted graphene-based superconducting electronics.” ”
His MIT co-authors are lead author Dahlia Klein PhD ’21, graduate student Li-Qiao Xia, former postdoc David MacNeill, and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science, Japan.
flip the switch
In 2019, a team at Stanford University discovered that magic-angle graphene can be forced into a ferromagnetic state. Ferromagnets are materials that retain their magnetic properties in the absence of an externally applied magnetic field.
Researchers have discovered that magic-angle graphene may exhibit ferromagnetic properties in ways that can be turned on and off. This occurred when a graphene sheet was stacked between two boron nitride sheets such that the crystal structure of the graphene was aligned with one of the boron nitride layers. The arrangement resembles a cheese sandwich, with the top slice of bread and cheese aligned, but the bottom slice of bread rotated at a random angle to the top slice. The results piqued the interest of his MIT group.
“By lining up both slices, we were trying to get a stronger magnet,” says Jarillo-Herrero. “Instead, I found something completely different.”
In the current study, the team created sandwiches of carefully angled and stacked material. The “cheese” of the sandwich consists of magic-angle graphene (two graphene sheets), with the top slightly rotated at a “magic” angle of 1.1 degrees to the bottom sheet. On top of this structure, we placed a layer of boron nitride precisely aligned with the upper graphene sheet. Finally, they placed his second layer of boron nitride under the entire structure, offset by 30 degrees against the top layer of boron nitride.
The team then measured the electrical resistance of the graphene layer when a gate voltage was applied. They found that twisted bilayer graphene switches electronic states, changing between insulating, conducting, and superconducting states, as others have done. at a certain known voltage.
What the group did not expect was that each electronic state would persist rather than disappear as soon as the voltage was removed. This is a property known as bistability. They found that at a certain voltage the graphene layer turned into a superconductor and remained superconducting even when the researchers removed this voltage.
This bistability effect suggests that superconductivity can be turned on and off with short electrical pulses instead of continuous electric fields, similar to flicking an optical switch. It is not clear what makes this switchable superconductivity possible, but the researchers suggest that the special alignment of both twisted graphene to the boron nitride layer and I believe there is a relationship. (Ferroelectric materials exhibit bistability in their electrical properties.)
“Paying attention to stacking adds another adjustment knob to the complexity of the magic angle superconducting device,” says Klein.
For now, the team sees the new superconducting switch as another tool researchers can consider as they develop materials for faster, smaller, and more energy-efficient electronics.
“People are trying to build electronic devices that do computation in a brain-inspired way,” says Jarillo-Herrero. “There are neurons in the brain that fire when a certain threshold is crossed. Similarly, we discovered how magic-angle graphene suddenly switches to superconductivity beyond a certain threshold. It is an important characteristic in realizing the performance.”
Original: Research: Superconductivity turns on and off in ‘magic angle’ graphene
Than: Massachusetts Institute of Technology
