MIT physicists have discovered a unique property in "magic-angle" graphene through precise manipulation and layering. This property is superconductivity which can be activated and deactivated using an electric pulse, similar to flipping a switch. The finding could create high-speed, energy-efficient superconducting transistors for neuromorphic electronics, which mimic the quick on/off firing of brain neurons.
Magic-angle graphene is a specific graphene arrangement, a single layer of carbon atoms arranged in a hexagonal pattern like chicken wire. This occurs when two sheets of graphene are stacked at a precise angle, causing a twisted structure with an offset "moiré" pattern, known as a superlattice. This structure exhibits unusual electronic properties, as reported by Phys.
In 2018, Pablo Jarillo-Herrero and his team at MIT made the initial discovery of magic-angle twisted bilayer graphene. They showed that the bilayer structure behaves as an insulator under a specific electric field, similar to wood. However, when the electric field is increased, the insulator transforms into a superconductor, enabling the electrons to flow without friction
The 'Magic' of Angle Graphene
The discovery of magic-angle graphene gave birth to "twistronics," a discipline that studies the emergence of electronic properties from the stacking and twisting 2D materials. Since then, researchers, including Jarillo-Herrero, have continued to uncover surprising properties in magic-angle graphene and various methods to switch the material between various electronic states. However, these switches have acted as dimmers, meaning that researchers need to continuously apply an electric or magnetic field to turn on superconductivity and maintain it.
Jarillo-Herrero and his team have now demonstrated that superconductivity in magic-angle graphene can be activated and maintained with a brief electric pulse instead of a constant electric field. They found that the combination of stacking and twisting was crucial. In a study published in Nature Nanotechnology, the team reports that by stacking magic-angle graphene between two layers of boron nitride, a 2D insulating material with an offset alignment, they could activate and deactivate graphene's superconductivity using a short electric pulse.
Jarillo-Herrero, who holds the position of Cecil and Ida Green Professor of Physics at MIT, stated that the electric state of most materials vanishes instantly when the electric field is removed. However, this discovery marks the first time that a superconducting material has been created that can be turned on and off electrically abruptly. This finding could lead to the developing of a new generation of superconducting electronics based on twisted graphene. The study was co-authored by Dahlia Klein, Li-Qiao Xia, and David MacNeill at MIT, as well as Kenji Watanabe and Takashi Taniguchi from the National Institute for Materials Science in Japan.
MIT physicists have found a new way to switch superconductivity on and off in magic-angle graphene.
Switching Superconductivity
In 2019, a team at Stanford University discovered that magic-angle graphene could be made into a ferromagnetic state, which are materials that retain their magnetic properties without an external magnetic field. The team found that the graphene's ferromagnetic properties could be turned on and off by layering it between two sheets of boron nitride and aligning the crystal structure of the graphene with one of the boron nitride layers. The arrangement was similar to a cheese sandwich where the top slice of bread and cheese are aligned, but the bottom slice is rotated randomly to the top slice. This discovery intrigued the MIT group, which initially aimed to increase the magnetic strength by aligning both slices but found something unexpected instead.
The team fabricated a sandwich structure using magic-angle graphene (two graphene sheets slightly rotated at a "magic" angle of 1.1 degrees), a layer of boron nitride aligned with the top graphene sheet, and a second layer of boron nitride below the entire structure offset by 30 degrees. They measured the electrical resistance of the graphene layers and applied a gate voltage, which showed that the twisted bilayer graphene switched between insulating, conducting, and superconducting states at specific voltages. The team discovered that each electronic form was persistent and remained even after removing the voltage, a property known as bistability. The graphene layers became a superconductor at a particular voltage and remained superconducting even after removing the voltage.
The researchers discovered that they could turn on and off superconductivity in magic-angle graphene with short electric pulses instead of a continuous electric field. The special alignment of the twisted graphene with the two layers of boron nitride is suspected to be the cause of this switchable superconductivity. The discovery can be used to developing faster, smaller, and more energy-efficient electronics for applications such as neuromorphic computing. The researchers see the new superconducting switch as a way to add another layer of complexity to magic-angle superconducting devices.
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