MIT team discovers new property in 'magic-angle' graphene

Researchers at the Massachusetts Institute of Technology (MIT) have discovered, what they call, an exciting new property in "magic-angle" graphene, according to an MIT News article published Jan. 30.

The researchers uncovered superconductivity that can be switched on and off with an electric pulse should the graphene be stacked and adjusted in a certain way.

According to a paper published in Nature Nanotechnology, the researchers reported the discovery could lead to hyper-fast, energy-efficient superconducting transistors for neuromorphic devices, or electronics made to operate in a way akin to the quick on/off firing of neurons in the human brain.

Magic-angle graphene refers to a specific stacking of graphene, an atom-thin material composed of carbon atoms that are linked in a hexagonal pattern resembling chicken wire. When one graphene sheet is stacked on a second sheet at a precise "magic" angle, the twisted structure creates a slightly offset "moiré" pattern, or superlattice, capable of supporting many surprising electronic behaviors.

In 2018, MIT Cecil and Ida Green Physics Professor Pablo Jarillo-Herrero and his team were the first to display magic-angle twisted bilayer graphene, showing that the new bilayer structure could behave as an insulator-like wood when a certain continuous electric field was applied. When the field was increased, the insulator transformed into a superconductor, allowing electrons to flow friction-free.

The discovery was a monumental moment in the field of "twistronics," which focuses on studying how specific electronic properties emerge from the twisting and layering of two-dimensional materials. 

Researchers such as Jarillo-Herrero have continued to reveal new properties in magic-angle graphene, such as multiple ways to switch the material between different electronic states. Currently, such "switches" have acted more akin to dimmers, in that researchers must constantly apply an electric or magnetic field to turn on and sustain superconductivity.

Recently, Jarillo-Herrero and his team members have proven that superconductivity in magic-angle graphene can be turned and kept on with only a short pulse, rather than a continuous electric field. They say the key was a combination of twisting and slacking.

In the paper, the team reported that by stacking magic-angle graphene between two offset layers of boron nitride, a two-dimensional insulating material, the unique alignment of the sandwich structure enabled the researchers to turn the graphene's superconductivity on and off with a short electric pulse.

“For the vast majority of materials, if you remove the electric field, 'zip,' the electric state is gone,” Jarillo-Herrero said. "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."

The discovery holds large implications for the field of neuromorphic computing, which focuses on developing machines that can perform complex calculations as efficiently as the human brain. Such devices require the ability to switch on and off at rapid speeds, which this discovery enables.

Dahlia Klein, a Ph.D, 2021 graduate student Li-Qiao Xia, former postdoc David MacNeill, and Japan's Kenji Watanabe and Takashi Taniguchi co-authored the paper alongside, Jarillo-Herrero.