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Magic-angle twisted bilayer graphene yields multiple electronic and exotic states

By Rachel Nuwer March 3, 2020
twisted bilayer graphene
Artistic illustration of the twisted bilayer graphene and the various different states of matter that have been discovered. ©ICFO/F. Vialla

Graphene made headlines about 15 years ago, when scientists discovered that the constituent one-atom thick layers of graphite were super-strong and flexible and conducted heat and electrical current. While graphene did not revolutionize electronics, as was originally predicted by some, the material has found some concrete applications.

Further applications could soon follow, thanks to research published last year. Scientists at the Massachusetts Institute of Technology (MIT) discovered that layering two sheets of graphene on top of each other to produce a moiré pattern and then rotating the sheets by 1.1° resulted in superconductivity when the device was cooled to just above absolute zero. They called this magic-angle graphene, because only at 1.1°—not at 1.0° or 1.2°—do the superconducting and correlated insulated phases appear. Since this breakthrough, new lines of investigations have begun rapidly progressing. But basic questions remain about magic-angle graphene’s superconductor properties, including over what range of angles, densities, and temperatures resistance drops to zero.

Now, an international team of researchers has begun to answer those questions by refining the quality of the MIT team’s original twisted magic-angle graphene device. As the researchers reported in Nature, their new device exhibits a novel suite of superconducting, magnetic, topological, and correlated states.  

“We improved on the initial results [from MIT] and by doing so, we got a much broader view of what’s going on in the system,” says Dmitri Efetov, a physicist at the Institute of Photonic Sciences and the Barcelona Institute of Science and Technology, and senior author of the article. “We found that correlated insulators and superconductors are much more universal to the system and appear at many places in the phase diagram.”  

Efetov and his colleagues turned to a “tear and stack” assembly technique that is regularly used to create van der Waals heterostructures, or other types of stacked monolayer materials. They used a mechanical cleaning process to get rid of impurities in the graphene interphase, such as air bubbles and strain, and slowly smoothed the second layer of graphene onto the first (as though they were applying a protective screen to a smart phone). The resulting twisted graphene bilayers exhibited significantly reduced disorder compared to the MIT device. “We just played a little bit by the book, going with the most obvious steps for improvements,” Efetov says. “We got lucky it worked out so well.”

The researchers attached electrodes to the two-layered twisted graphene by evaporating gold on its surface and attached an electronic gate underneath it. As they applied and tuned the voltage and varied the temperature, the graphene’s resistance changed from kilohms to megaohms to almost zero. This meant the same device behaved as an insulator and a superconductor. It also exhibited superconductivity at the lowest carrier densities known. The research team was able to achieve superconductivity at 3 K, about twice as high as the MIT team’s device.

The device could also enter into a magnetic state with an exotic property called orbital magnetism. Orbital magnetism induces current to form at the edges of a device rather than at its center, a phenomenon referred to as non-trivial topological texture. This is the first time, to the researchers’ knowledge, that such a phase has been observed. “Probably, the MIT devices contained these states, but they were not visible because they were masked by impurities,” Efetov says.

Cory Dean, a physicist at Columbia University who was not involved in the new study, says that Efetov and his colleagues moved the field forward by showing that the entire flat energy band that is induced by twisting to the magic angle can be superconducting. The first MIT study, and a subsequent study by Dean’s group, found evidence of superconductivity only when these bands were close to half full of electrons or holes. Efetov’s group, Dean says, has now shown superconductivity can manifest for nearly all fillings of the flat band—something that may be significant for understanding the origin of superconductivity in the system.

“This provides a major new data point in trying to understand, for example, the relationship between the superconducting phases and nearby spontaneously insulating phases,” Dean says.

One major problem that Efetov and his colleagues appear to have addressed, Dean continues, is the issue of local variations in the twist angle. The magic angle is defined for only a 0.1° total angle range, so small variations can lead to significant local strain and inhomogeneity, making it difficult for researchers to know what they are measuring. “Efetov’s results demonstrate that it is possible to fabricate highly uniform systems with very low disorder, suggesting that it is possible to uncover the intrinsic, or disorder-free, properties of this system,” Dean says. “This will play a major role in trying to bridge understanding between the experimental data and competing theoretical models.”   

The next challenge, he says, is for researchers to develop the capability to fabricate devices that explore the range of possible angles in a more precise way. “Efetov has shown that making a disorder-free magic-angle device is possible,” Dean says. “Whether we can intentionally make a device with an angle of, say, 1.05° instead of 1.08° remains to be seen.”

Efetov and his colleagues are already exploring these possibilities, including a device that could detect single photons. Uses for such a device would range from quantum computing and communications to research satellites and biomedical imaging. “If you have a better detector, it always improves many applications,” Efetov says. 

The study of twistronics—not just for graphene, but for other twisted, layered materials—could lead to many surprising new discoveries and real-world applications. “We can think outside the box and twist magnets, intrinsic superconductors, and topological insulators,” Efetov says. “In principle, the sky is the limit.”

Read the abstract in Nature.