Give double-layer graphene a twist and it superconducts

A ‘magic angle’ lets electrons flow freely

grapheneDOUBLE UP  A device made of two layers of graphene (illustrated) can conduct electricity without resistance when one layer is rotated relative to the other.

LOS ANGELES — Give a graphene layer cake a twist and it superconducts — electrons flow freely through it without resistance. Made up of two layers of graphene, a form of carbon arranged in single-atom-thick sheets, the structure’s weird behavior suggests it may provide a fruitful playground for testing how certain unusual types of superconductors work, physicist Pablo Jarillo-Herrero of MIT reported March 7 at a meeting of the American Physical Society.

The discovery, also detailed in two papers published online in Nature on March 5, could aid the search for a superconductor that functions at room temperature, instead of the chilly conditions required by all known superconductors. If found, such a substance could replace standard conductors in various electronics, promising massive energy savings.

Layered graphene’s superconductivity occurs when the second layer of graphene is twisted relative to the first, at a “magic angle” of about 1.1 degrees, and when cooled below 1.7 kelvins (about –271° Celsius). Surprisingly, Jarillo-Herrero and colleagues report, the same material can also be nudged into becoming an insulator — in which electrons are stuck in place — by using an electric field to remove electrons from the material. That close relationship with an insulator is a characteristic shared by certain types of high-temperature superconductors, which function at significantly warmer temperatures than other superconductors, although they still require cooling.

The discovery of a material with behavior parallel to that of certain high-temperature superconductors could help explain the still-murky physics behind them (SN: 1/20/18, p. 11).  “We’re a little bit stuck. We have studied these materials to death,” Jarillo-Herrero said during a news conference on March 6. Twisted graphene might help scientists make progress on the topic.

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