2018-12-25 L202

Yuan Cao’s teenage years were hardly typical. By age 18, he had already graduated from high school, completed an undergraduate degree at the University of Science and Technology of China in Hefei, and travelled to the United States to begin his PhD. He hasn’t slowed down since: this year, aged just 21, Cao had two papers published on strange behaviour in atom-thick layers of carbon that have spurred a new field of physics. Cao admits that his situation is unusual, but says he isn’t special. After all, he did spend a full four years at university: “I just skipped some of the boring stuff in middle school.”

Pablo Jarillo-Herrero’s group at the Massachusetts Institute of Technology (MIT) in Cambridge was already layering and rotating sheets of carbon at different angles when Cao joined the lab in 2014. Cao’s job was to investigate what happened in two-layer stacks when one graphene sheet was twisted only slightly with respect to the other, which one theory predicted would radically change the material’s behaviour.

Many physicists were sceptical about the idea. But when Cao set out to create the subtly twisted stacks, he spotted something strange. Exposed to a small electric field and cooled to 1.7 degrees above absolute zero, the graphene — which ordinarily conducts electricity — became an insulator . That by itself was surprising. “We knew already that it would have a big impact on the community,” says Cao. But the best was yet to come: with a slight tweak to the field, the twisted sheets became a superconductor, in which electricity flowed without resistance . Seeing the effect in a second sample convinced the team that it was real.

The ability to coax atom-thick carbon into a complex electronic state through a simple rotation now has physicists clamouring to engineer exciting behaviour in other twisted 2D materials. Some even hope that graphene could shed light on how more-complex materials super-conduct at much higher temperatures. “There are so many things we can do,” says Cory Dean, a physicist at Columbia University in New York City. “The opportunities at hand now are almost overwhelming.”

Hitting graphene’s ‘magic angle’ — a rotation between parallel sheets of around 1.1° — involved some trial and error, but Cao was soon able to do it reliably. His experimental skill was crucial, says Jarillo-Herrero. Cao pioneered a method of tearing a single sheet of graphene so that he could create a stack composed of two layers with identical orientation, from which he could then fine-tune alignment. He also tweaked the cryogenic system to reach a temperature that allowed superconductivity to emerge more clearly.

Cao loves to take things apart and rebuild them. At heart, he is “a tinkerer”, his supervisor says. On his own time, this means photographing the night sky using homemade cameras and telescopes — pieces of which usually lie strewn across Cao’s office. “Every time I go in, it’s a huge mess, with computers taken apart and pieces of telescope all over his desk,” says Jarillo-Herrero.

Despite his youth and shy manner, colleagues say that Cao’s maturity shines through in his persistence. Having missed out by a whisker on a place in MIT’s physics graduate programme, for example, Cao found a way to pursue the subject by joining Jarillo-Herrero’s team through the electrical-engineering department. Cao also shrugged off a disappointing start to his PhD, after realizing that seemingly exciting data that he had spent six months trying to understand were due to a quirk of the experimental set-up. “He wasn’t happy, but he just rolled up his sleeves and continued working,” Jarillo-Herrero says.

Cao, now 22, doesn’t yet know where he’d like his career to lead. “On magic-angle graphene, we still have a lot of things to do,” he says. But universities around the world are already eyeing him for not only postdoctorate jobs, but also faculty positions, says physicist Changgan Zeng, Cao’s undergraduate supervisor and mentor at the University of Science and Technology of China. “Among condensed-matter physicists in China, everybody knows his name,” Zeng says. The university would gladly have him back, but Zeng expects that Cao will stay in the United States for now. “There, it’s easier to see the stars.”