Getting Close To The Secrets Of Superconductivity

quantamagazine  |  For the last three years, electrons have been toying with physicists.

The game started in 2018 when the lab of Pablo Jarillo-Herrero announced the find of the decade: When the researchers stacked one flat sheet of carbon atoms on top of another, applied a “magic” 1.1-degree twist between them, then cooled the atomic wafers to nearly absolute zero, the sample became a perfect conduit of electrons.

How were the particles conspiring to slip flawlessly through the graphene sheets? The kaleidoscopic “moiré” pattern created by the skew angle seemed significant, but no one knew for sure. To find out, researchers started twisting and stacking every material they could get their hands on.

At first, the electrons played along. Experiment after experiment found that, in an array of flat materials, frigid temperatures brought plummeting electric resistance. A more profound understanding of the conditions necessary for ideal conduction felt close, and with it, a tantalizing step toward an electronics revolution.

“It seemed like superconductivity was everywhere,” said Matthew Yankowitz, a condensed matter physicist at the University of Washington, “no matter what system you looked at.”

But the electrons proved coy. As researchers inspected their samples more carefully, the instances of superconductivity vanished. In some materials, resistance wasn’t actually getting down to zero. In others, different tests offered conflicting results. Only in the original double-layered graphene did electrons regularly achieve a frictionless flow.

“We had this zoo of different twisted materials, and twisted bilayer graphene was the only one that was clearly a superconductor,” Yankowitz said.

Then in the past month, two papers published in the journals Nature and Science described a second related superconductor, a three-layer graphene sandwich with the “bread” sheets aligned and the filling sheet skewed by 1.56 degrees. The unmistakable electron-carrying prowess of twisted trilayer graphene confirms that the two-wafer system was not a fluke. “It was the first of a family of moiré superconductors,” said Jarillo-Herrero, a physicist at the Massachusetts Institute of Technology who also led one of the new experiments, “and this one is the second member of the family.”

Importantly, this second sibling has helped to illuminate an underlying mechanism that could be what powers the superconductivity of these materials.

In the months after the 2018 discovery, one group of theorists began to puzzle over the mechanism that made bilayer graphene superconduct. They suspected that one particular geometric trait might allow electrons to swirl into exotic maelstroms that behave in an entirely novel manner. This mechanism, which is unlike any of the (few) known schemes responsible for superconductivity, would explain the superconductive success of bilayer graphene, as well as the failure of other materials. It also predicted that graphene’s trilayer sibling would superconduct as well.