Harvard Researchers Achieve Tunable Superconductivity in Trilayer Graphene

Harvard scientists have developed a new twisted graphene configuration for achieving superconductivity that could help lead to the realization of superconductors at higher temperatures, according to a Feb. 4 paper published in Science.

Members of Physics professor Philip Kim’s group have developed a new superconductor — a material that conducts electricity with zero resistance — by stacking three layers of graphene and twisting them. Graphene is a material consisting of a single sheet of carbon atoms.

The new trilayer system builds upon the 2018 discovery by MIT Physics professor Pablo Jarillo-Herrero’s group that graphene could act as a superconductor when doubly stacked and twisted at a “magic” angle.

Compared to the previous bilayer system, the trilayer is a more robust superconductor; it can carry larger currents, and its layers can be stacked at larger angles, Kim’s group found.

The twisted graphene trilayer system is also capable of superconductivity at higher temperatures than previous bilayer systems. Conventional superconductors only work in extremely cold temperatures — below minus 140 degrees Celsius — at atmospheric pressure, rendering them impractical for everyday use. A better understanding of high-temperature superconductors, however, would allow for technological breakthroughs in many fields, ranging from power transmission and transportation to quantum computing.

This finding will allow scientists to use and study the trilayer graphene as a controllable model system in attempts to unlock the mysteries of high temperature superconductivity.

“It’s very hard to predict what materials are going to superconduct and at what temperature, and having this very tunable material where we can control the superconductivity lets us disentangle those,” co-lead author of the paper and postdoctoral researcher Andrew M. Zimmerman said.

Experimentally, stacking two layers of graphene was already “challenging,” and adding the third layer was even trickier, according to co-lead author and Harvard graduate student Zeyu Hao. Hao said the process required a lot of practice as well as the development of a new technique over the course of a year.

“We developed, based on some previous knowledge, on how to use a so-called atomic force microscope to cut this material — previously, people just tore the material,” Hao said. “When you tear materials, they tend to move, and we’d have these wrinkles that would mess up your twist angle.”

Jarillo-Herrro’s group also published its own realization of the trilayer system in a Feb. 1 paper in the journal Nature. The superconductivity in this graphene system can be controlled with “two knobs” that allow the variation of the charge density and electric field, according to Jarillo-Herrero.

“That is very rare — I’m not sure there is any other material that does that,” Jarillo-Herrero said. “It means you can make devices with it, and you can investigate the properties of the materials in much more detail and obtain much more insight.”

Xi Dai — a physics professor at The Hong Kong University of Science and Technology and materials professor at University of California, Santa Barbara — said the graphene system allows for new research possibilities.

“This is very unique, because we can realize many different interesting states in condensed matter,” Dai said. “Maybe in the next step, we can try some other ways of twisting the trilayer, and also we can try the four-layer system.”

—Staff writer Justin Lee can be reached at justin.lee@thecrimson.com.

—Staff writer Lauren L. Zhang can be reached at lauren.zhang@thecrimson.com.