A Graphene Superconductor That Plays More Than One Tune

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What’s thinner than a human hair but has a depth of special traits? A multitasking graphene device developed by researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). The superthin material easily switches from a superconductor that conducts electricity without losing any energy, to an insulator that resists the flow of electric current, and back again to a superconductor—all with a simple flip of a switch. Their findings were reported today in the journal Nature.

“Usually, when someone wants to study how electrons interact with each other in a superconducting quantum phase versus an insulating phase, they would need to look at different materials. With our system, you can study both the superconductivity phase and the insulating phase in one place,” said Guorui Chen, the study’s lead author and a postdoctoral researcher in the lab of Feng Wang, who led the study. Wang, a faculty scientist in Berkeley Lab’s Materials Sciences Division, is also a UC Berkeley physics professor.

Schematic of graphene/boron nitride moire’ superlattice superconductor/insulator device: The heterostructure material is composed of three atomically thin (2D) layers of graphene (gray) sandwiched between 2D layers of boron nitride (red and blue) to form a repeating pattern called a moiré superlattice. Superconductivity is indicated by the light-green circles, which represent the hole (positive charge) sitting on each unit cell of the moiré superlattice. (Credit: Guorui Chen/Berkeley Lab)

The graphene device is composed of three atomically thin (2D) layers of graphene sandwiched between 2D layers of boron nitride to form a repeating pattern called a moiré superlattice. The material could help other scientists understand the complicated mechanics behind a phenomenon known as high-temperature superconductivity, where a material can conduct electricity without resistance at temperatures higher than expected, though still hundreds of degrees below freezing.

In a previous study, the researchers reported observing the properties of a Mott insulator in a device made of trilayer graphene. A Mott insulator is a class of material that somehow stops conducting electricity at hundreds of degrees below freezing despite classical theory predicting electrical conductivity. But it has long been believed that a Mott insulator can become superconductive by adding more electrons or positive charges to make it superconductive, Chen explained.

For the past 10 years, scientists have been studying ways to combine different 2D materials, often starting with graphene – a material known for its ability to efficiently conduct heat and electricity. Out of this body of work, it was discovered that moiré superlattices formed with graphene exhibit exotic physics such as superconductivity when the layers are aligned at just the right angle.

“So for this study we asked ourselves, ‘If our trilayer graphene system is a Mott insulator, could it also be a superconductor?’” said Chen.

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