Material Changes Between Superconducting and Insulating with Flip of a Switch

Researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a graphene device that switches from superconducting to insulating with the flip of a switch. See schematic above of the graphene/boron nitride moire' superlattice superconductor/insulator device: The heterostructure material is made of three atomically-thin (2D) layers of graphene (gray) sandwiched between 2D layers of boron nitride (red and blue) in a repeating pattern called a moiré superlattice. Superconductivity is represented by the light-green circles, which show the hole (positive charge) sitting on each unit cell of the moiré superlattice (schematic courtesy of Guorui Chen/Berkeley Lab).

The super-thin material can switch from superconducting without any energy loss to insulating, resisting electric current flow, and it can even go back to superconducting, all with the flip of a switch.

The findings were reported 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, serves as a UC Berkeley physics professor.

The graphene device is made of three atomically-thin (2D) layers of graphene held between 2D layers of boron nitride in a repeating pattern called a moiré superlattice.

The material could help scientists understand the complicated mechanics behind high-temperature superconductivity, where a material conducts electricity without resistance at temperatures higher than expected. Such superconducting occurs at temperatures still hundreds of degrees below freezing.

In a previous investigation, the researchers reported witnessing the properties of a Mott insulator in a device composed of trilayer graphene.

A Mott insulator is a type of material that stops conducting electricity at hundreds of degrees below freezing despite classical theory predicting electrical conductivity. It has long been theorized that adding more electrons or positive charges to a Mott insulator can make it superconductive, Chen explained.

For about a decade, scientists have been studying ways to combine different 2D materials, often beginning with graphene. Graphene is known for efficiently conducting heat and electricity. Out of this work, it was found that moiré superlattices made of graphene exhibit exotic physical properties such as superconductivity when the layers are aligned at the proper 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.

Opening the gate to a new world of physics

Collaborating with David Goldhaber-Gordon of Stanford University and the Stanford Institute for Materials and Energy Sciences at SLAC National Accelerator Laboratory, and Yuanbo Zhang of Fudan University, the researchers utilized a dilution refrigerator which can reach extremely cold temperatures of 40mK or nearly -460°F to cool the graphene/boron nitride device to a temperature near the Mott insulator phase at which superconductivity is expected to appear, according to Chen.

Once the device reached a temperature of 4K (minus 452°F), the researchers applied a range of electrical voltages to the top and bottom gates of the device.

As expected, after applying a high vertical electrical field to the top and bottom gates, an electron filled each cell of the graphene/boron nitride device. This electrical field caused the electrons to stabilize and stay in place, turning the device into a Mott insulator.

Then, they applied an even higher electrical voltage to the gates.

A second reading revealed that the electrons were no longer stable. Rather, they were moving from cell to cell and conducting electricity without loss or resistance in a superconductor phase.

Chen explained that the boron nitride moiré superlattice somehow increases the electron-electron interactions that occur when an electrical voltage is applied, switching on its superconducting phase.

They found it is also reversible. When a lower electrical voltage is applied to the gates, the device switches back to insulating.

The multitasking device gives scientists a versatile playground for studying the delicate interaction among atoms and electrons in exotic new superconducting materials. The device can also be used to study new Mott insulator materials could theoretically be made into 2D Mott transistors for microelectronics.

High-temperature superconducting materials have numerous potential uses including applications in quantum computers that store and manipulate information in qubits,

"This result was very exciting for us. We never imagined that the graphene/boron nitride device would do so well," Chen said. "You can study almost everything with it, from single particles to superconductivity. It's the best system I know of for studying new kinds of physics," Chen said.

Support for the study came from the Center for Novel Pathways to Quantum Coherence in Materials (NPQC), an Energy Frontier Research Center led by Berkeley Lab and funded by the DOE Office of Science.

Researchers from Nanjing University, China and Shanghai Jiao Tong University; the National Institute for Materials Science, Japan; and the University of Seoul, Korea also contributed to the study.