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Multitasking Graphene Device can Help Study Exotic Quantum Physics

Scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have created a new multitasking graphene device that is thinner than a human hair but has a depth of unique traits.

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. (Image credit: Guorui Chen/Berkeley Lab)

This ultra-thin material conveniently changes from a superconductor to an insulator, and reverses again to a superconductor. All this occurs by simply flipping a switch. When the material is in a superconductor state, it conducts electricity without losing any energy and when it is in an insulator state, it resists the flow of electric current. The results of the study have been recently 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.

Guorui Chen, Study Lead Author and Postdoctoral Researcher, Department of Physics, Berkeley Lab

Chen works in the laboratory of Feng Wang, who headed the study. Wang is a faculty scientist in the Materials Sciences Division of Berkeley Lab and is also a UC Berkeley physics professor.

The new graphene device is made of three atomically thin (2D) layers of graphene closely packed between 2D layers of boron nitride to create a repeating pattern known as a moiré superlattice.

This material can allow other researchers to interpret the intricate mechanics behind a phenomenon called high-temperature superconductivity. In this phenomenon, electricity can be conducted by a material at temperatures higher than predicted—though still hundreds of degrees below freezing—without any resistance.

In an earlier study, the scientists reported visualizing the characteristics of a Mott insulator—a class of material—in a device composed of trilayer graphene. At hundreds of degrees below freezing, a Mott insulator does not conduct electricity in spite of conventional theory predicting electrical conductivity. However, it has been traditionally believed that a Mott insulator can become superconductive when more positive charges or electrons are added, Chen explained.

In the past decade, investigators have been exploring ways to integrate varied 2D materials, usually beginning with graphene—a material that can efficiently conduct electricity and heat. Based on this body of work, it was found that moiré superlattices created with graphene display unusual physics, like superconductivity, upon aligning the layers 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?’” stated Chen.

Opening the Gate to a New World of Physics

The researchers—in association 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—utilized a dilution refrigerator, which can reach extremely cold temperatures of 40 mK, or almost –460 ºF, to cool down the temperature of the graphene/boron nitride device at which the scientists anticipated superconductivity to occur close to the Mott insulator phase, informed Chen.

As soon as the graphene/boron nitride device reached a temperature of 4 K (–452 ºF), the scientists applied a range of electrical voltages to the device’s small top and bottom gates.

As predicted by the team, when a high vertical electrical field is applied to the top and bottom gates of the device, each cell of the graphene/boron nitride device is filled up with an electron. As a result, the electrons stabilize and stay in place, and it is this “localization” of electrons that converts the device into a Mott insulator.

Subsequently, the researchers applied an even greater electrical voltage to the gates of the device, but to their surprise, a second reading denoted that the electrons were not stable anymore.

Rather, they were shuttling about, traveling from one cell to another, and conducting electricity without any resistance or loss. To put this in simple terms, the device had changed from the Mott insulator phase to the superconductor phase.

The boron nitride moiré superlattice somehow boosts the interactions between electrons that occur upon applying an electrical voltage to the device—an effect that changes on its superconducting phase, explained Chen. It is also reversible, that is, the device changes back to an insulating state when a lower electrical voltage is applied to the gates.

The multitasking device provides researchers a small, versatile playground for exploring the unique interplay between electrons and atoms in new, exotic superconducting materials with promising applications in quantum computers and also new Mott insulator materials that are anticipated to make compact 2D Mott transistors for microelectronics a reality. Quantum computers are computers that store and exploit data in qubits, which are generally subatomic particles like photons or electrons,

This result was very exciting for us. We never imagined that the graphene/boron nitride device would do so well. 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.

Guorui Chen, Study Lead Author and Postdoctoral Researcher, Department of Physics, Berkeley Lab

The research was supported by 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.

NPQC brings together scientists at Argonne National Laboratory, Berkeley Lab, UC Santa Barbara, and Columbia University to study how quantum coherence underlies unanticipated phenomena in novel materials like trilayer graphene, with an aim toward upcoming applications in quantum information science and technology.

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


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