A graphene superconductor that plays more than one tune
Researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a graphene device that's thinner than a human hair but has a depth of special traits. It easily switches from a superconducting material 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.
The graphene device is composed of three atomically thin (2D) layers of graphene. When sandwiched between 2D layers of boron nitride, it forms 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, researchers 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, other researchers had 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.
Opening the gate to a new world of physics
Working 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 used a dilution refrigerator, which can reach intensely cold temperatures of 40 millikelvins - or nearly minus 460 degrees Fahrenheit - to cool the graphene/boron nitride device down to a temperature at which the researchers expected superconductivity to appear near the Mott insulator phase, said Chen.
Once the device reached a temperature of 4 kelvins (minus 452 degrees Fahrenheit), the researchers applied a range of electrical voltages to the tiny top and bottom gates of the device. As they expected, when they applied a high vertical electrical field to both the top and bottom gates, an electron filled each cell of the graphene/boron nitride device. This caused the electrons to stabilize and stay in place, and this "localization" of electrons turned the device into a Mott insulator.
Then, they applied an even higher electrical voltage to the gates. To their delight, a second reading indicated that the electrons were no longer stable. Instead, they were shuttling about, moving from cell to cell, and conducting electricity without loss or resistance. In other words, the device had switched from the Mott insulator phase to the superconductor phase.
Chen explained that the boron nitride moiré superlattice somehow increases the electron-electron interactions that take place when an electrical voltage is applied to the device, an effect that switches on its superconducting phase. It's also reversible - when a lower electrical voltage is applied to the gates, the device switches back to an insulating state.
The multitasking device offers scientists a tiny, versatile playground for studying the exquisite interplay between atoms and electrons in exotic new superconducting materials with potential use in quantum computers - computers that store and manipulate information in qubits, which are typically subatomic particles such as electrons or photons - as well as new Mott insulator materials that could one day make tiny 2D Mott transistors for microelectronics a reality.
"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.
Evidence of a gate-tunable Mott insulator in a trilayer graphene moiré superlattice
Guorui Chen, Lili Jiang, Shuang Wu, Bosai Lyu, Hongyuan Li, Bheema Lingam Chittari, Kenji Watanabe, Takashi Taniguchi, Zhiwen Shi, Jeil Jung, Yuanbo Zhang & Feng Wang
Nature Physics volume 15, pages 237–241 (21 Jan 2019)
Signatures of tunable superconductivity in a trilayer graphene moiré superlattice
Guorui Chen, Aaron L. Sharpe, Patrick Gallagher, Ilan T. Rosen, Eli J. Fox, Lili Jiang, Bosai Lyu, Hongyuan Li, Kenji Watanabe, Takashi Taniguchi, Jeil Jung, Zhiwen Shi, David Goldhaber-Gordon, Yuanbo Zhang & Feng Wang
Nature (17 July 2019)
Professor of Physics at UC Berkeley
Tel: (510) 643-3275
Professor of Physics at Stanford University
Tel: (650) 724-3709
Professor of Physics at Fudan University
U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab)