Mar 5 2020
Graphene was discovered in 2004 and since that time, researchers have looked at many different ways to put this atomically thin and gifted two-dimensional (2D) material to work.
An optical image of the graphene device (shown above as a square gold pad) on a silicon dioxide/silicon chip. Shining metal wires are connected to gold electrodes for electrical measurement. The tiny graphene device has a length and width of just one-tenth of a millimeter. Image Credit: Guorui Chen/Berkeley Lab.
Graphene is thinner than one DNA strand yet 200 times stronger than that of steel. It is also an excellent conductor of heat and electricity and can adapt to any kind of shape, from an electronic circuit to an ultrathin 2D sheet.
The previous year, a research team, headed by Feng Wang, a faculty scientist in the Materials Sciences Division at Berkeley Lab and a professor of physics at UC Berkeley, created a new multitasking graphene device that changes from a superconductor to an insulator and reverts to a superconductor. The superconductor efficiently conducts electricity, while the insulator resists the flow of electricity.
As described in the Nature journal recently, the scientists had tapped into the graphene system’s talent to juggle three properties, instead of the proverbial two properties—that is, insulating, superconducting, and a kind of magnetism known as ferromagnetism.
The multitasking graphene device could pave the way for novel physics experiments such as research in the quest for an electric circuit for next-generation and faster electronics, for example, quantum computing technologies.
So far, materials simultaneously showing superconducting, insulating, and magnetic properties have been very rare. And most people believed that it would be difficult to induce magnetism in graphene, because it’s typically not magnetic. Our graphene system is the first to combine all three properties in a single sample.
Guorui Chen, Study Lead Author, University of California, Berkeley
Chen is also a postdoctoral researcher in Wang’s Ultrafast Nano-Optics Group at the University of California, Berkeley (UC Berkeley).
Using Electricity to Turn On Graphene’s Hidden Potential
Graphene has plenty of potential in the realm of electronics. The material’s atomically thin structure, together with its powerful thermal and electronic conductivity, “could offer a unique advantage in the development of next-generation electronics and memory storage devices,” added Chen, who also worked as a postdoctoral researcher in the Materials Sciences Division at Berkeley Lab during the research.
The issue is that the magnetic materials, which are currently utilized in electronics, are composed of ferromagnetic metals such as cobalt or iron alloys.
Ferromagnetic materials are similar to the regular bar magnet and have a south pole and a north pole. When such materials are used for storing data on the hard disk of a computer, both north and south poles point in an upward or downward direction, representing zeros and ones—known as bits.
On the other hand, graphene is made of carbon and not composed of a magnetic metal. Therefore, the researchers developed an inventive alternative approach.
They eventually designed an ultrathin device, which had a thickness of just 1 nm containing three layers of atomically thin graphene. When these graphene layers are packed closely between 2D layers of boron nitride, they form a repeating pattern known as a moiré superlattice. These graphene layers are described as trilayer graphene in the research.
When electrical voltages were applied through the gates of the graphene device, the electrical force pushed the electrons in the device and allowed them to circle in the same direction, similar to small cars speeding around a track. This created a forceful momentum that converted the graphene device into a ferromagnetic system.
Further measurements showed a remarkable new set of characteristics—the interior of the graphene system had become both magnetic and insulating; and regardless of the magnetism, the outer edges of the graphene system morphed into electronic current channels that shift without any sign of resistance. Properties like these define an exceptional class of insulators called Chern insulators, stated the scientists.
Another aspect that was even more surprising was the calculations which showed that the graphene device has two conductive edges and not just only one. These calculations were made by co-author Ya-Hui Zhang from the Massachusetts Institute of Technology. This finding makes this graphene device the first observed “high-order Chern insulator,” an outcome of the robust interactions between the electrons in the trilayer graphene.
Researchers have been pursuing Chern insulators in an area of research called topology, which analyzes the unique states of matter. Chern insulators provide new, possible ways to exploit information in a quantum computer, in which data is stored in quantum bits, also known as qubits. A qubit can indicate a zero, a one, or a state where it is both a zero and a one simultaneously.
Our discovery demonstrates that graphene is an ideal platform for studying different physics, ranging from single-particle physics, to superconductivity, and now topological physics to study quantum phases of matter in 2D materials. It’s exciting that we can now explore new physics in a tiny device just 1 millionth of a millimeter thick.
Guorui Chen, Study Lead Author, University of California, Berkeley
The scientists are hoping to perform additional experiments with their graphene device to gain a deeper understanding of the evolution of the Chern insulator/magnet and the mechanics behind its unique properties.
Scientists from Stanford University; Berkeley Lab; UC Berkeley; Massachusetts Institute of Technology; SLAC National Accelerator Laboratory; China’s Shanghai Jiao Tong University, Collaborative Innovation Center of Advanced Microstructures, Japan’s National Institute for Materials Science; and Fudan University took part in the study.
The study was funded by the Center for Novel Pathways to Quantum Coherence in Materials, an Energy Frontier Research Center financed by the U.S. Department of Energy, Office of Science.