AGORACOM Small-cap Investor Relations Blog

SPONSOR: ZEN Graphene Solutions: An emerging advanced materials and graphene development company with a focus on new solutions using pure graphene and other two-dimensional materials. Our competitive advantage relies on the unique qualities of our multi-decade supply of precursor materials in the Albany Graphite Deposit. Independent labs in Japan, UK, Israel, USA and Canada confirm this. Click here for more information

The first demonstration of graphene double quantum dots in which it is possible to control the number of electrons down to zero has been reported in Nano Letters. Far from an abstract academic stunt, the results could prove key to future implementations of quantum computing based on graphene. “Having exact information and control over the number of electrons in the dots is essential for spin based quantum information technology,” says Luca Banszerus, a researcher at RWTH Aachen University in Germany and the first author of the paper reporting these results.

Although this level of control has been demonstrated in single quantum dots, this is the first demonstration in graphene double quantum dots, which are particularly useful as spin qubits. “Using a double dot heavily facilitates the readout of the electron’s spin state and the implementation of quantum gates,” Banszerus adds.

Less edgy quantum dots 

The idea of using graphene in quantum dots dates back almost as far as the first reports of the material’s isolation in 2004. Graphene has almost no spin-orbit interaction and very little hyperfine coupling, which would suggest that spin lifetimes can be extremely high. Unfortunately, quantum dots physically etched from larger graphene flakes run into problems due to the disorder at the dot’s edges disrupting the material’s behavior. As a result, the transport behavior of these quantum dots is dominated by localized states at the edges. “This leads to an unknown effective quantum dot size and an occupation of typically many electrons,” says Banszerus.

Instead, Banszerus and colleagues at RWTH Aachen and the National Institute of Materials Science in Japan work with bilayer graphene, which can be tuned to be a semiconductor. A voltage applied to specific regions of a bilayer graphene flake can switch those regions to behave as insulators, electrostatically defining a quantum dot that has no edge states nearby.

The Aachen researchers strip single flakes of bilayer graphene from graphite (mechanical exfoliation) and handle it using a dry pick-up technique that hinges on van der Waals interactions. They encapsulate the bilayer graphene in hexagonal boron nitride (hBN) crystal. They then place the structure on a graphite flake, which acts as the bottom electrode, and add chromium and gold split gates and finger gates separated from the split gates by a 30-nm-thick layer of atomic layer deposited Al2O3.

They were able to control the number of electrons on the quantum dots by applying a voltage, which also affected the tunneling coupling between the dots. As a result, once the total occupation of the two quantum dots exceeds eight electrons, they begin to behave as one single quantum dot, rather than a double quantum dot. Transport measurements also revealed that the number of electrons loaded on the quantum dot could be controlled down to zero electrons.

The idea of defining quantum dots in bilayer graphene electrostatically in this way is not new. However, although different groups have attempted this approach since 2010, the process required recently discovered tricks of the trade, such as better encapsulation in hBN and the use of graphite flakes as gates to get a clean band gap. Banszerus says these developments came as quite a surprise and revived interest in graphene quantum dots in 2018. He hopes the capabilities they have now demonstrated will further spark activity in this field.

Coupling control

“Even though being able to control the number of charges in a graphene double dot is a huge step forward, there are still many problems to be solved on the road toward spin-based quantum information technology in graphene,” says Banszerus. Next, he hopes to tackle the problem of controlling the coupling between the quantum dots and the reservoir, which he hopes to achieve by adding an additional layer of interdigitated finger gates on top.

Source: https://phys.org/news/2020-03-bilayer-graphene-quantum-dots-tune.html

SPONSOR: ZEN Graphene Solutions: An emerging advanced materials and graphene development company with a focus on new solutions using pure graphene and other two-dimensional materials. Our competitive advantage relies on the unique qualities of our multi-decade supply of precursor materials in the Albany Graphite Deposit. Independent labs in Japan, UK, Israel, USA and Canada confirm this. Click here for more information

• Berkeley Lab scientists tap into graphene’s hidden talent as an electrically tunable superconductor, insulator, and magnetic device for the advancement of quantum information science

Ever since graphene’s discovery in 2004, scientists have looked for ways to put this talented, atomically thin 2D material to work. Thinner than a single strand of DNA yet 200 times stronger than steel, graphene is an excellent conductor of electricity and heat, and it can conform to any number of shapes, from an ultrathin 2D sheet, to an electronic circuit.

Last year, a team of researchers led by Feng Wang, a faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor of physics at UC Berkeley, developed a multitasking graphene device that switches from a superconductor that efficiently conducts electricity, to an insulator that resists the flow of electric current, and back again to a superconductor.

Now, as reported in Nature today, the researchers have tapped into their graphene system’s talent for juggling not just two properties, but three: superconducting, insulating, and a type of magnetism called ferromagnetism. The multitasking device could make possible new physics experiments, such as research in the pursuit of an electric circuit for faster, next-generation electronics like quantum computing technologies.

Optical image of a trilayer graphene material sandwiched between boron nitride layers during the nanofabrication process (left); and the trilayer graphene/boron nitride device with gold electrodes (right). (Credit: Guorui Chen/Berkeley Lab)

“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,” said Guorui Chen, a postdoctoral researcher in Wang’s Ultrafast Nano-Optics Group at UC Berkeley, and the study’s lead author.

Using electricity to turn on graphene’s hidden potential

Graphene has a lot of potential in the world of electronics. Its atomically thin structure, combined with its robust electronic and thermal conductivity, “could offer a unique advantage in the development of next-generation electronics and memory storage devices,” said Chen, who also worked as a postdoctoral researcher in Berkeley Lab’s Materials Sciences Division at the time of the study.

The problem is that the magnetic materials used in electronics today are made of ferromagnetic metals, such as iron or cobalt alloys. Ferromagnetic materials, like the common bar magnet, have a north and a south pole. When ferromagnetic materials are used to store data on a computer’s hard disk, these poles point either up or down, representing zeros and ones – called bits.

Graphene, however, is not made of a magnetic metal – it’s made of carbon.

So the scientists came up with a creative workaround.

Illustration of the trilayer graphene/boron nitride moiré superlattice with electronic and ferromagnetic properties. (Credit: Guorui Chen/Berkeley Lab)

They engineered an ultrathin device, just 1 nanometer in thickness, featuring three layers of atomically thin graphene. When sandwiched between 2D layers of boron nitride, the graphene layers – described as trilayer graphene in the study – form a repeating pattern called a moiré superlattice.

By applying electrical voltages through the graphene device’s gates, the force from the electricity prodded electrons in the device to circle in the same direction, like tiny cars racing around a track. This generated a forceful momentum that transformed the graphene device into a ferromagnetic system.

More measurements revealed an astonishing new set of properties: The graphene system’s interior had not only become magnetic but also insulating; and despite the magnetism, its outer edges morphed into channels of electronic current that move without resistance. Such properties characterize a rare class of insulators known as Chern insulators, the researchers said.

Even more surprising, calculations by co-author Ya-Hui Zhang of the Massachusetts Institute of Technology revealed that the graphene device has not just one, but two conductive edges, making it the first observed “high-order Chern insulator,” a consequence of the strong electron-electron interactions in the trilayer graphene.

Scientists have been in hot pursuit of Chern insulators in a field of research known as topology, which investigates exotic states of matter. Chern insulators offer potential new ways to manipulate information in a quantum computer, where data is stored in quantum bits, or qubits. A qubit can represent a one, a zero, or a state in which it is both a one and a zero at the same time.

“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,” Chen said. “It’s exciting that we can now explore new physics in a tiny device just 1 millionth of a millimeter thick.”

The researchers hope to conduct more experiments with their graphene device to have a better understanding of how the Chern insulator/magnet emerged, and the mechanics behind its unusual properties.

Researchers from Berkeley Lab; UC Berkeley; Stanford University; SLAC National Accelerator Laboratory; Massachusetts Institute of Technology; China’s Shanghai Jiao Tong University, Collaborative Innovation Center of Advanced Microstructures, and Fudan University; and Japan’s National Institute for Materials Science participated in the work.

This work was supported by the Center for Novel Pathways to Quantum Coherence in Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.

Founded in 1931 on the belief that the biggest scientific challenges are best addressed by teams, Lawrence Berkeley National Laboratory and its scientists have been recognized with 13 Nobel Prizes. Today, Berkeley Lab researchers develop sustainable energy and environmental solutions, create useful new materials, advance the frontiers of computing, and probe the mysteries of life, matter, and the universe. Scientists from around the world rely on the Lab’s facilities for their own discovery science. Berkeley Lab is a multiprogram national laboratory, managed by the University of California for the U.S. Department of Energy’s Office of Science.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.

SOURCE: https://newscenter.lbl.gov/2020/03/04/2d-material-gets-a-new-gig/