Conductors

Researchers at MIT and National Institute for Materials Science in Tsukuba, Japan, have found a new and intriguing property of “magic-angle” graphene: superconductivity that can be turned on and off with an electric pulse, much like a light switch.

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The discovery could lead to ultrafast, energy-efficient superconducting transistors for neuromorphic devices — electronics designed to operate in a way similar to the rapid on/off firing of neurons in the human brain.

Skoltech researchers have patented a method that enables producing arbitrarily shaped functional graphene components on a transparent substrate with 100-nanometer resolution, which could be especially suited for flexible and transparent electronics. The new approach reportedly helps avoid defects that arise during graphene transfer between substrates and strongly affect the material’s quality.

“Flexible and transparent electronics is typically associated with wearable biosensors that monitor vital signs, such as heart rate, breathing, and blood oxygenation, and relay them to a smartphone or fitness band,” Skoltech PhD student Aleksei Shiverskii, one of the inventors, said. “An affordable and efficient technology that at first may seem impractical soon becomes a ubiquitous and indispensable appliance, like a bluetooth electric kettle or a wifi vacuum cleaner. I believe that someday flexible and transparent electronics will become a fixture, too.”

Researchers from Columbia University, Harvard University, Japan's National Institute for Materials Science and Austria's University of Innsbruck have studied the structural and electronic properties of twisted trilayer graphene using low-temperature scanning tunneling microscopy at twist angles for which superconductivity has been observed.

The discovery of superconductivity in two layers of graphene arranges in the "magic angle" of 1.1 degrees has become quite famous. With just two atom-thin sheets of carbon, researchers discovered a simple device to study the resistance-free flow of electricity, among other phenomena related to the movement of electrons through a material. Adding a third layer of graphene improves the odds of finding superconductivity, but the reason was unclear. Now, the researchers of the new study reveal new details about the physical structure of trilayer graphene that help explain why three layers are better than two for studying superconductivity.

Researchers from Science and Technology (IST) Austria, in collaboration with scientists from the Weizmann Institute of Science in Israel, have developed a theoretical framework of unconventional superconductivity, which addresses the questions raised by earlier work that detected unique superconductivity in 'magic angle' trilayer graphene.

Superconductivity relies on the pairing of free electrons in the material despite their repulsion arising from their equal negative charges. This pairing happens between electrons of opposite spin through vibrations of the crystal lattice. Spin is a quantum property of particles comparable, but not identical to rotation. The mentioned kind of pairing is the case at least in conventional superconductors. "Applied to trilayer graphene," co-lead-author from IST, Areg Ghazaryan, points out, "we identified two puzzles that seem difficult to reconcile with conventional superconductivity."

Researchers from Columbia University and collaborators from Korea's Sungkyunkwan University and Japan's National Institute for Materials Science have reported that graphene can be efficiently doped using a monolayer of tungsten oxyselenide (TOS) that is created by oxidizing a monolayer of tungsten diselenide.

The new results relied on a cleaner technique to manipulate the flow of electricity, giving graphene greater conductivity than metals such as copper and gold, and raising its potential for use in telecommunications systems and quantum computers.

Princeton researchers have dissipated some of the mystery around 'magic angle' graphene's superconductivity by showing an uncanny resemblance between it and the superconductivity of high temperature superconductors. Magic graphene may hold the key to unlocking new mechanisms of superconductivity, including high temperature superconductivity.

Ali Yazdani, Professor of Physics and Director of the Center for Complex Materials at Princeton University, led the research. He and his team have studied many different types of superconductors over the years and have recently turned their attention to magic bilayer graphene. Some have argued that magic bilayer graphene is actually an ordinary superconductor disguised in an extraordinary material, said Yazdani, but when we examined it microscopically it has many of the characteristics of high temperature cuprate superconductors. It is a déjà vu moment.

A team of researchers, led by Klaus Ensslin and Thomas Ihn at the Laboratory for Solid State Physics at ETH Zurich, together with colleagues at the University of Texas in Austin (USA), has observed a novel state in twisted bi-layer graphene. In that state, negatively charged electrons and positively charged (so-called) holes, which are missing electrons in the material, are correlated so strongly with each other that the material no longer conducts electric current.

Image by Peter Rickhaus / ETH Zurich (taken from Nanowerk)

In conventional experiments, in which graphene layers are twisted by about one degree with respect to each other, the mobility of the electrons is influenced by quantum mechanical tunneling between the layers, explains Peter Rickhaus, a post-doc and lead author of the study. In our new experiment, by contrast, we twist two double layers of graphene by more than two degrees relative to each other, so that electrons can essentially no longer tunnel between the double layers.

Researchers at MIT and Harvard University have previously found that graphene can have exotic properties when situated at a 'magic angle'. Now, a new study by some of the members of the same team shows that this material could also be a "spin-triplet" superconductor one that isn't affected by high magnetic fields which potentially makes it even more useful.

"The value of this experiment is what it teaches us about fundamental superconductivity, about how materials can behave, so that with those lessons learned, we can try to design principles for other materials which would be easier to manufacture, that could perhaps give you better superconductivity," says physicist Pablo Jarillo-Herrero, from the Massachusetts Institute of Technology (MIT).

Researchers at ETH Zurich, led by Klaus Ensslin and Thomas Ihn at the Laboratory for Solid State Physics, have succeeded in turning specially prepared graphene flakes either into insulators or into superconductors by applying an electric voltage. This technique is even said to work locally, meaning that in the same graphene flake regions with completely different physical properties can be realized side by side.

The material keyboard realized by the ETH Zurich researchers. Image by ETH Zurich/F. de Vries

The material Ensslin and his co--workers used is known as Magic Angle Twisted Bilayer Graphene. The starting point for the material is graphene flakes - the researchers put two of those layers on top of each other in such a way that their crystal axes are not parallel, but rather make a magic angle of exactly 1.06 degrees. That’s pretty tricky, and we also need to accurately control the temperature of the flakes during production. As a result, it often goes wrong, explains Peter Rickhaus, who was involved in the experiments.

Researchers from Spain, Finland and France have demonstrated that magnetism and superconductivity can coexist in graphene, opening a path towards graphene-based topological qubits.

Schematic illustration of the interplay of magnetism and superconductivity in a graphene grain boundary, a potential building block for carbon-based topological qubits Credit: Jose Lado/Aalto University

In the quantum realm, electrons can behave in interesting ways. Magnetism is one of these behaviors that can be seen in everyday life, as is the rarer phenomena of superconductivity. Intriguingly, these two behaviors are often antagonists - the existence of one of them often destroys the other. However, if these two opposite quantum states are forced to coexist artificially, an elusive state called a topological superconductor appears, which is useful for researchers trying to make topological qubits.