Princeton team gains better understanding of superconductivity in 'magic angle' graphene

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.

Read the full story Posted: Oct 21,2021

Researchers discover a correlated electron-hole state in double-bilayer graphene

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.

An insulator made of two conductors imageImage 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.

Read the full story Posted: Sep 10,2021

'Magic angle' trilayer graphene found to act as rare "spin-triplet" superconductor

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).

Read the full story Posted: Jul 22,2021

Researchers turn 'magic angle graphene' into insulator or superconductor by applying an electric voltage

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.

A material-keyboard made of graphene imageThe 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.

Read the full story Posted: May 05,2021

Researchers take a step towards achieving topological qubits in graphene

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 imageSchematic 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.

Read the full story Posted: Apr 29,2021

Researchers produce extremely conductive graphene-enhanced hydrogel for medical applications

An interdisciplinary research team of the Research Training Group (RTG) 2154 "Materials for Brain" at Kiel University (CAU) has developed a method to produce graphene-enhanced hydrogels with an excellent level of electrical conductivity. What makes this method special is that the mechanical properties of the hydrogels are largely retained. The material is said to have potential for medical functional implants, for example, and other medical applications.

"Graphene has outstanding electrical and mechanical properties and is also very light," says Dr. Fabian Schütt, junior group leader in the Research Training Group, thus emphasizing the advantages of the ultra-thin material, which consists of only one layer of carbon atoms. What makes this new method different is the amount of graphene used. "We are using significantly less graphene than previous studies, and as a result, the key properties of the hydrogel are retained," says Schütt about the current study, which he initiated.

Read the full story Posted: Mar 21,2021

Researchers find surprising electron interaction in ‘magic-angle’ graphene

A research team, led by Brown University physicists, has found a new way to precisely probe the nature of the superconducting state in magic-angle graphene. The technique enables researchers to manipulate the repulsive force - the Coulomb interaction - in the system. In their recent study, the researchers showed that magic-angle superconductivity grows more robust when Coulomb interaction is reduced, which could be an important piece of information in understanding how this superconductor works.

"This is the first time anyone has demonstrated that you can directly manipulate the strength of Coulomb interaction in a strongly correlated electronic system," said Jia Li, an assistant professor of physics at Brown and corresponding author of the research. "Superconductivity is driven by the interactions between electrons, so when we can manipulate that interaction, it tells us something really important about that system. In this case, demonstrating that weaker Coulomb interaction strengthens superconductivity provides an important new theoretical constraint on this system."

Read the full story Posted: Mar 19,2021

Researchers create tunable superconductivity in magic-angle twisted trilayer graphene

When two sheets of graphene are stacked atop each other at just the right angle, the layered structure morphs into an unconventional superconductor, allowing electric currents to pass through without resistance or wasted energy. This magic-angle transformation in bilayer graphene was observed for the first time in 2018 in the group of Pablo Jarillo-Herrero at MIT. Since then, scientists have searched for other materials that can be similarly twisted into superconductivity, but for the most part, no other twisted material has exhibited superconductivity other than the original twisted bilayer graphene.

Stacking order imageIllustrations of A-tw-A stacking (a) and A-tw-B stacking (b). Image from Nature

In a recent paper, Jarillo-Herrero and his group reported observing superconductivity in a sandwich of three graphene sheets, the middle layer of which is twisted at a new angle with respect to the outer layers. This new trilayer configuration reportedly exhibits superconductivity that is more robust than its bilayer counterpart.

Read the full story Posted: Feb 02,2021

Researchers track the path of calcium atoms added to graphene

Researchers from Australia's Monash University, U.S Naval Research Laboratory, University of Maryland and IMDEA Nanociencia in Spain have confirmed what actually happens to calcium atoms that are added to graphene in order to create a superconductor: surprisingly, the calcium goes underneath both the upper graphene sheet and a lower ‘buffer’ sheet, ‘floating’ the graphene on a bed of calcium atoms.

Injecting calcium into graphene creates a superconductor, but where does the calcium actually end up image

Superconducting calcium-injected graphene holds great promise for energy-efficient electronics and transparent electronics.

Read the full story Posted: Sep 17,2020

New work shows that superconductivity in twisted bilayer graphene can exist away from the magic angle

New study by Caltech shows that superconductivity in twisted bilayer graphene can exist away from the magic angle when coupled to a two-dimensional semiconductor

In 2018, researchers made the surprising discovery that when you layer two sheets of single-atom-thick graphene atop one another and rotate them by precisely 1.05 degrees with respect to one another, the resulting bilayer material takes on new properties: when the density of electrons in the material is increased through the application of a voltage on a nearby electrode, it becomes a superconductor—electrons can flow freely through the material, without resistance. However, with a slight change in electron density, the bilayer becomes an insulator and prevents the flow of electrons.

Read the full story Posted: Jul 17,2020