Since its discovery in 2004, scientists have believed that graphene may have the innate ability to superconduct. Now Cambridge researchers have found a way to activate that previously dormant potential.
Researchers have found a way to trigger the innate, but previously hidden, ability of graphene to act as a superconductor – meaning that it can be made to carry an electrical current with zero resistance.
The finding, reported in Nature Communications, further enhances the potential of graphene, which is already widely seen as a material that could revolutionise industries such as healthcare and electronics. Graphene is a two-dimensional sheet of carbon atoms and combines several remarkable properties; for example, it is very strong, but also light and flexible, and highly conductive.
Since its discovery in 2004, scientists have speculated that graphene may also have the capacity to be a superconductor. Until now, superconductivity in graphene has only been achieved by doping it with, or by placing it on, a superconducting material – a process which can compromise some of its other properties.
But in the new study, researchers at the University of Cambridge managed to activate the dormant potential for graphene to superconduct in its own right. This was achieved by coupling it with a material called praseodymium cerium copper oxide (PCCO).
Superconductors are already used in numerous applications. Because they generate large magnetic fields they are an essential component in MRI scanners and levitating trains. They could also be used to make energy-efficient power lines and devices capable of storing energy for millions of years.
Superconducting graphene opens up yet more possibilities. The researchers suggest, for example, that graphene could now be used to create new types of superconducting quantum devices for high-speed computing. Intriguingly, it might also be used to prove the existence of a mysterious form of superconductivity known as “p-wave” superconductivity, which academics have been struggling to verify for more than 20 years.
The research was led by Dr Angelo Di Bernardo and Dr Jason Robinson, Fellows at St John’s College, University of Cambridge, alongside collaborators Professor Andrea Ferrari, from the Cambridge Graphene Centre; Professor Oded Millo, from the Hebrew University of Jerusalem, and Professor Jacob Linder, at the Norwegian University of Science and Technology in Trondheim.
“It has long been postulated that, under the right conditions, graphene should undergo a superconducting transition, but can’t,” Robinson said. “The idea of this experiment was, if we couple graphene to a superconductor, can we switch that intrinsic superconductivity on? The question then becomes how do you know that the superconductivity you are seeing is coming from within the graphene itself, and not the underlying superconductor?”
Similar approaches have been taken in previous studies using metallic-based superconductors, but with limited success. “Placing graphene on a metal can dramatically alter the properties so it is technically no longer behaving as we would expect,” Di Bernardo said. “What you see is not graphene’s intrinsic superconductivity, but simply that of the underlying superconductor being passed on.”
PCCO is an oxide from a wider class of superconducting materials called “cuprates”. It also has well-understood electronic properties, and using a technique called scanning and tunnelling microscopy, the researchers were able to distinguish the superconductivity in PCCO from the superconductivity observed in graphene.
Superconductivity is characterised by the way the electrons interact: within a superconductor electrons form pairs, and the spin alignment between the electrons of a pair may be different depending on the type – or “symmetry” – of superconductivity involved. In PCCO, for example, the pairs’ spin state is misaligned (antiparallel), in what is known as a “d-wave state”.
By contrast, when graphene was coupled to superconducting PCCO in the Cambridge-led experiment, the results suggested that the electron pairs within graphene were in a p-wave state. “What we saw in the graphene was, in other words, a very different type of superconductivity than in PCCO,” Robinson said. “This was a really important step because it meant that we knew the superconductivity was not coming from outside it and that the PCCO was therefore only required to unleash the intrinsic superconductivity of graphene.”
It remains unclear what type of superconductivity the team activated, but their results strongly indicate that it is the elusive “p-wave” form. If so, the study could transform the ongoing debate about whether this mysterious type of superconductivity exists, and – if so – what exactly it is.
In 1994, researchers in Japan fabricated a triplet superconductor that may have a p-wave symmetry using a material called strontium ruthenate (SRO). The p-wave symmetry of SRO has never been fully verified, partly hindered by the fact that SRO is a bulky crystal, which makes it challenging to fabricate into the type of devices necessary to test theoretical predictions.
“If p-wave superconductivity is indeed being created in graphene, graphene could be used as a scaffold for the creation and exploration of a whole new spectrum of superconducting devices for fundamental and applied research areas,” Robinson said. “Such experiments would necessarily lead to new science through a better understanding of p-wave superconductivity, and how it behaves in different devices and settings.”
The study also has further implications. For example, it suggests that graphene could be used to make a transistor-like device in a superconducting circuit, and that its superconductivity could be incorporated into molecular electronics. “In principle, given the variety of chemical molecules that can bind to graphene’s surface, this research can result in the development of molecular electronics devices with novel functionalities based on superconducting graphene,” Di Bernardo added.
Learn more: Graphene’s sleeping superconductivity awakens
Researchers at the University of Valencia show that the superconducting state can be maintained even when the material in question is reduced from three to two dimensions, making the efficiency gains needed for technologies like those underlying the frictionless train possible.
An international research team led by Eugenio Coronado, of the Univeristy of Valencia’s Institute of Molecular Science (ICMol) has shown that it is possible to maintain superconductivity at the two-dimensional limit, currently one of the most hotly debated issues in solid state physics. This finding allows us to advance our understanding of superconductivity and paves the way for the miniaturisation of ultrasensitive magnetic field detectors. The work was published in Nature Communications.
Superconductivity is one of the most fascinating quantum phenomena in physics. In the superconducting state, materials conduct electricity without energy loss, which makes them very efficient for many applications including the manufacture of the strongest known magnets, ultrasensitive magnetic field detectors, efficient energy conduction and frictionless transportation (levitating trains).
Since its discovery in 1911, one of the issues that has most intrigued scientists is whether it is possible to maintain the superconducting state even when the material is reduced from three to two dimensions. Intuitively we expect that it would be more difficult to stabilise the superconducting state when the dimensionality is reduced. With the isolation of graphene, the first two-dimensional material, made up of a single layer of carbon atoms, the issue been pushed resolutely to the fore. However, despite graphene’s extraordinary mechanical, electrical and magnetic properties, superconductivity has so far remained an elusive property.
Based at the UV’s Science Park, ICMol researchers have shown that superconductivity can indeed be maintained at this two-dimensional limit. The researchers have studied layered materials similar to graphene, but which become superconductors when cooled to low temperatures. Specifically, they have studied the electrical properties of a large family of layered materals known as metal dichalcogenides.
A research team led by the U.S. Department of Energy’s (DOE’s) Argonne National Laboratory has discovered that only half the atoms in some iron-based superconductors are magnetic, providing a conclusive demonstration of the wave-like properties of metallic magnetism in these materials.
The discovery allows for a clearer understanding of the magnetism in some compounds of iron, the iron arsenides, and how it helps induce superconductivity, the resistance-free flow of electrical current through a solid-state material, which occurs at temperatures up to 138 degrees Kelvin, or minus -135 degrees Celsius.
“In order to be able to design novel superconducting materials, one must understand what causes superconductivity,” said Argonne senior physicist Raymond Osborn, one of the project’s lead researchers. “Understanding the origin of magnetism is a first vital step toward obtaining an understanding of what makes these materials superconducting. Given the similarity to other materials, such as the copper-based superconductors, our goal was to improve our understanding of high-temperature superconductivity.”
Understanding the origin of magnetism is a first vital step toward obtaining an understanding of what makes these materials superconducting.
From an applied perspective, such an understanding would allow for the development of magnetic energy-storage systems, fast-charging batteries for electric cars and a highly efficient electrical grid, said Argonne senior physicist Stephan Rosenkranz, the project’s other lead researcher.
Superconductors reduce power loss. The use of high-temperature superconducting materials in the electrical grid, for example, would significantly reduce the large amount of electricity that is lost as it travels though the grid, enabling the grid to operate more efficiently.
The researchers were able to show that the magnetism in these materials was produced by mobile electrons that are not bound to a particular iron atom, producing waves of magnetization throughout the sample. They discovered that, in some iron arsenides, two waves interfere to cancel out, producing zero magnetization in some atoms. This quantum interference, which has never been seen before, was revealed by Mössbauer spectroscopy, which is extremely sensitive to the magnetism on each iron site.
Superconductivity could have implications for creating technologies like ultra-efficient power grids and magnetically levitating vehicles
Physicists at the have led an international team that has come closer to understanding the mystery of how superconductivity, an exotic state that allows electricity to be conducted with practically zero resistance, occurs in certain materials.
Physicists all over the world are on a quest to understand the secrets of superconductivity because of the exciting technological possibilities that could be realized if they could make it happen at closer to room temperatures. In conventional superconductivity, materials that are cooled to nearly absolute zero ( ?273.15 Celsius) exhibit the fantastic property of electrons pairing up and being able to conduct electricity with practically zero resistance. If superconductivity worked at higher temperatures, it could have implications for creating technologies such as ultra-efficient power grids, supercomputers and magnetically levitating vehicles.
The new findings from an international collaboration, led by Waterloo physicists David Hawthorn, Canada Research Chair Michel Gingras, doctoral student Andrew Achkar and post-doctoral student Zhihao Hao,present direct experimental evidence of what is known as electronic nematicity – when electron clouds snap into an aligned and directional order – in a particular type of high-temperature superconductor. The results, published in the prestigious journal Science, may eventually lead to a theory explaining why superconductivity occurs at higher temperatures in certain materials.