Already renowned for its potential to revolutionize everything from light bulbs and dental fillings through to semiconductors and motorcycle helmets, graphene can now add innate superconductivity to its repertoire. Scientists at the University of Cambridge claim to have discovered a method to trigger the superconducting properties of graphene without actually altering its chemical structure.
Light, flexible, and super-strong, the single layer of carbon atoms that makes up graphene has only been rendered superconductive previously by doping it with impurities, or by affixing it to other superconducting materials, both of which may undermine some of its other unique properties.
However, in the the latest research conducted at the University of Cambridge, scientists claim to have found a way to activate superconduction in graphene by coupling it with a material known as praseodymium cerium copper oxide (Pr2−xCexCuO4) or PCCO. PCCO is from a wider class of superconducting materials known as cuprates (derived from the Latin word for copper), known for their use in high-temperature superconductivity.
"It has long been postulated that, under the right conditions, graphene should undergo a superconducting transition, but can't," said Dr Jason Robinson, one of the leaders of the study from the University of Cambridge. "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?"
Using PCCO, however, which has properties well known in its long-term use in superconduction research, and by using both scanning and tunnelling microscopes to observe the effects, the scientists were able to differentiate the superconductivity generated in the PCCO from the superconductivity seen in the graphene sample.
Superconductivity generates superconductor electrons that form into pairs, and the spin alignment of the electron pairs is dependent upon the type of superconductivity (and therefore the material) involved. PCCO has pairs of electrons with a spin state that is antiparallel – known as a "d-wave state."
The superconductivity measured in the graphene, however, was different to the d-state wave and so must have been a different type, thereby showing that the graphene was generating its own superconductivity.
"What we saw in the graphene was, in other words, a very different type of superconductivity than in PCCO," said Robinson. "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."
Even more tantalizing than the fact that the researchers had managed to initiate the innate superconductivity of graphene, however, was the type of wave generated using this new method. What they seemed to have produced may be the elusive "p-wave" – where electrons exhibit a spin-triplet pairing excited to a higher energy state by the absorption of radiation. This is something that physicists have been trying to prove exists for more than 20 years.
At the moment, however, it is unclear exactly what type of superconductivity occured in the graphene, but it is certain that it did generate its own form of the phenomenon. Whether it was the elusive p-wave form remains to be verified by further experimentation.
"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," said Robinson. "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."
By being able to consistently trigger the innate superconducting properties of graphene at will, the researchers believe that it may be possible to produce transistor-like devices in superconducting circuits, molecular electronics, and possibly new types of superconducting components for high-speed quantum computing.
The results of this research were recently published in the journal Nature Communications.
Source: University of Cambridge
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