Since the discovery of graphene in 2004, the scientific community has been waiting to prove that, besides all other characteristics, it has the capability to be a superconductor. This material is a grid of two-dimensional carbon atoms arranged in a hexagonal structure, with extraordinary properties that could revolutionize the electronic industry, among others. Being a superconductor at room temperature would add to its unique properties. Until now, however, promoting the innate ability of graphene to act as a superconductor has been a failure. Superconductivity in graphene has been achieved only when the material is doped, as a typical semiconductor, or when placing it on a superconducting material.
This week, however, a team of researchers at the University of Cambridge in the UK have found a unique way to trigger superconductivity in its own right. This was achieved by coupling it with a material called praseodymium cerium copper oxide (PCCO). 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?”
In the past, metallic-based superconductors have been used to trigger superconductivity, but with dubious 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.”
This research has not only proved the possibility of converting graphene into a superconductor, but it is also proof of the existence of a very unique and mysterious type of superconductivity called p-wave superconductivity that has evaded researchers for over 20 years.
PCCO is a copper oxide from an excellent class of superconductors called “cupprates.” Once the PCCO was coupled with graphene, the researchers, using scanning and tunneling microscopy, were able to distinguish the superconductivity coming from PCCO from the one produced by graphene. This was an important distinction. In a superconductor, electrons tend to form pairs and the spin alignment of the pair may be different depending on the superconductivity. In PCCO, that pairs’ spin state is a type known as a “d-wave state.”
However, when graphene was coupled with PCCO in the Cambridge experiment, the researcher found that the spin state in graphene was indeed 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.”
“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.”
Going further, we can imagine this superconducting graphene can be used to make transistor devices in a superconducting circuit that easily can be incorporated into molecular electronics; a big step in electronic devices. “In principle, given the variety of chemical molecules that can bind to graphene's surface, this research can result in the development of molecular electronic devices with novel functionalities based on superconducting graphene,” Di Bernardo added.
The research was published in Nature Communications on January 19, 2017.