Graphene is a honeycomb-like lattice made of a one-atom-thick layer of carbon atoms. It is the thinnest, lightest, and strongest known material and offers extremely high electrical and thermal conductivity. Recently, researchers are trying to add superconductivity to its unique set of properties.
How Does Superconductivity Happen?
A superconductor achieves zero electrical resistance below a certain temperature which may be as low as -269 degrees Celsius. Such superconductors, called low-temperature superconductors [PDF], were discovered nearly 100 years ago. On the other hand, high-temperature superconductors, which has a transition temperature of about -135 degree Celsius, were not discovered until about 30 years ago.
A low-temperature superconductor using liquid nitrogen. Photo courtesy of Camilla Hoel [CC BY-SA 2.0]
In a metal, electrons move on their own, repelling and colliding with each other. However, in a superconductor, they travel in pairs and move more smoothly. Suchitra Sebastian, an applied physicist at the University of Cambridge, envisions that in a superconductor electrons travel in lanes. Today, scientists have a deeper understanding of low-temperature superconductors. They know that the crystal structure of these materials forces the electrons to travel in pairs.
Applications of Superconductors
Magnetic levitation is one of the most well-known applications of superconducting where the strong superconducting magnets are used to make a vehicle, such as a train, float in the air and travel at extremely high speeds. In April 2015, the MLX01 test vehicle achieved an incredible speed of 603 kph.
The medical applications of superconductors are magnetic resonance imaging (MRI) and the SQUIDs. The latter can be used to examine certain depths of the body without applying a strong magnetic field like that of MRI. Another interesting application of this technology is superconductor-based electric generators which are estimated to have a worldwide market of $20-30 billion in the next decade.
Petaflop computers, ultra-high-performance filters, very low-frequency antennas, and E-bombs are just a few of other applications of this technology which would be impossible otherwise. Superconductivity was observed in graphite years ago and, even before experimental verifications, scientists believed that incorporating the right additives must lead to superconductivity in graphene.
Superconductivity of Lithium-Coated Graphene
Less than two years ago, researchers incorporated lithium atoms to make the world’s first graphene superconductor. The international research team created graphene sheets and coated them with lithium atoms.
Andrea Damascelli, director of the University of British Columbia's Quantum Matter Institute in Vancouver who was involved in this research, noted that the way samples are prepared is a key factor. Before this, several other groups had been trying to create superconducting lithium-coated graphene; however, they always faced sources of instability which made success elusive.
Damascelli and his colleagues experimented in ultra-high-vacuum conditions at about minus 268 degrees Celsius.
Calcium Atoms Sandwiched with Graphene Sheets
Nearly one year ago, researchers from Tohoku University and the University of Tokyo placed calcium atoms between sheets of graphene which were grown on a silicon carbide crystal. They achieved superconductivity at -269 degrees Celsius.
A representation of the developed material. Image courtesy of Tohoku University.
Obviously, these super cold temperatures are not suitable for applications such as superconductor-based power lines. However, according to Tohoku University, these studies pave the way for ultra high-speed superconducting nanodevices which can be used in quantum computing.
PCCO Unleashes the Superconductivity of Graphene
While the above experiments relied on doping graphene to achieve a superconductor, researchers from the University of Cambridge have recently developed a graphene-based superconductor without altering the material.
Jason Robinson, involved in the project, notes that, apparently, the study has achieved a rare type of superconductivity called p-wave state. However, he adds that further experiments are required to confirm this.
According to Angelo di Bernardo, methods which place graphene on other materials change its properties. On the other hand, although they achieve superconductivity, it is not necessarily from graphene but simply that of the underlying superconductor being passed on.
The Cambridge team incorporates a material called praseodymium cerium copper oxide (PCCO) to awaken graphene’s dormant superconductivity. While the experiment may look like previous ones where a second material was required to achieve superconductivity, the new method is quite different from previous techniques. In this recent experiment, the achieved superconductivity is clearly distinguished from that of the added material, i.e. PCCO. In a PCCO, electron pairs are in a state called d-wave; however, the spin state of electron pairs in the new superconductor was observed to be p-wave which is a rare and still unverified type of superconductivity first proposed by Japanese researchers in 1994.
According to Robinson, the superconductivity was not from PCCO and PCCO was simply required to unleash the intrinsic superconductivity of graphene.
The experiment is a big deal because it can prove that the elusive p-wave superconductivity really exists and, consequently, give the researchers the chance to properly investigate this type of superconductivity. With a better understanding of p-wave superconductivity, researchers may find a whole new spectrum of superconductors.
Uses for Superconducting Graphene
Superconducting graphene may not be a good choice to develop more efficient power lines, but researchers believe that it suits applications such as SQUIDs (superconducting quantum interference devices). SQUIDs, which are capable of sensing a change in a magnetic field over a billion times weaker than the force that moves the needle on a compass, can scan brain activities with a great precision.
Damascelli believes that graphene-based superconductors could lead to a 100-fold increase in the sensitivities which are currently achievable.
Unfortunately, there are many mysterious unknowns about how superconductivity is achieved in general, especially in graphene-based materials. However, all these endeavors seem to be quite rewarding and many research groups are intrigued to discover this territory.
The details of this research are published in Nature Communications.