Already renowned for its potential to revolutionize everything from light bulbs anddental 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.
Scientists Observed Conventional Superconductivity at Minus 70 Degrees Celsius
Up until now, no material has been able to conduct current with no resistance at such high temperatures: Researchers at the Max Planck Institute for Chemistry in Mainz and the Johannes Gutenberg University Mainz observed that hydrogen sulfide becomes superconductive at minus 70 degree Celsius—when the substance is placed under a pressure of 1.5 million bar. This corresponds to half of the pressure of the earth’s core. With their high pressure experiments the researchers in Mainz have thus not only set a new record for superconductivity—their findings have also highlighted a potential new way to transport current at room temperature with no loss.
For many solid-state physicists, superconductors that are suitable for use at room temperature are still a dream. Up until now, the only materials known to conduct current with no electrical resistance and thus no loss did so only at very low temperatures. Accordingly, special copper ceramics (cuprates) took the leading positions in terms of transition temperature—the temperature at which the material loses its resistance. The record for a ceramic of this type is roughly minus 140 degrees Celsius at normal air pressure and minus 109 degrees Celsius at high pressure. In the ceramics, a special, unconventional form of superconductivity occurs. For conventional superconductivity, temperatures of at least minus 234 degrees Celsius have so far been necessary.
A team led by Mikhael Eremets, head of a working group at the Max Planck Institute for Chemistry, working in collaboration with the researchers at Johannes Gutenberg University Mainz has now observed conventional superconductivity at minus 70 degrees Celsius, in hydrogen sulfide (H2S). To convert the substance, which is a gas under normal conditions, into a superconductor the scientists did however have to subject it to a pressure of 1.5 megabar (1.5 million bar), as they describe in the latest edition of the science magazine Nature.
The transition temperature of conventional superconductivity knows no limits
“With our experiments we have set a new record for the temperature at which a material becomes superconductive,” says Mikhael Eremets. His team has also been the first to prove in an experiment that there are conventional superconductors with a high transition temperature. Theoretical calculations had already predicted this for certain substances including H2S. “There is a lot of potential in looking for other materials in which conventional superconductivity occurs at high temperatures,” says the physicist. “There is theoretically no limit for the transition temperature of conventional superconductors, and our experiments give reason to hope that superconductivity can even occur at room temperature.”
The researchers generated the extremely high pressure required to make H2S superconductive at comparatively moderate negative temperatures in a special pressure chamber smaller than one cubic centimeter in size. The two diamond tips on the side, which act as anvils, are able to constantly increase the pressure that the sample is subjected to. The cell is equipped with contacts to measure the electrical resistance of the sample. In another high-pressure cell, the researchers were able to investigate the magnetic properties of a material that also change at the transition temperature.
After the researchers had filled the pressure chamber with liquid hydrogen sulfide, they increased the pressure acting on the sample gradually up to roughly two megabar and changing the temperature for each pressure level. They took measurements of both resistance and magnetization to determine the material’s transition temperature. The magnetization measurements provide very useful information, because a superconductor possesses ideal magnetic properties.
Hydrogen atoms facilitate superconductivity at high temperatures
The researchers believe that it is mainly hydrogen atoms that are responsible for hydrogen sulfide losing its electrical resistance under high pressure at relatively high temperatures: Hydrogen atoms oscillate in the lattice with the highest frequency of all elements, because hydrogen is the lightest. As the oscillations of the lattice determine the conventional superconductivity—and do this more effectively the faster the atoms oscillate—materials with high hydrogen content exhibit a relatively high transition temperature. In addition, strong bonds between the atoms increase the temperature at which a material becomes superconductive. These conditions are met in H3S, and it is precisely this compound that develops from H2S at high pressure.
Mikhael Eremets and his team are now looking for materials with even higher transition temperatures. Increasing the pressure acting on the hydrogen sulfide above 1.5 megabar is not helpful in this case. This has not only been calculated by theoretical physicists, but now also confirmed in experiments performed by the team in Mainz. At even higher temperatures the electron structure changes in such a way that the transition temperature slowly begins to drop again.
Wanted: hydrogen-rich materials with a higher transition temperature
“An obvious candidate for a high transition temperature is pure hydrogen,” says Mikhael Eremets. “It is expected that it would become superconductive at room temperature under high pressure.” His team has already begun experimenting with pure hydrogen, but the experiments are very difficult as pressures of three to four megabar are required.
“Our research into hydrogen sulfide has however shown that many hydrogen-rich materials can have a high transition temperature,” says Eremets. It may even be possible to realize a high-temperature superconductor worth the name in terms of common temperature perception without high pressure. The researchers in Mainz currently need the high pressure to convert materials that act electrically insulating like hydrogen sulfide into metals. “There may be polymers or other hydrogen-rich compounds that can be converted to metals in some other way and become superconductive at room temperature,” says the physicist. If such materials can be found, we would finally have them: superconductors that can be used for a wide range of technical applications.
Key compound for high-temperature superconductivity found
A research group in Japan found a new compound H5S2 that shows a new superconductivity phase on computer simulation. Further theoretical and experimental research based on H5S2 predicted by this group will lead to the clarification of the mechanism behind high-temperature superconductivity, which takes place in hydrogen sulfide .
Superconductivity is the total disappearance of electrical resistance when an object is cooled below a definite temperature. If superconductor is used for electric wire, it becomes possible to carry electricity without loss. That's why superconductivity has been drawing attention as an important physical phenomenon for solving environmental and energy problems.
However, the superconducting critical temperature, the temperature at which superconductivity takes place, is so low that its practical realization is difficult. Last year, a striking news came out that H2S broke the record for superconducting critical temperature under high-pressure. However, the chemical composition ratio of sulfur and hydrogen and the crystal structure during the process in which superconductivity takes place have not been well understood.
A research group led by Takahiro Ishikawa, Specially Appointed Assistant Professor, and Katsuya Shimizu, Professor, at Center for Science and Technology under Extreme Conditions, Graduate School of Engineering Science, Osaka University, Tatsuki Oda, Professor at School of Mathematics and Physics, Kanazawa University, and Naoshi Suzuki, Professor at Faculty of Engineering Science, Kansai University predicted a new superconductivity phase of hydrogen sulfide (H5S2), which was presented at a pressure of 1.1 million bar on computer simulation. The superconducting critical temperature obtained from H5S2, whose calculated value was the same as the experimental value. This result may lead to the clarification of the mechanism behind high-temperature superconductivity, which takes place in hydrogen sulfide by further theoretical and experimental research based on H5S2.
Furthermore, by applying methods used and knowledge obtained by this group to other light element hydrides, it will become possible to establish guidelines for enhancing superconducting critical temperature to near room temperature.
This research was featured in the electronic version of Scientific Reports on Thursday, March 17, 2016.