Graphene: A Giant leap Towards Room Temperature Superconductors
Recent ground-breaking study has shown the unique properties of material graphene for a long-term possibility of finally developing economical and practical-to-use superconductors.
A superconductor is a material which can conduct (transmit) electricity without resistance. This resistance is defined as some loss of energy which occurs during this process. So, any material becomes superconductive when it is able to conduct electricity, at that particular ‘temperature’ or condition, without release of heat, sound or any other form of energy. Superconductors are 100 percent efficient but unfortunately, most materials require to be in an extremely low energy state in order to become superconductive, which basically means that they have to be very cold. Most superconductors need to be cooled with liquid helium to very low temperature of about -270 degrees Celsius. Thus, any superconducting application is generally coupled with some sort of active or passive cryogenic/low temperature cooling. This cooling procedure requires an excessive amount of energy in itself and liquid helium is not only very expensive but also non-renewable. Therefore, most conventional or “low temperature” superconductors are inefficient, have their limits, are uneconomical, expensive and impractical for large scale use.
The field of superconductors took a major leap in mid 1980s when a copper oxide compound was discovered which could superconduct at -238 degrees Celsius. This is still cold, but much warmer than liquid helium temperatures. This was known as the first “high-temperature superconductor” (HTC) ever discovered, winning the Nobel prize, though its “high” only in a greater relative sense. Therefore, it occurred to scientists that they could focus on eventually finding superconductors which work, let’s say with liquid nitrogen (-196° C) having the plus that its available in plenty and is also cheap. High temperature superconductors also have applications where very high magnetic fields are required. Their low-temp counterparts stop working at around 23 teslas (tesla is a unit of magnetic field strength), so they can’t be used to make more stronger magnets. But high temperature superconducting materials can work at more than twice that field, and likely even higher. Since superconductors generate large magnetic fields they are an essential component in scanners and levitating trains. For example, the MRI today (Magnetic Resonance Imaging) is a technique which uses this quality to look at and study materials, disease and complex molecules in the body. Also, other applications include grid scale storage of electricity by having energy-efficient power lines (example, superconducting cables can provide 10 times as much power as cooper wires of the same size), wind power generators and also supercomputers. The devices which are capable of storing energy for millions of years can be created with superconductors.
The current high temperature superconductors have their own limitations and challenges. Apart from being very expensive because of requiring a cooling device, these superconductors are made of brittle materials and are not easy to shape and thus cannot be used to make electrical wires. The material also might be chemically unstable in certain environments and also extremely sensitive to impurities from atmosphere and water and thus it has to be generally encased. Then there is only a maximum current that superconducting materials can carry and above a critical current density, superconductivity breaks down limiting the current. Huge costs and impracticalities are hindering the usage of good superconductors especially in developing countries. The engineers, in their imagination, would really want a soft, malleable, ferromagnetic superconductor which is impervious to impurities or applied current and magnetic fields. Too much to ask for!
Graphene could be it!
The central criterion of a successful superconductor is to find a high temperature superconductor, the ideal scenario being room temperature. However, newer materials are still limited and are very challenging to make. There is still continuous learning in this field about the exact methodology these high-temperature superconductors adopt and how scientists could arrive at a new design which is practical. One of the challenging aspects in high-temperature superconductors is that it’s very poorly understood what really helps the electrons in a material to pair up. In a recent ground-breaking study, it has been shown for the first time that the material graphene has intrinsic superconducting quality and we can really make a graphene superconductor in the material’s own natural state. Graphene, a purely carbon-based material, was discovered only in 2004 and is the thinnest material known. It is also light and flexible with each sheet comprised of carbon atoms arranged hexagonally. It is seen to be stronger than steel and also it expresses much better electrical conductivity compared to copper. Thus, it’s a multidimensional material with all these promising properties.
Physicists at Massachusetts Institute of Technology and Harvard University, USA, whose work is published in two papers1,2 in Nature, have reported that they are able to tune the material graphene to show two extreme electrical behaviour – as an insulator in which it doesn’t allow any current to pass and as a superconductor in which is allows current to pass without any resistance. A “superlattice” of two graphene sheets was created stacked together rotated slightly at a “magic angle” of 1.1 degrees. This particular overlaying hexagonal honeycomb pattern arrangement was done so as to potentially induce “strongly correlated interactions” between the electrons in the graphene sheets. And this did happen because graphene could conduct electricity with zero resistance at this “magic angle” while any other stacked arrangement kept graphene as distinct and there was no interaction with the neighbouring layers. Therefore, they showed a way to make graphene adopt an intrinsic quality to super conduct on its own. Why this is highly relevant is because, the same group had previously synthesized graphene superconductors by placing graphene in contact with other superconducting metals allowing it to inherit some superconducting behaviours but could not achieve with graphene alone. This is a ground-breaking report because graphene’s conductive abilities have been known for a while but it’s the first time ever that graphene’s superconductivity has been achieved without altering or adding other materials to it. Thus, graphene could be used to make a transistor-like device in a superconducting circuit and the superconductivity expressed by graphene could be incorporated into molecular electronics devices with novel functionalities.
This brings us back to all the talk on high-temperature superconductors and though this system still needed to be cooled to 1.7 degrees Celsius, producing and using graphene for large projects looks achievable now by investigating its unconventional superconductivity. Unlike the conventional superconductors, graphene’s activity cannot be explained by the mainstream theory of superconductivity. Such unconventional activity has been seen in complex copper oxides called cuprates, known to conduct electricity at up to 133 degrees Celsius, and has been the focus of research for multiple decades. Though, unlike these cuprates, a stacked graphene system is quite simple and the material is also understood better. Also, only now graphene has been discovered as a pure superconductor, but the material in itself has many outstanding capabilities which are previously known. This work paves way for a stronger role of graphene and development of high-temperature superconductors that are environment-friendly and more energy efficient and most importantly function at room temperature eliminating the need for expensive cooling. This could revolutionize energy transmission, research magnets, medical devices especially scanners and could really overhaul how energy is transmitted in our homes and offices.
- Yuan Cao et al., 2018, ‘Correlated insulator behaviour at half-filling in magic-angle graphene superlattices’, Nature, DOI: https://doi.org/10.1038/nature26154
- graphene: A giant leap towards room temperature superconductors Yuan Cao et al., 2018, ‘Unconventional superconductivity in magic-angle graphene superlattices’, Nature, DOI: https://doi.org/10.1038/nature26160
Vol.1 Issue 4 April 2018