One of the most remarkable and unexpected discoveries of the 20th century is that some materials can become "superconductors" when cooled down to very low temperatures. This means that they can conduct electricity with no resistance and are used in applications ranging from MRI scanners and particle accelerators to the "maglev trains" that move without touching the ground.
Liquid helium or nitrogen are used to cool most materials down to low enough temperatures so they become superconductors and then they stay that way. But as this is expensive and impractical, physicists have for decades tried to find new materials where the phenomenon exists at room temperature, which has proven difficult. An international research team I am a part of has now come up with a new technique to induce superconductivity at high temperatures by shining lasers on the material, which could pave the way for superconductors that can work at room-temperature.
While this is just a first step, the returns could one day be huge. Just as the creation of semiconductors laid the foundations for the entire digital world, a room-temperature superconductor could launch a similar technological revolution. It would make electronic devices more efficient and which require less power consumption and could even herald new technologies such as ultra-fast switches that could replace transistors, currently used to flip electrical signals in computers.
A century of superconductors
Materials are often categorised by their ability to conduct electricity. Metals allow electrons to move freely and carry with them electrical charge. In insulators, such as rubber or wood, electrons are stuck so no electrical current can flow.
Normal conductors always have some resistance because the mobile electrons within the material bounce off jiggling positive ions (atoms that have lost their mobile electrons) which slows down the current. But in 1911, Kamerlingh Onnes discovered that this resistance vanished abruptly in mercury when it was cooled close to absolute zero (or -273.15°C). A superconductor also expels magnetic fields which is crucial to magnetic levitation (the "maglev" effect in those trains).
It took over 50 years – and a number of Nobel prizes – to explain this effect. It is caused by mobile electrons pairing up together when the temperature is cooled enough. This makes electrical resistance vanish because, unlike single electrons, these pairs tend to be deflected by ions in a way which keeps them harnessed together and allows them to continue their journey essentially unscathed.
In the 1980s an entirely new class of superconductor was discovered in ceramic-like materials called cuprates that were capable of operating at 138K (-135.15°C). This higher temperature meant that cheap coolants like liquid nitrogen could be used to cool it down enough to become a superconductor, rather than using more expensive liquid helium.
Rattling the cage
Our experiment investigated a particular material built out of bucky-balls, which are large molecules composed of 60 carbon atoms arranged in a football-like cage. When squashed together, bucky-balls form a regular solid, which is a fairly boring insulator. But if alkali atoms like potassium are added to fill up the spaces between the carbon cages, things change. Potassium atoms have a single "loose" electron far from the nucleus, which can easily transfer to the bucky-balls. There, it can hop from one bucky-ball to another – making the material metallic at room temperature. However, the most striking feature occurs when the material is cooled down. Below 20K (-253.15°C) it is naturally a superconductor.
A useful feature of molecular solids like this is that they can vibrate in a number of different ways, corresponding to the cage being stretched or compressed. In our new study my colleagues used a powerful laser that generated very short pulses at a frequency resonant with one of the cage's vibrational modes, making the bucky-balls shake in unison. They then used another much weaker laser pulse to probe how the material behaved.
This is where things started to get curious. While the material naturally acts as a superconductor below 20K (-253.15°C), the experiment showed that it can still be a superconductor at 100K (-173.15°C) – where it wouldn't naturally be superconducting. This worked as long as the bucky-balls were vibrating. However, below 20K, they found that driving the vibration actually destroyed superconductivity. This suggests that the "laser-induced" superconductor at 100K is not just the naturally occurring one on steroids. Rather it is a completely different state, better suited to higher temperatures.
So what is going on here? The short answer is that we do not know, but my colleagues and I are trying to find out. Shaking the bucky-balls may be enhancing the "glue" between electron pairs either by distorting the lattice structure very slightly or by causing the mobile electrons to slosh around the surface of the bucky-ball, changing the way they interact with one another.
Rattling the carbon cage thus appeared to have forced the material to behave for a short time like a superconductor at five times higher temperatures than it does naturally. For instance, if we could raise the temperature at which cuprate superconductors work by only a factor of two, they would comfortably operate at room temperature. So the race is now on to find suitable materials that naturally superconduct at higher temperatures that are susceptible to the same effect.
Perhaps the most important feature of this experiment is that it opens up new knobs and levers from which we can control and manipulate superconductivity.
Hopefully, further research with this technique will help us reveal useful clues about how to fabricate a material that superconducts at high temperatures without even needing a laser – taking us even closer to a real superconductor revolution.
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