A heated debate rises over high pressure superconductivity
- Publication Date
- Professor Graeme Ackland FRSE
Sometimes theoretical physics must wait a very long time for technology to catch up. Acclaimed Hungarian physicist Eugene Wigner first predicted hydrogen, for instance, would turn into a metal at high enough pressure, way back in 1935. And it wasn’t until the 1960s that people predicted it would become a ‘superconductor’ – a material which conducts electricity with no magnetic resistance.
Today superconductors are, of course, widely used. When you have an MRI scan, for example, you are literally placed in a tube of helium at minus 270C so the powerful magnets can induce nuclear magnetic resonances in your body. But there lies their still-unresolved greatest restriction – they often use more electricity keeping themselves suitably cold to work, than everything else put together.
If only a superconductor could be transformed into a material capable of conducting electricity efficiently in everyday room-temperature conditions, its use could be instantly opened up to multiple technologies, potentially unlocking endless possibilities from how your mobile phone works better, to magnetically levitating trains, to future fusion power plants.
Under normal conditions hydrogen is a molecule with its electrons locked in a chemical bond. To conduct, those electrons have to be removed from that bond, and the real key to making a superconductor is to break those bonds. Super-high pressure achieves this by pushing the molecules so close together that the electrons can jump easily between molecules – once the electron escapes from the bond, there is no bond. In theory, the bonds can also be disrupted if more electrons are added to the hydrogen.
Rare earth metals are especially good at donating electrons, and Edinburgh theorists suggested lutetium. This combined ‘squeeze and add electrons’ was first achieved in Germany 2015 by adding sulphur to hydrogen, creating a material which superconducts at -70 degrees, and half the pressure predicted for pure hydrogen.
Since this discovery in hydrogen disulphide – the molecule responsible for the smell of rotten eggs – more exotic materials have been predicted theoretically and synthesized experimentally, lowering the required pressure and raising the temperature ever closer to normal conditions. We have already replicated the sulphide superconductor experiment, which theory suggests has an extra hydrogen – three per sulphur rather than two.
At the American Physical Society’s annual meeting in March, in Las Vegas, a team from the University of Rochester claimed they had made such a revolutionary advance, which has divided the global research community. Its new superconductor consists of lutetium hydrogen and nitrogen.
Because the new lutetium-based material is superconducting at much lower pressures, research groups around the world – including my own Centre for Science at Extreme Conditions in Edinburgh – have been scrambling since to reproduce the experiment or to understand it theoretically. Although the lutetium compound has been recreated in experiments, the sample was not superconducting, and while widely studied theoretically, nobody has succeeded in understanding how superconductivity can persist to low pressure.
The world’s premier high-pressure physics conference comes to the Edinburgh Conference Centre at the end of July, so we can be sure high-pressure superconductivity will be the major topic of debate. Most believe that room temperature superconductors at high pressure can be made. More hotly debated is whether they have, or even could be recovered to atmospheric pressure. And whether lutetium hydride is a new wonder-material, the subject of an experimental error, or as some now suggest, the latest scientific fraud?
And Wigner? A year after his metallic hydrogen prediction, he was fired by Princeton University. But he then went on to work on the Manhattan project at the University of Chicago, where he played a pivotal role in the creation of the atomic bomb, build the world’s first nuclear reactor, and win a Nobel Prize for physics in 1963.
Like many scientists before and after him, Wigner had to bide his time for global recognition.
Professor Graeme Ackland FRSE is Professor of Computer Simulation at The University of Edinburgh, a researcher at the Higgs Centre for Theoretical Physics, and a Fellow of the Royal Society of Edinburgh.
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