Crystal structures of the competing phases.
The quantum behaviour of hydrogen affects the structural properties of hydrogen-rich compounds, which are possible candidates for the elusive room temperature superconductor. New theoretical results suggest that the quantum nature of hydrogen – meaning that it can behave like a particle or a wave — strongly affects the recently discovered hydrogen sulphur superconductor, a compound that when subjected to extremely high pressure, is the highest-temperature superconductor yet identified. This new step towards understanding the underlying physics of high temperature superconductivity may aid in the search for a room temperature superconductor, which could be used for applications such as levitating trains, lossless electrical grids and next-generation supercomputers.
Last year, German researchers identified the highest temperature superconductor yet – hydrogen sulphide, the same compound that gives rotten their distinctive odour. When subjected to extreme pressure – 1 million times higher than Earth’s atmospheric pressure – this compound has superconducting behaviour as high as 203K, far higher than any other high temperature superconductor yet discovered.
Hydrogen, being the lightest element of the periodic table, is the atom most strongly subjected to quantum behaviour. Its quantum nature affects structural and physical properties of many hydrogen compounds. An example is high-pressure ice, where quantum fluctuations of the proton lead to a change in the way that the molecules are held together, so that the chemical bonds between atoms become symmetrical. A similar quantum H-bond symmetrisation occurs in the hydrogen sulphide superconductor.
Theoretical models that treat hydrogen atoms as classical particles predict that at extremely high pressures the atoms sit exactly halfway between 2 sulphur atoms, making a fully symmetrical structure. However, at lower pressures, hydrogen atoms move to an off-centre position, forming one shorter and one longer bond. The researchers have found that when considering H atoms as quantum particles behaving like waves, they form symmetrical bonds at much lower pressures, meaning that quantum physics, and symmetrical hydrogen bonds, were behind the record-breaking superconductivity.
“That we are able to make quantitative predictions with such a good agreement with the experiments is exciting and means that computation can be confidently used to accelerate the discovery of high temperature superconductors,” said study co-author Professor Chris Pickard of Cambridge’s Department of Materials Science & Metallurgy.
According to calculations, the quantum symmetrisation of the hydrogen bond has a tremendous impact on the vibrational and superconducting properties of hydrogen sulphide. “In order to theoretically reproduce the observed pressure dependence of the superconducting critical temperature the quantum symmetrisation needs to be taken into account,” said Ion Errea.
The discovery of such a high temperature superconductor suggests that room temperature superconductivity might be possible in other hydrogen-rich compounds. The current theoretical study shows that in all these compounds, the quantum motion of hydrogen can strongly affect the structural properties, even modifying the chemical bonding, and the electron-phonon interaction that drives the superconducting transition. “Theory and computation have played an important role in the hunt for superconducting hydrides under extreme compression,” said Pickard. “The challenges for the future are twofold — increasing the temperature towards room temperature, but, more importantly, dramatically reducing the pressures required.” http://www.cam.ac.uk/research/news/quantum-effects-at-work-in-the-worlds-smelliest-superconductor
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