Monday, September 25, 2017

Theoretical Progress Toward Room Temperature Superconductors

Room temperature superconductors would be a game-changer in our energy-hungry world. Even as we integrate alternative energy sources into the electric grid, create more energy efficient devices, and reduce demand where we can, we still battle resistance. Electrical resistance, that is.

Every time electricity travels through a wire, it loses energy to resistance, which usually comes off as heat. The one exception to this loss is in superconductors. Electricity can travel through superconducting materials with 100% efficiency because it encounters zero resistance. The problem is that, so far, the superconducting materials scientists have discovered are only superconducting at extremely cold temperatures. They require expensive cooling systems that make them impractical for anything but a few specialized applications.

When superconductivity is useful, though, it's useful. The cables on top in this image carry the same current as the one on bottom.
Image Credit: CERN's "Microcosm" exhibit. (CC BY-SA 2.0 fr)
But in a new article published in the American Physical Society’s journal Physical Review B, a team of scientists from the University of Saskatchewan in Canada show theoretical progress toward achieving higher-temperature superconductors. In this study, Kaori Tanaka, John Tse, and Hanyu Liu (now at the Carnegie Institution for Science) establish guidelines that can help scientists design and synthesize chemical compounds that are predicted to be superconductors at higher temperatures, perhaps even near room temperature.

Hydrogen, the lightest of the elements, has played one of the lead roles in superconductivity research. More than 80 years ago, physicists predicted that molecular hydrogen could become metallic if you cool it down and put it under extremely high pressure. Solid metallic hydrogen is thought to be superconducting and meta-stable, which means that once created, it would remain superconducting at room temperature and pressure. Earlier this year, a team from Harvard announced the creation of metallic hydrogen at an astounding pressure of nearly 5 million atmospheres and a temperature of about -450°F (5.5K), but this result has yet to be verified. (For more on the experiment, check out Metallic Hydrogen at Last?)

The highest-temperature superconductor discovered to date is hydrogen sulfide (H2S). In 2015, scientists at the Max Planck Institute for Chemistry in Germany discovered that this compound of hydrogen and sulfur became superconducting when subject to a pressure of about 1.5 million atmospheres and then cooled to a temperature below around -94°F (203K). This surprisingly high temperature, say the scientists behind this new research, increases the possibility that even higher transition temperatures could exist in compounds of hydrogen and other elements, called hydrides.

In the last ten years, hydrides have become an increasingly popular area of interest for superconductivity researchers. Computational studies of how electrons are arranged in hydrogen-rich compounds suggest that several hydrides will become high temperature superconductors at high pressures. For example, CaH6 and YH6 are expected to become superconducting at temperatures above -100°F (200K). Of course, even 0°F is not what most of us would call “high temperature,” but it brings us closer to a reality in which superconductors could be cooled with ice instead of expensive liquid helium or liquid nitrogen systems.

This image shows one of the crystal structures of the strontium-hydrogen compound SrH10. It shows planes of Sr (green) sandwiched between every two “puckered honeycomb” layers of H. This hydride is stable under high pressure and has a structure similar to one of the metallic phases of solid hydrogen, indicating that it could be a good high temperature superconductor.
Image Credit: Kaori Tanaka, John Tse, and Hanyu Liu.
There have been several theoretical and experimental studies exploring the superconductivity of individual hydrides, but this new research aims to build a more general framework describing the underlying mechanisms of high temperature superconductivity in hydrogen-rich materials. The ultimate goal is to create a set of rules that scientists can use to design and synthesize new hydrogen compounds that are likely to be superconducting at high temperatures.

In order to do this, Tanaka and his colleagues went back to physics fundamentals. Using the conventional theory of superconductivity, they studied different kinds of hydrides that newer computational models predict will become superconducting at high temperatures—including one made from lead and one made of yttrium, among others. Based on their analysis, the team determined some of the physical characteristics of hydrides that seem to be most effective in increasing the temperature at which a compound becomes superconducting. For example, their work suggests that the molecular structure of a hydride is important, and that certain geometrical arrangements of hydrogen atoms seem to promote high temperature superconductivity.

In light of their results, the researchers propose possible paths to designing a hydrogen compound that is superconducting near room temperature. Although the path is not as simple as just following a recipe, the team’s holistic view offers direction that could help experimentalists reach higher temperature superconductors more quickly.

On the discovery of superconductors, author and physicist Stephen Blundell wrote in his book Superconductivity: A Very Short Introduction, “Superconductors were not just better than ordinary conductors of electricity, they were of a completely different order, as strange and mysterious as a visitor from the planet Krypton wearing underpants over his trousers." The hope is that in the near future, this strange discovery can be an ally in our quest to use energy more efficiently and responsibly.

Kendra Redmond

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