Metal hydrides for high temperature superconductivity

01-29-2010: An international research team, comprising scientists working in the Sweden, UK, USA & South Korea and led by Professor Rajeev Ahuja of Uppsala University now presents new findings for better understanding of superconducting high-pressure metal hydride systems. The study is published in the Proceedings of the National Academy of Sciences of the United States of America ( PNAS ).

High-temperature superconductivity continues to fascinate the scientific world since its discovery, not least because the prospect of a material that could remain superconducting even at room temperature would have far-reaching technological consequences. Hydrogen, the lightest element of the periodic table, was predicted to be such a room-temperature superconductor, albeit only for very high pressure which is yet out of range for experimentalists. Nevertheless, the study of pressurized hydrogen-rich materials may offer a glimpse at what properties could be expected for solid hydrogen in this high-pressure regime. The reason for this is that in some metal hydride alloys, a so-called "chemical pre-compression" from the metal atoms combines with the experimentally applied pressure to facilitate hydrogen densities in a range where metallization (and hence, at sufficiently low temperatures, superconductivity) can occur.

In the present work, the research team utilized some of the most sophisticated implementations of theoretical methods available to carry out an in-depth study of superconductivity in the metal hydrides ScH3, YH3, and LaH3 over a wide pressure range. This approach enabled the scientists to identify a general trend in the behavior for all three metal hydrides: the superconducting state is strongest when the material is weakest. Put more precisely, this means that near those pressures where a structural transformation is about to set in and the material becomes dynamically unstable, the electron-phonon coupling (which is a measure of the strength of a superconducting phase) is highest and drives the critical temperature of superconductivity to its maximal value.

But the scientists also discovered several differences between the three metal hydrides, despite their chemical similarities. Thus, it was found that a secondary superconducting phase, which was earlier predicted by researchers of the same team to exist in YH3, is mysteriously absent in ScH3 and LaH3. What is the explanation? It could be a case of happenstance: the mass of the Y atom appears to be just right to generate vibrations that can cause this phenomenon, while Sc is too light and La too heavy.

"As with all theoretical predictions, we hope that our work will lead to experimental tests of the data.", says Professor Rajeev Ahuja. "The pressures for which we have predicted superconductivity to occur is well within the reach of contemporary techniques."

"Nevertheless, we believe that experimentalists will embrace the challenge to test these predictions, since there is tremendous merit in understanding the underlying physics of superconducting metallic hydrogen-rich systems."

The extensive calculations were performed on several supercomputer sites in Sweden administered by the Swedish National Infrastructure for Computing (SNIC), including Uppsala University's Multidisciplinary Center for Advanced Computational Science (UPPMAX).