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Simpler superconducting

发布时间:2019-03-07 01:18:01来源:未知点击:

By Duncan Graham-Rowe A previously ignored “off-the-shelf” chemical compound has been found to superconduct at far higher temperatures than believed possible for simple metallic compounds. The discovery has created a huge buzz among superconductivity researchers and opens up the possibility of making cheap superconductors that operate at far higher temperatures than before. “It’s terribly exciting,” says Lesley Cohen an experimental physicist at Imperial College London. “At room temperature it’s a very clean metal so that immediately makes it easier to use.” Many ceramic superconductors are highly toxic. Superconductors are materials that have virtually no resistance to electrical current flow but they only do this when the temperature is below a certain critical temperature (Tc). Ceramic superconductors have been developed that have critical temperatures far higher than metallic compounds, in some cases more than 90 Kelvin higher, but these ceramics materials also tend to be expensive, complex to make and difficult to use. Because of this metal compounds have been favoured for practical applications, such as making wires for magnetic resonance imaging (MRI) scanners. Until now such metal compounds have only been found to operate at around 23K. But Jun Akimitsu and his colleagues at Aoyama-Gakuin University in Tokyo, Japan, have made magnesium diboride (MgB2) superconduct at 39K. “We believe it to be the highest [temperature] yet determined for a [non-ceramic] bulk superconductor,” Jun Akimitsu explained in Nature, where the research is published. Borides have been long suspected as potentially good superconductors but no one had looked to see how MgB2 fared. It is readily available chemical reagent sold in kilogram-sized jars. Materials superconduct when electrons overcome their usual charge repulsion and team up into pairs. This allows them to pass through the crystalline lattices of the material without colliding with atoms. But as the temperature of the material rises thermal vibrations cause the pairs to break apart and superconducting ceases. The theoretical upper temperature for this to happen with metallic compounds is about 40K. So the race is now on to try to understand why MgB2 is able to superconduct so close to this maximum. It might even be possible to push materials beyond the 40K maximum by tinkering with MgB2. “This holds tremendous promise for even higher temperatures in conventional materials,” says Robert Cava at the Department of Chemistry and Materials Institute, Princeton University in New Jersey. Akimitsu first presented his results at a conference held in Sendai, Japan in January. Since then other groups were quick to reproduce his results. And one team lead by Paul Canfield at Iowa State University are reported to have already made MgB2 superconducting wires. More at: Nature (vol 410,