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Another superconductor shows up

16 Dec 2003 Belle Dumé

Physicists at the University of Tokyo have discovered a new superconductor made of potassium, osmium and oxygen. The material - KOs2O6 - has a superconducting transition temperature of 9.6 K, remains a superconductor in high magnetic fields, and is the second superconductor with a so-called pyrochlore structure to be discovered by the group. The nature of the superconductivity in the new material remains a mystery, say the researchers (J. Phys.: Condens. Matter to be published)

Recent advances in superconductivity – the complete loss of electrical resistance in certain metals when cooled to low temperatures – continues to hold surprises. Three years ago, researchers discovered surface superconductivity in carbon-60 and, in 2001, bulk superconductivity in magnesium diboride. They have since found that certain elements, including boron and lithium, become superconductors under extreme pressures and that an alloy of plutonium containing cobalt and gallium can also superconduct.

Two years ago Zenji Hiroi and colleagues discovered the first pyrochlore superconductor – a class of material that usually has the general formula A2B2O7, where A and B are positive metal ions. Their material – Cd2Re2O7 – lost all electrical resistance when cooled to 1K. They then realised that replacing the rhenium with osmium would create a metal that becomes insulating at 225 K. It seemed that the number of d-electrons on the cations – two for rhenium and three for osmium – has a big effect on the properties of the material, as do electron correlations near the metal-insulator transition. This inspired them to create the new material KOs2O6, which is the second pyrochlore superconductor. It is a “defect pyrochlore”, having only six oxygen atoms and one A-type ion.

Hiroi and co-workers synthesized KOs2O6 from potassium oxide and osmium oxide. X-ray diffraction measurements showed that the compound crystallizes into the pyrochlore structure and the researchers observed superconductivity in measurements of resistivity and magnetic susceptibility. The team now hopes to perform measurements on single crystals of the material to further investigate its properties.

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