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.