A superconducting material invented by Chinese scientists could help to answer long-standing puzzles about how certain materials can carry electricity without resistance at surprisingly high temperatures.

Superconductors are already found in powerful electromagnets and magnetic sensors, but their uses are limited by the fact that they need to be cooled to extremely low temperatures to work. Even ‘high-temperature’ superconductors, such as cuprate compounds, must be cooled to at least –135 °C before they start to superconduct, and physicists are still unclear exactly how the phenomenon arises.

Fig. 1: Replacing iron atoms with iridium can induce superconductivity in the SmFeAsO system.

In 2008, the discovery of a new class of iron-based high-temperature superconductors caused great excitement, offering an alternative route to study the phenomenon and potentially leading to even higher superconducting temperatures.

The performance of some of these compounds has been improved by doping — replacing a small proportion of the constituent iron atoms — with cobalt or nickel, which have very similar magnetic properties to iron.

A team led by Yong Zhao, director of the superconductivity R&D Center in Southwest Jiaotong University, China1, has now developed a series of new superconducting compounds in which the non-magnetic metal iridium partially replaces iron.

The team made their compounds by reacting iridium with samarium arsenide, iron and iron oxide to produce samarium-iron-iridium-arsenic oxide (SmFe1-xIrxAsO). They found that an iron:iridium ratio of 85:15 was the ideal recipe, producing a material that began to superconduct when cooled to below –257 °C, just 16° above absolute zero.

This temperature is relatively low, admits Yong, “but by optimizing the processing conditions, the temperature may be increased.” There are more important implications, however. “The significance here is more about fundamental physics than the potential applications. It may provide important insights into the superconductivity mechanism of iron-based systems.”

The team studied the atomic structure of their compounds and showed how iridium distorts the arrangement of atoms in different ways compared to the cobalt-doped compound. These structural studies could help develop a firm link between the superconducting temperature and the atomic structure of a material — a key part of formulating a theory that describes high-temperature superconductivity accurately.

The discovery should also prompt scientists to widen their search beyond cobalt or nickel for other elements to incorporate into their iron-based superconductors, says Yong.