Since they were discovered a little more than a year ago, iron-arsenide superconductors have raised hopes that physicists will soon crack the difficult problem of explaining why certain materials remain superconducting at relatively high temperatures, while others do not. Now, a team of physicists in the US and China has shown that the superconductivity in a specific iron-arsenide material does not depend on the orientation of an applied magnetic field. The finding could challenge a long-standing belief among some physicists that high-temperature superconductivity only occurs when electrons are confined to move in two directions.

Superconductivity occurs when a material is cooled below a certain temperature and its conduction electrons form a condensate that can flow without any resistance. In a conventional, low-temperature superconductor, such as lead, this process (which involves electron pairs) is described by BCS theory, developed in 1957 by John Bardeen, Leon Cooper and Robert Schrieffer. However, BCS theory cannot explain why superconductivity persists in certain cuprate materials, some of which remain superconductors at temperatures above 100 K.

The new family of iron-arsenide-based high-temperature superconductors that were discovered last year do not, however, appear to fit either the BCS or cuprate models. Some physicists think that the mystery of why the cuprates are superconductors at such high temperatures could be cracked by comparing the physical properties of these new materials to known superconductors.

Now, Huiqiu Yuan and colleagues at Los Alamos National Laboratory in the US and the Chinese Academy of Science in Beijing have added an important piece to the puzzle by measuring the electrical resistivity of Ba_0.6K_0.4Fe₂As₂ by placing a single crystal of it in a strong magnetic field that could be varied between 0 and 60 T (Nature 457 565).

2D or not 2D

In all types of superconductors, the critical temperature (Tc) at which a material ceases to be a superconductor falls as the magnetic field is increased. In conventional superconductors only the strength of the field matters, not its direction. In cuprates, both the strength and direction of the field relative to the crystal lattice will affect superconductivity — suggesting that the superconductivity occurs in special 2D planes in the material.

The researchers found that the Tc of Ba_0.6K_0.4Fe₂As₂ fell from 28K as the field was increased from zero. Surprisingly, however, the material’s Tc did not depend much on the orientation of the magnetic field relative to material — in other words the superconductivity is 3D. It appears therefore that the iron arsenides are more like conventional superconductors in this respect.

Directionality in the cuprates suggests that the electrons move without resistance through planes of copper and oxygen atoms, which has led some physicists to conclude that the “quasi-2D” nature of these electrons is necessary for high-Tc superconductivity. But because the material studied by Yuan’s team contains planes of iron-arsenide — yet does not have the directionality of the cuprates — the link between high-Tc superconductivity and two-dimensionality could be a “red herring”, according to Jan Zaanen of Leiden University in the Netherlands.

Not very high-Tc

Others, however are more cautious in how they interpret Yuan’s data. Nigel Hussey of the University of Bristol, UK, points out that with a Tc of 28K, Ba_0.6K_0.4Fe₂As₂ cannot really be compared to the cuprates as is not really a high-temperature superconductor. Instead, he points out that such a Tc is more in line with other “3D” superconductors such as the perovskite Ba_0.6K_0.4BiO₃ and the fulleride superconductors, which have Tc values as high as 30K.

Hussey adds that some iron-arsenide superconductors similar to Yuan’s sample, but with Tc values as high as 56K, appear to be more 2D in nature.

So the plot thickens for iron-arsenide superconductors and the mystery of high-Tc superconductivity lives on.