Theory forms for superconducting pnictides

Physicists from Rice and Rutgers universities have published a theory that explains some of the complex electronic and magnetic properties of iron ‘pnictides’. In a series of discoveries this spring, pnictides were shown to superconduct at relatively high temperatures.

High-temperature superconductivity – a phenomenon first documented in 1986 – remains one of the unexplained mysteries of condensed-matter physics. Until the discovery of the iron pnictides, the phenomena was limited to a class of copper-based compounds called ‘cuprates’. The new pnictide theory appears in this week’s issue of Physical Review Letters.

“There is a great deal of excitement in the quantum condensed matter community about the iron pnictides,” said paper co-author Qimiao Si, Rice University theoretical physicist. “For more than 20 years, our perspective was limited to cuprates, and it is hoped that this new class of materials will help us understand the mechanism for high-temperature superconductivity.”

In February 2008, a group from Japan discovered superconductivity above 20K in iron pnictides. In March and April, several research groups from China showed that related iron pnictides superconduct at temperatures greater than 50K. Some cuprates have been found to superconduct at temperatures higher than 140K.

Si and Rutgers University theorist Elihu Abrahams have proposed an explanation of some of the similarities and differences between cuprates and pnictides. The arrangement of atoms in both types of materials creates a “strongly correlated electron system” in which electrons interact in a co-ordinated way and behave collectively.

Si and Abrahams proposed that the pnictides exhibit a property called “magnetic frustration”, an atomic arrangement that suppresses the natural tendency of iron atoms to magnetically order themselves in relation to each other. These frustration effects enhance magnetic quantum fluctuations, which may be responsible for the high-temperature superconductivity.

“Precisely how this happens is one of the challenging questions in strongly correlated electron systems,” Abrahams said. “But even though we don't know the precise mechanism, we are still able to make some general predictions about the behaviour of pnictides, and we've suggested a number of experiments that can test these predictions.”

The tests include some specific forms of the electronic spectrum and spin states.

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