Superconductivity mystery deepens

By Michael Banks

I have been closely following events concerning a new class of iron-based superconductors ever since Physics World broke the story about their discovery in March. The new materials, containing planes of iron and arsenic separated by planes of lanthanum and oxygen, offer high temperature superconductivity without the need of copper-oxide planes as in the cuprates.

The challenge now is to understand how these superconductors work, i.e. what the responsible pairing mechanism is. Early calculations showed that the superconductivity cannot be described by electron-phonon coupling. The mechanism could therefore be similar to cuprate-based superconductors, which currently hold the record for the highest superconducting transition temperature (although the mechanism in the cuprates is still not understood).

Now, however, a paper published in Nature suggests that SmFeAsOF, which is the same as the material in the story we reported in March but with the lanthanum replaced by samarium, may behave quite differently to the cuprates. The paper’s authors, who are based in the US and China, show that SmFeAsOF has a ‘single gap’ in the density of states of the Cooper-pair fluid (an energy gap originates since there is a finite amount of energy needed to break the pair of electrons held in a Cooper-pair). The temperature dependence of the gap was found to obey conventional BCS predictions — the theory named after Bardeen, Cooper and Schrieffer that proposes electron attraction, via phonons, to form cooper-pairs.

This is all different from the cuprates, which don’t follow BCS predictions and also have a so-called ‘pseudo-gap’, which as I understand only allows certain electrons to ‘see’ a gap depending on how the travel with respect to the crystal lattice. The authors found no evidence of a ‘pseudo-gap’ in the new materials. So it seems that the materials follow BCS predictions, but with a superconducting transition temperature that is too high to be explained via electron-phonon coupling. The mystery deepens.

In another recent development, researchers in Switzerland have managed to grow single crystals of the Sm based iron superconductor. All research done before was performed on polycrystalline samples, but now opening research into single crystals means finding those elusive mechanisms may be a step closer.