The pluses and minuses of iron superconductors

STM data showing quasiparticle scatterings q2 and q3 (Courtesy:Science)

By Hamish Johnston

Almost exactly 24 years ago to the day, Georg Bednorz and Alex Müller of IBM Zurich submitted a paper describing the first high-Tc superconductor. The new copper-based (or cuprate) material had zero electrical resistance at temperatures up to Tc = 35K – shattering the previous record by more than ten degrees.

Bednorz and Müller won the 1987 Nobel Prize in Physics for the discovery, which spurred intense international effort to understand the physics of high-Tc materials and pushed superconducting temperatures to 100K and beyond.

However, physicists still struggle to understand much of the physics behind the cuprates and other high-Tc materials – making it one of the great unsolved problems of physics.

The discipline received a huge morale boost in 2007-08 with the discovery of the first iron-based high-Tc material. Many more iron superconductors have been found since, and physicists hope that comparisons between these new materials and the cuprates will lead to a breakthrough.

So what do we know so far? Superconductivity arises when conduction electrons form pairs – which, unlike single electrons, can condense at low temperatures into a superfluid that travels through the material without any resistance.

The pairing mechanism in conventional low-temperature superconductors such as lead or mercury is well understood – lattice vibrations called phonons mediate a spherically symmetric interaction between electrons. This is relatively easy to describe mathematically in conventional superconductors – but calculations are much more difficult for cuprates because the electrons interact much more strongly with each other. Furthermore, physicists don’t know exactly what mediates the pairing in cuprates – it’s unlikely to be phonons and could be the electron–electron interactions themselves.

Physicists do know that the pairing interaction in cuprates is not spherically symmetric (or s-wave) but rather has a pronounced lobes at right angles to each other (d-wave).

The big question is whether the new iron-based materials are also d-wave – and evidence is mounting that the answer is no. Instead, the interaction appears to be perfectly symmetric in terms of its magnitude but involves a reversal of phase. First theorized by Igor Mazin and colleagues in 2008, this “S± symmetry” is supported by a growing number of experiments.

The latest is published today in the journal Science, where Tetsuo Hanaguri and colleagues at RIKEN in Japan present scanning tunneling microscopy (STM) studies of a superconductor made of iron, selenium and tellurium.

The team looked at the interference patterns that arise when an electron – or more precisely an electron-like quasiparticle – in the superconductor scatters from one state to another. The scattering is caused by superconducting magnetic vortices in the sample and the measurement gives the phase difference between the quasiparticle states.

The result backs Mazin’s S± theory in which the pairing interaction is mediated by spin fluctuations. This magnetic origin for superconductivity is perhaps not that surprising in iron-based materials.

As for the cuprates…the work continues!