Iron-based superconductors: Pluses and minuses

Scanning tunneling microscopy experiments point to spin fluctuations as the paring mechanism in an iron-based superconductor.

Just over two years ago, a class of iron-based compounds was found to exhibit superconductivity at up to a few tens of degrees Kelvin — relatively high temperatures by superconducting standards. The superconductivity community immediately focused on trying to understand the physical mechanism at the source of the phenomenon.

Superconductivity stems from the formation of pairs of electrons or electron ‘holes’, but the mechanism behind this pairing appears to differ among the various superconducting systems. For example, in the conventional class of low-temperature superconductors, the electron pairing is known to be mediated by lattice vibrations, known as phonons. In the case of iron-based superconductors, on the other hand, it has been proposed that the pairing is mediated by strong spin fluctuations. Tetsuo Hanaguri and colleagues from the RIKEN Advanced Science Institute in Japan¹ have now provided strong experimental evidence that this is indeed the case.

The energy required to break an electron or hole pair in a superconductor is known as the ‘superconducting gap’. In phonon-mediated superconductivity, known as ‘s-wave’ type, the superconducting gap is intrinsic to the system and remains uniform with respect to the movement of pairs. In systems characterized by spin fluctuations, however, the sign of the gap changes depending on the momentum of superconducting pairs, and whether the pairs are electrons or holes. This type of superconductivity is known as s_±.

Fig. 1: Magnetic field-induced change in 'quasi-particle inteference' intensities showing opposite signals from two distinct scattering groups (q₂ and q₃). The result can only be explained by s±-type superconductivity.From Ref. 1. Reproduced with permission. © 2010 AAAS

To confirm whether this gap sign reversal occurs in iron-based superconductors, and thereby confirm that spin fluctuations are the origin of superconductivity in such systems, Hanaguri and his colleagues explored the sign reversal of the superconducting gap in an iron–selenium–tellurium superconductor using a scanning tunneling microscope in combination with spectroscopic imaging. By analyzing the scattering between pair states with different momentums, they were able to observe a distinct dissimilarity in behavior between two types of scattering in the presence of a magnetic field (Fig. 1) — a clear signature of a change in sign. “Our results provide a crucial clue to establishing a theoretical model that describes iron-based superconductivity, and it may help to ‘design’ a new superconductor,” says Hanaguri.