This is an amazing experiment in more ways than one.
Abstract: The superconducting state is characterized by a pairing of electrons with a superconducting gap on the Fermi surface. In iron-based superconductors, an unconventional pairing state has been argued for theoretically. We used scanning tunneling microscopy on Fe(Se,Te) single crystals to image the quasi-particle scattering interference patterns in the superconducting state. By applying a magnetic field to break the time-reversal symmetry, the relative sign of the superconducting gap can be determined from the magnetic-field dependence of quasi-particle scattering amplitudes. Our results indicate that the sign is reversed between the hole and the electron Fermi-surface pockets (s±-wave), favoring the unconventional pairing mechanism associated with spin fluctuations.
T. Hanaguri et al., Science v.328, p.474 (2010).
What is amazing is that they not only managed to determine the pairing symmetry for this family of superconductors, but also were able, using STM no less, to detect the s±-wave pairing symmetry, which is not easy! See J.E. Hoffman article reviewing this work in the same issue of Science to understand why.
Is this the first ever experiment to actually make an observation of this pairing symmetry? I'm sure that it is the first ever using an STM.