Probing order parameter structure in iron-based superconductors using vortices

Impurities, inevitably present in all samples, induce elastic transitions between quasiparticle states on the contours of constant energy. These transitions may be seen in Fourier-transformed scanning tunneling spectroscopy experiments, sorted by their momentum transfer. In a superconductor, anomalous scattering in the pairing channel may be introduced by magnetic field. When a magnetic field is applied, vortices act as additional sources of scattering. These additional transitions may enhance or suppress the impurity-induced scattering. We find that the vortex contribution to the transitions is sensitive to the momentum-space structure of the pairing function. In the iron-based superconductors, there are both electron and hole pockets at different regions of the Brillouin zone. Scattering processes therefore represent intrapocket or interpocket transitions, depending on the momentum transfer in the process. In this work we show that while in a simple s-wave superconductor all transitions are enhanced by vortex scattering, in an s_± superconductor only intrapocket transitions are affected. We suggest this effect as a probe for the existence of the sign change in the order parameter