  Based on a phenomenological model with $s_{\pm}$ or s-wave pairing symmetry,
the mixed-state effect on quasiparticle interference in iron-based
superconductors is investigated by solving large-scale Bogoliubov-de Gennes
equations based on the Chebyshev polynomial expansion. Taking into account the
presence of magnetic field, our result for the $s_{\pm}$ pairing is in
qualitative agreement with recent scanning tunneling microscopy experiment
while for the s-wave pairing, the result is in apparent contradiction with
experimental observations, thus excluding the s-wave pairing. Furthermore, we
treat the effect of vortices rigorously instead of approximating the vortices
as magnetic impurities, thus our results are robust and should be more capable
of explaining the experimental data.

Superconductivity