K_z selective scattering within quasiparticle interference measurements of FeSe

Quasiparticle interference (QPI) provides a wealth of information relating to the electronic structure of a material. However, it is often assumed that this information is constrained to two-dimensional electronic states. We show that this is not necessarily the case. For FeSe, a system dominated by surface defects, we show that it is actually all electronic states with negligible group velocity in the z axis that are contained within the experimental data. By using a three-dimensional tight-binding model of FeSe, fit to photoemission measurements, we directly reproduce the experimental QPI scattering dispersion, within a T-matrix formalism, by including both k_z=0 and k_z=π electronic states. This result unifies both tunnelling based and photoemission based experiments on FeSe and highlights the importance of k_z within surface sensitive measurements of QPI.