Three dimensional 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 limited to two-dimensional electronic states. Here, we provide direct evidence that out-of-plane scattering vectors can also be observed within QPI experiments. We focus on the case of FeSe, where detailed photoemission and QPI data are available. By using a three-dimensional tight binding model, fit directly to photoemission measurements, we show that the highly anisotropic scattering vectors, measured by QPI, can be directly reproduced using a model of FeSe which includes both $k_z = 0$ and $k_z = \pi$ states. This result unifies both tunnelling and photoemission based experiments on FeSe and highlights the importance of out-of-plane scattering vectors within the surface sensitive technique of QPI.