  Fourier transform scanning tunneling spectroscopy (FT-STS) measures the
scattering of conduction electrons from impurities and defects, giving
information about the electronic structure of both the host material and
adsorbed impurities. We interpret such FT-STS measurements in terms of the
quasiparticle interference (QPI), investigating the QPI due to magnetic
impurities adsorbed on a range of representative non-magnetic host surfaces,
and contrasting with the case of a simple scalar impurity or point defect. We
demonstrate how the electronic correlations present for magnetic impurities
markedly affect the QPI, showing e.g. a large intensity enhancement due to the
Kondo effect, and universality at low temperatures/scanning-energies. The
commonly-used joint density of states (JDOS) interpretation of FT-STS
measurements is also considered, and shown to be insuffcient in many cases,
including that of magnetic impurities.
