Similarly to simpler systems like Cu(111) and Au(111), the oscillation here seems to also propagate in the form of a trigonometric function. However, the sinusoidal form, as proposed by the authors, seems to only be supported by the minimal electron density at the impurity, which is likely a result of a phase shift of a cosine function, as described in the theoretical section 1.4. 2 Sprunger et al. , have studied FOs on Be(0001) surface. Using FT-STM, they have found kF to be 0.945 Å -1 , which agrees with a slightly anisotropic value of 0.93 and 0.96 Å -1 obtained by photoemission (for more information on the anisotropicity of FOs and kF values, please see subsection 2.7). The amplitude of the imaged oscillations, however, is much bigger than that observed for other metals and, therefore, as proposed by the authors, might be a result of electron-phonon coupling or a property of the Be electron gas.