Surface vacancies at the fivefold icosahedral Al-Pd-Mn quasicrystal surface: A comparison ofab initiocalculated and experimental STM images

Abstract

Scanning tunneling microscopy (STM) images of the fivefold surface of the icosahedral $(i)$ Al-Pd-Mn quasicrystal have been calculated using ab initio density functional methods. The STM signal is formed predominantly by the Al atoms in the surface plane. The Pd atoms are seen in the STM image as dark spots. The reason for their small contribution to the STM current is a small local density of states around the Fermi level and the localized $d$-character of the Pd states. On the other hand, the local paramagnetic density of states on Mn atoms is much higher than that on Pd atoms and therefore Mn atoms are seen in the STM images as bright spots. A direct comparison of the experimental images obtained from the STM with the ab initio calculated STM images reveals that the characteristic features of the images---the dark pentagonal holes---can also be created by surface vacancies and not only by truncated Bergman clusters as it has been supposed up to now.