We perform a first-principles study of Mg adatom and adislands on the
Mg(0001) surface, and Be adatom on Be(0001), to obtain further insights into
the previously reported energetic preference of the fcc faulty stacking of Mg
monomers on Mg(0001). We first provide a viewpoint on how Friedel oscillations
influence ionic relaxation on these surfaces. Our three-dimensional
charge-density analysis demonstrates that Friedel oscillations have maxima
which are more spatially localized than what one-dimensional average density or
two-dimensional cross sectional plots could possibly inform: The well-known
charge-density enhancement around the topmost surface layer of Mg(0001) is
strongly localized at its fcc hollow sites. The charge accumulation at this
site explains the energetically preferred stacking fault of the Mg monomer,
dimer and trimer. Yet, larger islands prefer the normal hcp stacking.
Surprisingly, the mechanism by which the fcc site becomes energetically more
favorable is not that of enhancing the surface-adatom bonds but rather those
between surface atoms. To confirm our conclusions, we analyze the stacking of
Be adatom on Be(0001) - a surface also largely influenced by Friedel
oscillations. We find, in fact, a much stronger effect: The charge enhancement
at the fcc site is even larger and, consequently, the stacking-fault energy
favoring the fcc site is quite large, 44 meV.Comment: Submitted to Physical Review
