  The nature of the alpha-Sn/Ge(111) surface is still a matter of debate. In
particular, two possible configurations have been proposed for the 3x3 ground
state of this surface: one with two Sn adatoms in a lower position with respect
to the third one (1U2D) and the other with opposite configuration (2U1D). By
means of first-principles quasiparticle calculations we could simulate STM
images as a function of bias voltage and compare them with STM experimental
results at 78K, obtaining an unambiguous indication that the stable
configuration for the alpha-Sn/Ge(111) surface is the 1U2D. The possible
inequivalence of the two down Sn adatoms is also discussed.
