First-principles calculations and bias-dependent STM measurements at the   alpha-Sn/Ge(111) surface: a clear indication for the 1U2D configuration
  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. 
P. Gori, F. Ronci, S. Colonna, A. Cricenti, O. Pulci and G. Le Lay