First-principles calculations and bias-dependent STM measurements at the α-Sn/Ge(111) surface

The nature of the α-Sn/Ge(111) surface is still a matter of debate. In particular, two possible configurations have been proposed for the 3×3 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). We report ab initio GW calculations for the α-Sn/Ge(111) system, simulating STM images as a function of bias voltage and comparing them with STM experimental results at 78 K. The concurrent application of theory and experiments and the very good match between their results provide unambiguous indications that the stable configuration for the α-Sn/Ge(111) surface is the 1U2D. The possible inequivalence of the two down Sn adatoms is also discussed.