We have determined the atomic structure of the (√3×√3)R30° phase of the 1/3 monolayer Sn/Ge(111) surface by using chemical-shift resolved photoelectron diffraction. The nature of this phase has been highly controversial since it was found that it undergoes a reversible transition to a (3x3) phase upon cooling below 220 K.
The experimental and theoretical results collected up to now indicate that the dynamical-fluctuations model correctly describes the atomistic mechanism of the transition. In this model the stable surface is considered to be a (3x3) one, along the whole temperature range. At LT the system is frozen in the stable reconstruction, while, above the transition temperature, it starts oscillating with increasing frequency, between the two well differentiate vertical positions occupied at LT.
Regarding the atomic structure of the LT phase, several diffraction experiments have shown that the (3x3) reconstruction is corrugated with the Sn adatoms distributed between two inequivalent vertical positions. The most popular model for the (3x3) reconstruction is the 1U2D, where one Sn atom is displaced upwards and the other two displaced downwards. However, very recently, several experiments have reported that the 2U1D model is the correct one, coinciding with what we have initially suggested, based on early X-Ray Surface Diffraction and Photodiffraction results.
We shall discuss the atomic structure of the fluctuating (√3×√3)R30° phase and the origin of the two components of the two components of the Sn-4d photoemission peak. Our results clearly favour the 2Up-1Down model.
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