A mechanism of dimer flipping on the Si(001) surface has been investigated by means of ab initio pseudopotential calculations. These studies clearly demonstrate a tied flipping behavior of two consecutive dimers: the buckling of adjacent outer dimers becomes stronger while two inner dimers switch their orientation, which facilitates the inner dimer flipping by relieving the sublayer strain. Contrary to popular speculation, in most cases, the dimers undergo thermal fluctuations between two unequal energy minima. Hence, a clean Si(001) surface can preserve the 2× anticorrelation of dimer buckling along a row at room temperature.
Bibliographical noteFunding Information:
This work was carried out at the Max-Planck-Institute fuer Festkoerperforschung in Germany, and supported by the Electrochemical Society through the F.M. Becket Memorial award to G.S.H. The author is very thankful to Professor M. Parrinello for his hospitality and general advice, P. Silvestrelli for his critical reading of this manuscript and J. Hutter for his guidance on the cpmd code. Helpful discussions with S. Goedecker, M. Krack and C. Mundy are also gratefully acknowledged.