-Phase of Pb on Ge(111): The competing roles of electronic bonding and thermal fluctuations

We have studied the low temperature phases of Pb/Ge(111) at Pb coverages =1/3, 1, and 4/3 using ab initio molecular dynamics. Stable phases are found to occur at =1/3 and 4/3. For the low-coverage (=1/3) phase, we confirm the model of Pb atoms adsorbed at T4 sites of the Ge(111) substrate. For the high coverage (=4/3) phase and very low temperature, we determine a structure characterized by the presence of covalently bonded Pb chains parallel to 110 substrate directions. At room temperature, fluctuations are found to disrupt long-range Pb chains and restore the threefold symmetry of the ideal Ge(111) surface. The calculated structure factors and local density of states agree well with x-ray diffraction and STM data.