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