Atomic dynamics and structure of the Ge(111)c(2×8) surface

We present results of a first-principles molecular-dynamics study of the surface structure and atomic dynamics of Ge(111)c(2×8) at T=0 and T=300 K. The T=0 ground-state structure shows asymmetries between the two adatoms and the two restatoms in the surface unit cell, which are seen experimentally in low-energy electron diffraction. These asymmetries, which are found to survive at T∼300 K, lead to zone folding and splitting of the surface phonon modes. Two pairs of prominent surface modes are identified, mostly localized on the adatoms and second-layer atoms just below. The values of the mean square displacements for the various layers at different temperatures moreover indicate an enhanced surface anharmonicity with respect to the bulk.