Anharmonicity and disorder on the Cu(110) surface

The results of a molecular-dynamics simulation of the finite-temperature properties of the Cu(110) surface are presented. The interatomic interactions have been calculated using the effective-medium theory. The temperature dependence of the interlayer spacings, phonon frequencies and lifetimes, and the structure factor is calculated. The anharmonic effects are shown to be considerably stronger than in the bulk, giving rise to a large thermal expansion coefficient and to a phonon density of states with large peak shifts and widths, in good agreement with experiment. Anharmonicity is also shown to be responsible to the loss of order above 550 K observed in a number of scattering experiments. Above 1000 K an additional loss of order sets in. Here the in-plane order disappears due to rearrangements of the surface atoms around the adatoms and vacancies that are formed at these temperatures. As the temperature is increased further toward the melting point, the disorder moves deeper into the crystal.