A Model for the Thermal Expansion of Ag(111) and other Metal Surfaces

We develop a model to study the thermal expansion of surfaces, wherein phonon frequencies are obtained from ab initio total energy calculations. Anharmonic effects are treated exactly in the direction normal to the surface, and within a quasiharmonic approximation in the plane of the surface. We apply this model to the Ag(111) and Al(111) surfaces, and find that our calculations reproduce the experimental observation of a large and anomalous increase in the surface thermal expansion of Ag(111) at high temperatures [P. Statiris, H.C. Lu and T. Gustafsson, Phys. Rev. Lett. 72, 3574 (1994)]. Surprisingly, we find that this increase can be attributed to a rapid softening of the in-plane phonon frequencies, rather than due to the anharmonicity of the out-of-plane surface phonon modes. This provides evidence for a new mechanism for the enhancement of surface anharmonicity. A comparison with Al(111) shows that the two surfaces behave quite differently, with no evidence for such anomalous behavior on Al(111).Comment: 17 pages, 4 figures, to appear in Z. Chem. Phy

Steering and ro-vibrational effects in the dissociative adsorption and associative desorption of H_2/Pd(100)

The interaction of hydrogen with many transition metal surfaces is characterized by a coexistence of activated with non-activated paths to adsorption with a broad distribution of barrier heights. By performing six-dimensional quantum dynamical calculations using a potential energy surface derived from ab initio calculations for the system H_2/Pd(100) we show that these features of the potential energy surface lead to strong steering effects in the dissociative adsorption and associative desorption dynamics. In particular, we focus on the coupling of the translational, rotational and vibrational degrees of freedom of the hydrogen molecule in the reaction dynamics.Comment: 8 pages, 5 figures, subm. to the Proceedings of ISSP-18, June 1996, Poland, to appear in Prog. Surf. Sc

Mechanism of Poisoning the Catalytic Activity of Pd(100) by a Sulfur Adlayer

The modification of the potential-energy surface (PES) of H_2 dissociation over Pd(100) as induced by the presence of a (2x2) S adlayer is investigated by density-functional theory and the linear augmented plane wave method. It is shown that the poisoning effect of S originates from the formation of energy barriers hindering the dissociation of H_2. The barriers are in the entrance channel of the PES and their magnitude strongly depends on the lateral distance of the H_2 molecule from the S adatoms.Comment: RevTeX, 14 pages, 3 figure

Bridging the length and time scales: from ab initio electronic structure calculations to macroscopic proportions

Density functional theory (DFT) primarily provides a good description of the electronic structure. Thus, DFT primarily deals with length scales as those of a chemical bond, i.e. 10^-10 meter, and with time scales of the order of atomic vibrations, i.e. 10^-13 seconds. However, several interesting phenomena happen and/or become observable on different scales, namely meso- or macroscopic lengths and on time scales of seconds or even minutes. To bridge the gap between 10^-13 seconds and a second or between 10^-10 meter and 10 and more nano meters is one of the important challenges we are facing today. In this paper we show how we are overcoming these time and size problems for the example of crystal growth and the evolution of nano-scale structures. The key is a kinetic Monte Carlo approach with detailed input from DFT calculations of the relevant atomistic processes.Comment: 13 pages, 5 figures, to be published in Comments on Condens. Matt. Phys. (1998). Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

The equilibrium shape of InAs quantum dots grown on a GaAs(001) substrate

The equilibrium shape of strained InAs quantum dots grown epitaxially on a GaAs(001) substrate is derived as a function of volume. InAs surface energies are calculated within density-functional theory, and a continuum approach is applied for the elastic relaxation energies.Comment: 4 pages, 1 figure, to appear in "The Physics of Semiconductors

Electronic and structural properties of GaN by the full-potential LMTO method : the role of the dd electrons

The structural and electronic properties of cubic GaN are studied within the local density approximation by the full-potential linear muffin-tin orbitals method. The Ga $3d$ electrons are treated as band states, and no shape approximation is made to the potential and charge density. The influence of $d$ electrons on the band structure, charge density, and bonding properties is analyzed. It is found that due to the energy resonance of the Ga 3$d$ states with nitrogen 2$s$ states, the cation $d$ bands are not inert, and features unusual for a III-V compound are found in the lower part of the valence band and in the valence charge density and density of states. To clarify the influence of the Ga $d$ states on the cohesive properties, additional full and frozen--overlapped-core calculations were performed for GaN, cubic ZnS, GaAs, and Si. The results show, in addition to the known importance of non-linear core-valence exchange-correlation corrections, that an explicit description of closed-shell repulsion effects is necessary to obtain accurate results for GaN and similar systems. In summary, GaN appears to be somewhat exceptional among the III-V compounds and reminiscent of II-VI materials, in that its band structure and cohesive properties are sensitive to a proper treatment of the cation $d$ bands, as a result of the presence of the latter in the valence band range.Comment: ( 20 REVTEX-preprint pages (REVTEX macros are included) 8 figures available upon reques