Electron-phonon contribution to hole linewidth of the surface state on Al(0 0 1)

We report an ab initio study of the electron-phonon interaction and its contribution to the excited hole linewidth of the surface state on Al(0 0 1). We have also evaluated the phonon-induced contribution to the linewidth of the occupied energy bands of Δ₁ symmetry in bulk Al. The calculations were carried out using a linear response approach in the plane-wave pseudopotential representation.     

Electron-phonon interaction in magnesium: From the monolayer to the Mg(0 0 0 1) surface

We report ab initio study of the electron-phonon coupling in a free standing magnesium monolayer and at the Mg(0 0 0 1) surface. The calculations were carried out using a linear-response approach in the plane-wave pseudopotential representation. Eliashberg spectral function α²F(ω) averaged over electron states at the Fermi surface is presented for the monolayer while for the Mg(0 0 0 1) surface, we compute the electron-phonon spectral function α²F_k,i(ω) for surface states at the and points.     

Diffusions of small clusters on Pt(1 1 1) and Cu(1 1 1) surfaces

Diffusions of small cluster Pt₆ on Pt(1 1 1) surface and Cu₆ on Cu(1 1 1) are studied by molecular dynamics simulation, respectively. The atomic interaction is modeled by the semiempirical potential. The results show that the diffusion processes in the two systems are far different. For example, on Pt(1 1 1) surface, the hopping of single atom and the shearing of two atoms of hexamer only occur on the adatom(s) adsorbed at B-step, while on Cu(1 1 1) surface they can appear on the adatom(s) either at A-step or B-step. To the concerted translation of the parallelogram hexamer, the anisotropy in the diffusion path is observed in the two systems, the mechanisms and then the preferential paths, however, are completely different. The reasons for these diffusion characteristics and differences are discussed.     

Electron-phonon interaction in a free standing beryllium monolayer

We report ab initio study of the electron-phonon coupling in a free standing Beryllium monolayer. The calculations were carried out using a linear-response approach in the plane-wave pseudopotential representation. The Eliashberg spectral function α²F(ω) and the electron-phonon coupling parameter λ are evaluated at the Fermi level. The obtained results show a large contribution to the electron-phonon coupling from the low-energy transverse mode scattering.     

Systematical study of dimer diffusion on metal fcc(0 0 1) surfaces

We study systematically the dimer diffusion on a series of metal fcc(0 0 1) surfaces. The atomic interactions are modeled by the realistic model potentials including embedded-atom method potential, surface-embedded-atom method potential, and Rosato-Guillopé-Legrand potential. Based on the results of the static calculations and the molecular dynamics simulations, three different kinds of fcc(0 0 1) surfaces can be distinguished named hard, middle, and soft. On the different kind of surfaces, not only the dominant diffusion mechanism but also the physical model for exchange mechanism is different. In addition, besides the conventional hopping and exchange mechanisms, some other interesting diffusion mechanisms for dimers are observed in our molecular dynamics simulations such as exchange rotation mechanism, cooperative hopping mechanism, and cooperative exchange mechanism.     

The stability of fcc (1 1 1) metal surfaces

The results of a theoretical study on the stability of fcc (1 1 1) metal surfaces to certain commensurate-incommensurate reconstructive phase transformations is presented. Specifically, we have performed computer simulation studies of the 22×√3 surface reconstruction of Au(1 1 1). This reconstruction involves a uniaxial contraction of the top monolayer corresponding to a surface strain of about 4.3% and has been observed to be the stable structure for the clean surface at low temperatures. The driving force for the reconstruction has been associated with the quantity (f−γ), where f is the surface stress and γ is the surface free energy, while the opposing force is due to the disregistry with the underlying lattice. A continuum model yields a stability criterion that depends on the knowledge of a small number of physical quantities: f, γ, the equilibrium nearest-neighbor spacing r₁ and the shear modulus G. We have performed molecular dynamics simulations as a general stability analysis of these types of reconstructions. The results are in excellent agreement with the continuum model. The simulations using embedded-atom method potentials also accurately reproduce many observed features of the reconstruction on Au(1 1 1).     

Anharmonicity in Al vicinal surfaces of (1 0 0) with (1 1 1) step

Molecular dynamics simulations incorporating an analytic embedded atom potential have been used to investigate the atomic structure and surface order of the Al vicinal surfaces for the temperature up to 900 K. The relaxation, mean square vibrational amplitude, and structure factor as a function of temperature, and of the terrace width for the p(1 0 0) × (1 1 1) surfaces (2 ≤ p ≤ 10) are discussed. The obtained structure factor indicates that the anharmonic effect reduces with increasing terrace widths. The decrease of surface energy with increasing terrace width also supports this conclusion.     

Quantum states of hydrogen (H, D, T) atoms on Cu(1 0 0) and (1 1 0) surfaces

We investigate the quantum mechanical behavior of adsorbed hydrogen (H, D, T) on Cu(1 0 0) and (1 1 0) surfaces. We construct potential energy surfaces (PESs) for the motion of the hydrogen H atom on Cu(1 0 0) and (1 1 0) surfaces within the framework of density functional theory. The potential energy takes a minimum value on the hollow site of Cu(1 0 0) and on the short bridge site of Cu(1 1 0). Moreover, we calculate the quantum states of hydrogen atom motion on these calculated PESs. The ground state wave function of the hydrogen atom motion is strongly localized around the hollow site on the Cu(1 0 0) surface. On the other hand, the ground state wave function of the hydrogen atom motion on Cu(1 1 0) is distributed from the short bridge site to two neighboring pseudo-threefold sites. We finally show isotope effects on the quantum states of the motion of hydrogen on both surfaces.     

CF interaction with Si(1 0 0)-(2 × 1): Molecular dynamics simulation

In this study, the interaction of CF with the clean Si(1 0 0)-(2 × 1) surface at normal incidence and room temperature was investigated using molecular dynamics simulation. Incident energies of 2, 12 and 50 eV were simulated. C atoms, arising from dissociation, preferentially react with Si to form Si-C bonds. A Si_xC_yF_z interfacial layer is formed, but no etching is observed. The interfacial layer thickness increases with increasing incident energy, mainly through enhanced penetration of the silicon lattice. Silicon carbide and fluorosilyl species are formed at 50 eV, which is in good agreement with available experimental data. The level of agreement between the simulated and experimental results is discussed.     

Vacancy charging on Si(1 1 1)-(7 × 7) investigated by density functional theory

The structure and energetics of charged vacancies on Si(1 1 1)-(7 × 7) are investigated using density functional theory calculations supplemented by estimates of ionization entropy. The calculations predict multiple possible charge states for the unfaulted edge vacancy in the adatom layer, although the −2 state is most stable on real Si(1 1 1) surfaces for which the Fermi level lies near the middle of the band gap.     

Density functional theory investigation of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0)

The adsorption of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0) has been investigated using density functional theory (DFT). While experimental studies of CN on Cu(1 1 1) show the molecular axis to be essentially parallel to the surface, the normally-preferred DFT approach using the generalised gradient approximation (GGA) yields a lowest energy configuration with the C-N axis perpendicular to the surface, although calculations using the local density approximation (LDA) do indicate that the experimental geometry is energetically favoured. The same conclusions are found for CN on Ni(1 1 1); on both surfaces bonding through the N atom is always unfavourable, in contrast to some earlier published results of ab initio calculations for Ni(1 1 1)/CN and Ni(1 0 0)/CN. The different predictions of the GGA and LDA approaches may lie in subtly different relative energies of the CN 5σ and 1π orbitals, a situation somewhat similar to that for CO adsorbed on Pt(1 1 1) which has proved challenging for DFT calculations. On Ni(1 0 0) GGA calculations favour a lying-down species in a hollow site in a geometry rather similar to that found experimentally and in GGA calculations for CN on Ni(1 1 0).     

Morphological evidence for surface pre-melting on Si(1 1 1)

We present what we believe to be the first morphological evidence for the occurrence of surface pre-melting on the Si(1 1 1) surface. Our results complement the extensive previous evidence from diffraction and ion scattering techniques for the presence of pre-melted (liquid-like) layers on Si(1 1 1) below the bulk melting temperature and also suggest how atomic steps are involved in the initiation of such layers. Our results are based on atomic force microscopy studies of morphologies that are preserved when surfaces are annealed in a range of high temperatures and then rapidly cooled to room temperature for observation. A unique feature of the experiments is the use of specially prepared atomically flat or very low step density surfaces; this allows us to see how the liquid-like morphologies are associated with the steps and also allows the high temperature structures to survive the cooling process without being absorbed into the steps which normally would exist on a surface vicinal to (1 1 1). Quenched-in structures ascribed to pre-melting also act as sinks for diffusing ‘excess’ adatoms generated by the (1 × 1) to (7 × 7) transition and this leads to the formation of dendritic islands.     

Theoretical and experimental studies of the structure and dynamics of the CaF2(1 1 1) surface

The structure and dynamics of the CaF₂(1 1 1) surface were investigated by means of low-energy electron diffraction (LEED) and molecular dynamics (MD) simulations at 300 K. LEED beam intensities were recorded as a function of electron energy and were analyzed with the tensor LEED approach. Positions as well as mean square amplitudes of the ions in the first layers were fitted to the experimental I(E) curves. According to both LEED and MD, the CaF₂(1 1 1) surface structure is similar to the bulk-terminated structure with only small relaxation of the outermost ions. Moreover, both methods show an enhancement of vibrational amplitudes in the outermost F-Ca-F triple layer.     

First-principles study on field evaporation of surface atoms from W(0 1 1) and Mo(0 1 1) surfaces

The simulations of field-evaporation processes for surface atoms on W(0 1 1) and Mo(0 1 1) surfaces are implemented using first-principles calculations based on the real-space finite-difference method. The threshold values of the external electric field for evaporation of the surface atoms, which are ∼6 V/Å for tungsten and ∼5 V/Å for molybdenum, are in agreement with the experimental results. While the threshold value of the electric field and the local-field enhancement around the evaporating atoms agree with those expected from the conclusion of the previous study using structureless jellium, the induced charge around the surface atom has a significant difference from that obtained by the jellium model.     

The interaction of Fe on MgO(1 0 0) surfaces

The atomic interaction and magnetic properties of ultrathin Fe films grown on cleaved and polished MgO(1 0 0) surfaces were studied by conversion electron Mössbauer spectroscopy (CEMS). ⁵⁷Fe layers were deposited as probe atoms in different layer positions in 10 ML thick Fe films. Fe layers of different thicknesses were formed on polished and cleaved substrate surfaces at RT deposition. The analysis of the spectra showed no Fe-O^2- interaction in MgO/Fe interface. FeO phase formation was excluded. The Mössbauer spectrum of 5 ML ⁵⁷Fe sample showed enhanced internal magnetic field at 80 K. No interdiffusion of ⁵⁷Fe and ⁵⁶Fe atoms was observed between the layers at room temperature.     

Vibrational entropy-driven dealloying of Mo(1 0 0) and W(1 0 0) surface alloys

The formation and stability of Cu, Ag and Au-induced c(2 × 2) alloys at the Mo(1 0 0) and W(1 0 0) surfaces have been investigated with low-energy electron microscopy and diffraction. The ordered alloys transform to disordered overlayer structures at elevated temperature. Comparison of the transformation temperatures with energetics obtained from first principles calculations reveals the vibrational entropic contribution to the system free energy that defines alloy thermal stability. Effective Debye temperatures for metal adatoms are determined that exhibit the expected mass and bond strength dependence.     

Structures of CoAl(1 1 1) surface: A first principles study

The structures of the CoAl(1 1 1) surface are studied by first principles calculations. Our calculations show that the surface layer is always occupied by pure Al for all concentrations studied here, which indicates the dominant role of the Al segregation tendency. This is different from the CoAl(0 0 1) surface, where a number of Co anti-sites are found on the top most layer. The calculated surface phase diagram of ground states shows that there are three stable structures. The diffusion barriers of the metastable structure evolving to the stable structure are also calculated. The high diffusion barrier can explain the appearance of metastable structures at low temperature in experiment.     

Surface and subsurface oxide formation on Ni(1 0 0) and Ni(1 1 1)

The oxidation of Ni(1 0 0) and Ni(1 1 1) at elevated temperatures and large oxygen exposures, typical of the methods used in the preparation of NiO(1 0 0) films for surface studies, has been investigated by medium energy ion scattering (MEIS) using 100 keV H⁺ incident ions. Oxide film growth proceeds significantly faster on Ni(1 1 1) than on Ni(1 0 0), but on both surfaces oxide penetration occurs to depths significantly greater than 100 Å with total exposures of 1200 and 6000 L respectively. The metal/oxide interface is extremely rough, with metallic Ni extending to the surface, even for much thicker oxide films on Ni(1 1 1). On Ni(1 1 1), NiO growth occurs with the (1 0 0) face parallel to the Ni(1 1 1) surface and the close-packed 〈1 1 0〉 directions parallel. On Ni(1 0 0) the MEIS blocking curves cannot be reconciled with a single orientation of NiO(1 0 0) (with the 〈1 1 0〉 directions parallel) on the surface, but is consistent with the substantial orientational disorder (including tilt) previously identified by spot-profile analysis LEED.     

Faceting of nanoscale fingers on the (1 1 1) surface of gold

Gold fingers, one atomic layer (0.25 nm) high, 4-5 nm wide, and several hundred nm long are formed on the (1 1 1) surface of gold at room temperature by a combination of atomic manipulation and surface self-organisation. Each finger has two parallel edges (type A and type B, respectively) running along its length. The type A step is found to have higher step energy and become nanofaceted when disturbed by either thermal energy or the electric field under the STM tip, leading to the transformation of fingers to “nano-knives”. Our findings reveal the important role of step energy in the process of nanostructure fabrication on surfaces. The gold fingers also provide an ideal system for the investigation of meta-stable nanostructures.