Calculation of core-level-shifts from an alternative ionicity scale

Shevchik et al.^1,2 proposed an electrostatic model relating charge transfer between ions and chemical shifts in ionic compounds. The charge transfer can be calculated from the ionicity. If one uses Phillips ionicities one obtains the wrong sign for the calculated core-level-shifts of copper halides as it is pointed out by Shevchik et al.^1,2. We show that the shifts can be computed fairly successfully if one uses an alternative definition of ionicity recently proposed⁴. Using this definition it is further shown that the charge transfer is identical with the longitudinal (Callen) charge of a crystal.     

Ag-Co clusters deposition on Ag(100): an atomic scale study

The slowing down of Co₁₀Ag₁₉₁ and Co₂₈₅Ag₃₀₁ nanoclusters on a Ag (100) surface is studied at the atomic scale by means of classical Molecular Dynamics simulations. The slowing down energy, 0.25 to 1.5 eV/atom, is characteristic of low energy cluster beam deposition and aerosol focused beam techniques. The two clusters differentiate by their size, stoechiometry and structure. While Co forms one or several groups just beneath the cluster surface in Co₁₀Ag₁₉₁, Co₂₈₅Ag₃₀₁ displays a core-shell structure where Ag forms one complete monolayer around the Co core. As a consequence of the impact, the smallest cluster undergoes deep reorganization and becomes fully epitaxial with the substrate. The larger one only undergoes partial accommodation and partially retains the memory of its initial morphology. For both, after impact, the Co forms one group covered by Ag. The substrate damage is significant and depends on the slowing down energy. It results in a Ag step surrounding the cluster which may be more than one atomic layers high and isolated add-atoms or small monolayer islands apart from the step. The latter originate from the cluster and the former from the substrate. Further details in the consequences of the impact are given, concerning the cluster penetration, its deformation and lattice distortions, with emphasis on the cluster size and stoechiometry.Received: 11 June 2004, Published online: 31 August 2004PACS: 35.40.-c - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 07.05.Tp Computer modeling and simulation     

Prospects for supersymmetric SO(10) models with an intermediate mass scale

We investigate possible patterns of SO(10) gauge symmetry breaking compatible with supersymmetry, limiting ourselves to the cases with one intermediate breaking scale. It is found that the one where a 54 representation breaks SO(10) into a Pati-Salam group SU(4)_C×SU(2)_L×SU(2)_R and the one where a 210 breaks it into SU(3)_C× U(1)_C×SU(2)_L×SU(2)_R are the most preferable patterns when supersymmetry is taken into account. Two models with the Pati-Salam intermediate symmetry are studied in more detail.     

Buried Ni/Cu(001) interface at the atomic scale

We present a quantitative surface x-ray analysis of the buried Ni/Cu(001) interface structure after deposition of 3 and 5 monolayers of Ni at room temperature. Interface mixing is found where 27+/-10% of top layer Cu atoms are exchanged by Ni. Atomic scale simulations reveal a kinetic pathway for the Ni/Cu-exchange process and explain the observed limited degree of intermixing. A disperse distribution of Ni within the Cu surface with a preferential Ni-Ni separation of 3-4 nearest neighbor distances is determined.     

Calculation of the atomic radius from shielding considerations

A simple empty core Thomas Fermi pseudopotential is used to calculate the value of the radius of the atom in the simple and transition metals. In a given atom the extent of the atomic core is taken equal to the distance from the nucleus at which the outermost node in the wave function of an s valence electron occurs. It is found that this parameter is simply related to the depth of the potential well for s electrons obtained by the model potential authors for the simple and transition metals.     

Coexistence of two different Peierls distortions within an atomic scale wire: Si(553)-Au

The ground state property of a Au-induced atomic wire array on a stepped Si(553) surface with interesting 1D metallic bands was investigated. Electron diffraction and scanning tunneling microscopy reveal an intriguing coexistence of triple- and double-period lattice distortions at low temperature. Angle-resolved photoemission observes both the nearly 1/3- and 1/2-filled bands to gradually open energy gaps upon cooling. We explain these unusual findings as due to the occurrence of Peierls distortions of triple and double periods on the two different atomic-scale chain elements, respectively, within a single unit wire. The two Peierls distortions are suggested to have different transition temperatures and little lateral correlation between each other.     

Computer simulation of the quenched condensation of an atomic system

The quenched condensation of an atomic system, consisting of 500 Lennard-Jones particles, is simulated by using the method of molecular dynamics. The particles are deposited onto a plane substrate with a smooth adhesion potential. The cooling is reached by reducing the velocities of those particles situated near the substrate. At the end of the quenching process the system exists at low temperatures in the amorphous state. At higher temperatures the system changes into the stable fcc lattice. The application of the Monte Carlo method does not result in a crystallization of the amorphous configuration. During the relaxation of the amorphous state at low temperatures, a slight structural change towards the fcc lattice takes place, indicating that the amorphous configuration of the Lennard-Jones system is less stable against crystallization.     

Graphene on Ru(0001): contact formation and chemical reactivity on the atomic scale

Graphene on Ru(0001) is contacted with Au tips of a cryogenic scanning tunneling microscope. The formation and conductance of single-atom contacts vary within the moiré unit cell. Density functional calculations reveal that elastic distortions of the graphene lattice occur at contact due to a selectively enhanced chemical reactivity of C atoms at hollow sites of Ru(0001). Concomitant quantum transport calculations indicate that the graphene-Ru distance determines the conductance variations.     

HREM study of the YBCO/MgO interface on an atomic scale

The film-substrate interface of c oriented YBCO thin films grown by sputtering or laser ablation on (001) MgO substrate has been investigated with high-resolution electron microscopy. The first atomic plane of the YBCO lattice is a CuO chain layer. Two interface configurations occur: (1) the YBCO lattice and the MgO lattice continue up to the interface (this configuration is occasionally associated with some periodic strain in the MgO lattice; (2) the YBCO lattice and the MgO lattice are separated by an (almost) amorphous layer with a thickness of the order of two atomic layers. This amorphous layer is found to lead to the absence of strain. In some cases the surface roughness coincided with misoriented grains but most of the steps in the MgO substrate were accommodated by steps in the YBCO of one or more complete unit cells in height and some lattice bending in the YBCO film.     

Calculation of the viscosity of SU(2) gluodynamics with the lattice simulation

The viscosity of SU(2) gluodynamics within the simulation of the lattice quantum chromodynamics at a temperature of T/T _c = 1.2 has been calculated with the Kubo formula relating the viscosity to the spectral function of the correlation function of the energy-momentum tensor. The correlation function of the energy-momentum tensor has been calculated using the numerical simulation of the lattice SU(2) gluodynamics on supercomputers.     

Distinguishability ofSU(2)×U(1)×U′(1) andSO(10) gauge models from the Weinberg-Salam model

The predictions ofSU(2)×U(1)×U′(1) andSO(10) gauge models for the asymmetry parametersA-,B-,C _L andC _R in the deep inelastic scattering of polarized electrons and positrons by unpolarized protons and deuterons are compared with those calculated in the Weinberg Salam model for different values ofy. The model based on,SU(2)×U(1)×U′(1) group has been found almost indistinguishable from the Weinberg Salam model with regard to the parametersA-,B- andC _L (except forB- in the region 0≦y≦0.2) althoughC _R exhibits marked distinguishability. TheSO(10) model, for certain choice of its model parameters, can be distinguished from the Weinberg Salam model through measurement of the asymmetry parameters for different values ofy.     

Evaluation of the magnetic dipolar fields from layered systems on atomic scale

Exact analytical expressions for the magnetic dipolar fields produced by a plane square lattice with localized magnetic moments are derived. Basing on these expressions surface roughness of a magnetic thin film is considered. Influence of the bipolar fields on the surface anisotropy and hyperfine fields is discussed.     

Remarks on (0,2) models and intermediate scale scenarios in string theory

Problems associated with large intermediate scales in string theory are discussed. It is shown that, in σ-model perturbation theory, the superpotential of theories with (2,2) supersymmetry is purely cubic. As a result, to exhibit flat directions with (0,2) supersymmetry, one needs to study only a small number of Yukawa couplings. Usually these flat directions are lifted by nonperturbative effects in the σ model. Examples where this is not the case, due to Cvetic and to Distler and Greene, are discussed. It is shown that in the former case, there are not, generically, extra massless particles to play the role of Higgs fields. The problems of proton decay, neutrino masses, and obtaining light Higgs fields in models with large intermediate scales are considered, and various difficulties with existing proposals are pointed out.     

Control of conductance in a quasi-1D channel in the presence of spin-orbit coupling

The electron energy spectrum and conductance in a quasi-1D channel with spin-orbit coupling is calculated. The channel is divided into two parts by a potential barrier. It is shown that a change in the barrier electric potential yields additional extema of the function ?(k). This results in modification of the conductance quantization law. The quantum states and conductance are calculated for the parameters of real structures with weak both and relatively strong spin-orbit coupling.     

Efficient channel of the associative oxygen desorption from Pt(1 1 1)

Mechanism of the associative desorption of oxygen from the Pt(1 1 1) surface has been studied on atomic level by means of density-functional calculations. Key to the association of two oxygen adatoms into the O₂ molecule is the excitation of one of the adatoms to on-top site, where it becomes essentially neutral. The related redistribution of the electronic density at the O adatom leads to the appearance of the lateral attraction with the other O atom, thus providing an efficient channel for associative desorption. Calculated local densities of states resemble the transformation of the electronic structure of adsorbed O adatoms from the reactive to bound state in the course of association.     

Study of an order-disorder phase transition on an atomic scale: the example of decagonal Al-Ni-Co quasicrystals

In the present paper we succeeded in studying structural phase transitions from an atomistic point of view by positron annihilation Doppler broadening. This differs and is complementary to conventionally used diffraction experiments with large coherence lengths. In the exemplary case of the 1140 K order-disorder transition in decagonal Al71.5Ni14Co14.5 quasicrystals the importance of this atomistic approach and its wide scope of application is demonstrated.