Metallicity of atomic wires

The variety of atomic “dimensional” wires can now be synthesized on furrowed and stepped surfaces. These adlayers provide a variety of opportunities for systematically tailoring the surface properties. One of key issue is the metallicity of an atomic wire (even a “supported” atomic wire). Monte-Carlo simulations provide insight into the parameters of indirect interaction that are the basis for the formation of the atomic wires and their stability. In some cases, these results can be directly compared with density functional theory (DFT) calculations of energies of the lateral interactions between adsorbed atoms—one of the most transparent example of Sr/Mo(1 1 2) is presented here as well. It is the surface band structure calculations that provide insights on how metallicity in such surface structures might be altered.Surprisingly, like most of “metallic” wires on semiconductor surfaces, linear chains of alkaline earth on the furrowed transition metal surfaces, such as the Mo(1 1 2) surface, also do not exhibit strong metallic character but, rather, may be considered dielectric atomic chains. The adsorption bonds result in a loss in electron itinerancy, leading to greater valence electron localization in the adlayer in some cases. The localized character of the bands near the Fermi level, associated with the adlayer, is replaced by a metallic band structure when the lattice period of the adsorbed layer becomes incommensurate with the substrate periodicity along the furrows with increasing coverage of the adlayer. With changes in adlayer coverage, both theory and experiment indicate that the adsorbed layers can undergo a Wilson type nonmetal-to-metal transition.     

Current-induced embrittlement of atomic wires

Recent experiments suggest that gold single-atom contacts and atomic chains break at applied voltages of 1 to 2 V. In order to understand why current flow affects these defect-free conductors, we have calculated the current-induced forces on atoms in a Au chain between two Au electrodes. These forces are not by themselves sufficient to rupture the chain. However, the current reduces the work to break the chain, which results in a dramatic increase in the probability of thermally activated spontaneous fracture of the chain. This current-induced embrittlement poses a fundamental limit to the current-carrying capacity of atomic wires.     

Magnetism of Zr atomic wires

By using the full-potential linearized augmented plane wave method to perform ab initio total energy calculations, we have explored magnetic ordering in one-dimensional Zr wires. The result shows that Zr can form linear, or dimerized, or zigzag wires, and the magnetic properties strongly depend on their geometric structures. The linear and zigzag wires exhibit ferromagnetic ground states at the equilibrium bonding distance, while the dimerized wire, despite its higher stability than that of the linear one, exhibits nonmagnetic ground states. The most stable geometry is shown to be the zigzag wire with a magnetic moment of 0.26μB per atom.     

Electronic conductance via atomic wires: a phase field matching theory approach

A model is presented for the quantum transport of electrons, across finite atomic wire nanojunctions between electric leads, at zero bias limit. In order to derive the appropriate transmission and reflection spectra, familiar in the Landauer-Büttiker formalism, we develop the algebraic phase field matching theory (PFMT). In particular, we apply our model calculations to determine the electronic conductance for freely suspended monatomic linear sodium wires (MLNaW) between leads of the same element, and for the diatomic copper-cobalt wires (DLCuCoW) between copper leads on a Cu(111) substrate. Calculations for the MLNaW system confirm the correctness and functionality of our PFMT approach. We present novel transmission spectra for this system, and show that its transport properties exhibit the conductance oscillations for the odd- and even-number wires in agreement with previously reported first-principle results. The numerical calculations for the DLCuCoW wire nanojunctions are motivated by the stability of these systems at low temperatures. Our results for the transmission spectra yield for this system, at its Fermi energy, a monotonic exponential decay of the conductance with increasing wire length of the Cu-Co pairs. This is a cumulative effect which is discussed in detail in the present work, and may prove useful for applications in nanocircuits. Furthermore, our PFMT formalism can be considered as a compact and efficient tool for the study of the electronic quantum transport for a wide range of nanomaterial wire systems. It provides a trade-off in computational efficiency and predictive capability as compared to slower first-principle based methods, and has the potential to treat the conductance properties of more complex molecular nanojunctions.     

Oscillatory magnetic anisotropy in one-dimensional atomic wires

One-dimensional Co atomic wires grown on Pt(997) have been investigated by x-ray magnetic circular dichroism. Strong changes of the magnetic properties are observed as the system evolves from 1D- to 2D-like. The easy axis of magnetization, the magnetic anisotropy energy, and the coercive field oscillate as a function of the transverse width of the wires, in agreement with theoretical predictions for 1D metal systems.     

Observation of a parity oscillation in the conductance of atomic wires

Using a scanning tunnel microscope or mechanically controllable break junctions atomic contacts for Au, Pt, and Ir are pulled to form chains of atoms. We have recorded traces of conductance during the pulling process and averaged these for a large number of contacts. An oscillatory evolution of conductance is observed during the formation of the monoatomic chain suggesting a dependence on the numbers of atoms forming the chain being even or odd. This behavior is not only observed for the monovalent metal Au, as was predicted, but is also found for the other chain-forming metals, suggesting it to be a universal feature of atomic wires.     

Mechanism of gap opening in a triple-band Peierls system: in atomic wires on Si

One dimensional (1D) metals are unstable at low temperature undergoing a metal-insulator transition coupled with a periodic lattice distortion, a Peierls transition. Angle-resolved photoemission study for the 1D metallic chains of In on Si(111), featuring a metal-insulator transition and triple metallic bands, clarifies in detail how the multiple band gaps are formed at low temperature. In addition to the gap opening for a half-filled ideal 1D band with a proper Fermi surface nesting, two other quasi-1D metallic bands are found to merge into a single band, opening a unique but k-dependent energy gap through an interband charge transfer. This result introduces a novel gap-opening mechanism for a multiband Peierls system where the interband interaction is important.     

Onset of energy dissipation in ballistic atomic wires

Electronic transport at finite voltages in free-standing gold atomic chains of up to seven atoms in length is studied at low temperatures using a scanning tunneling microscope. The conductance vs voltage curves show that transport in these single-mode ballistic atomic wires is nondissipative up to a finite voltage threshold of the order of several mV. The onset of dissipation and resistance within the wire corresponds to the excitation of the atomic vibrations by the electrons traversing the wire and is very sensitive to strain.     

Fabrication of atomic wires based on self-organization

One-dimensional chain-like structures consisting of rows of single chromium adatoms have been obtained by annealing a submonolayer chromium film deposited under ultra-high vacuum (UHV) conditions onto a Cr(100) single crystal substrate. The self-organization of the Cr atoms under the influence of the substrate surface potential leads to two kinds of atomic rows orientated in orthogonal directions, thereby reflecting the symmetry of the Cr(100) surface potential. The formation of these atomic-scale wires has been monitored in-situ by means of a UHV scanning tunneling microscope (STM).     

Spectroscopic and theoretical study in substituted N-phenoxyethylanilines

Infrared spectroscopic studies, assigned with the aid of density functional calculations, and ab initio theoretical calculations of N-(2-phenoxyethyl)aniline and their derivatives allow us to have an insight into the vibrational, geometrical and electronic properties of N-(2-phenoxyethyl)aniline, N-(2-(4-nitrophenoxy)ethyl)aniline, 4-methoxy-N-(2-(4-chlorophenoxy)ethyl) aniline, 4-methoxy-N-(2-(2-nitrophenoxy)ethyl)aniline and 4-bromo-N-(2-(2-bromo-4-chlorophenoxy)ethyl)aniline. Our calculations indicate that the gauche conformation around the (CH₂)₂ chain is the lowest-energy for all the molecules.We further investigated the vibrational behavior of N-(2-phenoxyethyl)aniline, N-(2-(4-nitrophenoxyethyl)aniline and 4-methoxy-N-(2-(4-chlorophenoxy)ethyl)aniline compounds at 300 and 77 K where several peaks corresponding to normal modes associated with the lineal chain showed reinforcement and loosening of their force constants. The peak corresponding to N–H stretching mode has been found dependent on their position and intensity upon the substituents and derivatives. We find that the splitting at 3413–3381 and 3403–3374 cm⁻¹ of the N–H band in the spectra of the NO₂ derivatives cannot be associated with the existence of a conformational equilibrium, but may be assigned to a free N–H and to a NH⋯O bonded form, respectively.An analysis of the Mulliken charges on the atoms that constitute the aliphatic chain of the different compounds shows a difference on the acidic capacity of the hydrogen atom attached to the nitrogen atom.     

Inelastic scattering and local heating in atomic gold wires

We present a method for including inelastic scattering in a first-principles density-functional computational scheme for molecular electronics. As an application, we study two geometries of four-atom gold wires corresponding to two different values of strain and present results for nonlinear differential conductance vs device bias. Our theory is in quantitative agreement with experimental results and explains the experimentally observed mode selectivity. We also identify the signatures of phonon heating.     

Thermal activation in atomic friction: revisiting the theoretical analysis

The effect of thermal activation on atomic-scale friction is often described in the framework of the Prandtl-Tomlinson model. Accurate use of this model relies on parameters that describe the shape of the corrugation potential β and the transition attempt frequency f(0). We show that the commonly used form of β for a sinusoidal corrugation potential can lead to underestimation of friction, and that the attempt frequency is not, as is usually assumed, a constant value, but rather varies as the energy landscape evolves. We partially resolve these issues by demonstrating that numerical results can be captured by a model with a fitted β and using harmonic transition state theory to develop a variable form of the attempt frequency. We incorporate these developments into a more accurate and generally applicable expression relating friction to temperature and velocity. Finally, by using a master equation approach, we verify the improved analytical model is accurate in its expected regime of validity.     

Magnetic focusing of cold atomic beam with a 2D array of current-carrying wires

A new scheme to realize a two-dimensional (2D) array of magnetic micro-lenses for a cold atomic beam,formed by an array of square current-carrying wires,is proposed.We calculate the spatial distributions of the magnetic fields from the array of current-carrying wires and the magnetic focusing potential for cold rubidium atoms,and study the dynamic focusing processes of cold atoms passing through the magnetic micro-lens array and its focusing properties by using Monte-Carlo simulations and trajectory tracing method.The result shows that the proposed micro-lens array can be used to focus effectively a cold atomic beam,even to load ultracold atoms or a BEC sample into a 2D optical lattice formed by blue detuned hollow beams.     

Self-alignment of Co adatoms on in atomic wires by quasi-one-dimensional electron-gas-meditated interactions

Low-density Co atoms are found to self-align on the Si(111)-(4 x 1)-In surface in the direction of In atomic wires at incommensurate adsorption sites. Indirect interaction between a pair of Co adatoms is investigated through a site distribution function of adatoms determined with scanning tunneling microscopy. In the direction of self-alignment, the potential of the mean force between two Co adatoms is long-range and oscillatory with multiple frequencies, which correlate strongly to the electronic scattering vectors of the surface-state bands at the Fermi level. We thus attribute the Co-Co interaction to that mediated by a quasi-one-dimensional electron gas confined within the In atomic wires.     

Band-structure engineering of gold atomic wires on silicon by controlled doping

We report on the systematic tuning of the electronic band structure of atomic wires by controlling the density of impurity atoms. The atomic wires are self-assembled on Si(111) by substitutional gold adsorbates and extra silicon atoms are deposited as the impurity dopants. The one-dimensional electronic band of gold atomic wires, measured by angle-resolved photoemission, changes from a fully metallic to semiconducting one with its band gap increasing above 0.3 eV along with an energy shift as a linear function of the Si dopant density. The gap opening mechanism is suggested to be related to the ordering of the impurities.     

The effect of hydrogen on the recombination of Frenkel pair in tungsten: A theoretical insight

<正>Tungsten (W), with its primary advantages, is considered as the most promising candidate for plasma facing materials (PFMs) for the next generation of fusion devices such as ITER. However, continuous bombardment with 14.1 MeV neutron introduces Frenkel defects as the primary damage in W [1]. The Frenkel defects, composed of self-interstitial atoms (SIAs) and vacancies, can develop to extended defects such as voids and interstitial clusters, resulting in hardening, swelling and embrittlement of W, thus degrading the properties of W [2]. The recombination of SIAs and vacancies is an effective way to reduce the Frenkel defects in bulk W, which enhances the radiation resistance of W based on recent theoretical calculations [3,4]. The moving of the SIA to the vacancy could finish the recombination process through instantaneous or thermally activated way [3]. The instantaneous recombination region is an ellipse with the semi-minor axis of 5.4 ? and semi-major axis of 18 ? according to the molecular dynamics calculation [4].     

Self-assembly of parallel atomic wires and periodic clusters of silicon on a vicinal Si111 surface

Silicon self-assembly at step edges in the initial stage of homoepitaxial growth on a vicinal Si(111) surface is studied by scanning tunneling microscopy. The resulting atomic structures change dramatically from a parallel array of 0.7 nm wide wires to one-dimensionally aligned periodic clusters of diameter approximately 2 nm and periodicity 2.7 nm in the very narrow range of growth temperatures between 400 and 300 degrees C. These nanostructures are expected to play important roles in future developments of silicon quantum computers. Mechanisms leading to such distinct structures are discussed.     

Measurement of surface defects on thin steel wires by atomic force microscopy

The characterisation of surface defects on thin metallic wires is very important for the industrial applications of these wires. The physical dimensions of the surface defects presented by several thin steel wires of different diameters have been measured using Atomic Force Microscopy (AFM). The measurements made show two main defects in thin steel wires: holes and scratches, but other defects like porosity or protuberances have also been observed. We have found an empirical relationship between the physical dimensions of the scratches and the wire diameter.     

A theoretical insight into the solid-state optical properties of luminescent materials: the supermolecular approach

The way interchain interactions affect the absorption and luminescence properties of organic conjugated materials is addressed by means of correlated quantum-chemical calculations on molecular aggregates. Special emphasis is given to the influence of chain length and relative positions of the interacting units. The consideration of α -oligothiophenes in their crystalline structure allows us to shed light into the chain-length evolution of the optical splitting of the lowest optically-allowed excited state and to validate the theoretical approach by confronting the theoretical predictions to experimental results. Various strategies are proposed to prevent luminescence quenching when going from solutions to films or crystals.     

Singular value decomposition of energy matrices in theoretical atomic structure calculations

The singular value decomposition, SVD, is applied to the linear eigenvalue problem in atomic structure calculations. By comparing with recent calculations of energy levels in neutral Ca, it is shown that the SVD can give quite accurate results and much faster than normal diagonalization techniques, even of the Davidson type. However, the energy levels calculated in this approach are more strongly bound than the real eigenvalues, and this is ascribed to an artefact of the SVD, caused by the use of a discretized continuum in the calculations. This effect can lead to fairly large errors if there is strong CI present. The property of the linear eigenvalue problem that the spectrum is unchanged when a constant is added to the diagonal does not apply to the SVD. This means that it is impossible to solve the problems connected with a discretized continuum simply by shifting the spectrum.