International Journal of Modern Physics B

The deposition of single Si ad-dimer on the p(2×2) reconstructed Si(100) surface has been simulated by an empirical tight-binding (ETB) method. Using the clean and defective Si surfaces as the deposition substrates, the deposition energies are mapped out around the clean surface and dimer vacancies. From the calculated energy plots, the binding sites and several possible diffusion paths are achieved. It is easy for the ad-dimer to form stable state A1 and metal stable state A2 on the clean surface. The change from A1 to A2 state appears as the rotation of the ad-dimer, which has been observed experimentally. The ad-dimers are found to prefer filling into the missing dimer vacancies. Finally, the diffusion traces of the ad-dimers on the surfaces have been simulated.

Absorption spectra of Si_0.6Ge_0.4/Si quantum wells are characterized by photocurrent measurements. The absorption coefficients of two different transitions, namely the transition between the Si band states and the discrete energy level in quantum wells, and the interlevel transition in quantum wells are deduced. They are directly proportional to (ℏω-ΔE)^3/2 and δ(ℏω-E_eh), respectively. The valence band offsets of Si_0.6Ge_0.4/Si interface are 297 meV. The ground state energy levels in valence band and conduction band Si_0.6Ge_0.4/Si quantum wells are 37 meV and 23 meV, respectively.

A method for interpreting elastic-lidar return signals in heavily-polluted atmospheres is presented. It is based on an equation derived directly from the classic lidar equation, which highlights gradients of the atmospheric backscattering properties along the laser optical path. The method is evaluated by comparing its results with those obtained with the differential absorption technique. The results were obtained from locating and ranging measurements in pollutant plumes and contaminated environments around central México.

Effects of finite spatial size of boson assemblies in traps are studied in a self-consistent lattice theory by modeling the trap as a bounded harmonic potential of size R₀. The thermodynamic quantities exhibit scaling and crossover from ideal gas behaviour at small (R₀/a₀) to that appropriate to an unbounded harmonic potential at large (R₀/a₀) with a crossover parameter , a₀ being the harmonic oscillator length, and τ denoting the dimensionless thermal energy. The numerical results obtained earlier by computing the energy levels of the bounded harmonic oscillator fit the general structure predicted by the theory very well. For a₁>10, the spatial size effects are negligible but for a₁<10 they become appreciable and experimentally measurable in suitably designed traps. At low temperatures the self consistent cell size is found to be about 2.5a₀ implying that the condensate is essentially a single coherent state contained in the central cell.

Robot arm positioning is an important factor in the robotic process. However, the robot manipulator experiences positioning inaccuracies. This positioning error is due to the dynamic inefficiencies of its actuator: DC servomotor. In a bid to resolve this actuator problem, an electro-rheological (ER) clutch-brake mechanism is employed. This clutch-brake mechanism can actuate and halt the motion of the robot arm. This rotary mechanism consists of two similar clutches that are driven to rotate in the opposite directions and an individual ER brake that provides braking torques to halt the manipulator at the required positions. The main aim of this paper is to establish a control strategy for the ER actuated robot arm by means of model validation with the experimental results. This study is conducted to understand the ER robotic positioning control for future applications.

Thin films of pure silver were deposited on glass substrate by thermal evaporation process at room temperature. Surface characterization of the films was performed using X-ray diffraction (XRD) and atomic force microscopy (AFM). Thickness of the films varied between 20 nm and 72.8 nm. XRD analysis provided a sharp peak at 38.75° from silver. These results indicated that the films deposited on glass substrates at room temperature are crystalline. Three-dimension and top view pictures of the films were obtained by AFM to study the grain size and its dependency on various factors. Average grain size increased with the thickness of the deposited films. A minimum grain size of 8 nm was obtained for 20 nm thick films, reaching 41.9 nm when the film size reaches 60 nm. Grain size was calculated from the information provided by the XRD spectrum and averaging method. We could not find any sequential variation in the grain size with the growth rate.

The power spectrum of quantum fluctuations of the electromagnetic field produced by an elementary particle is determined. It is found that in a wide range of practically important frequencies the power spectrum of fluctuations exhibits an inverse frequency dependence. The magnitude of fluctuations produced by a conducting sample is shown to have a Gaussian distribution around its mean value, and its dependence on the sample geometry is determined. In particular, it is demonstrated that for geometrically similar samples the power spectrum is inversely proportional to the sample volume. It is also argued that the magnitude of fluctuations induced by an external electric field is proportional to the field strength squared. A comparison with experimental data on flicker noise measurements in continuous metal films is made.

Transparent mesoporous TiO₂ films consisting of uniform anatase particles have been deposited on glass slides by using sol-gel technique. A Europium complex Eu(TTA)₃(bpy), where the TTA ligand is β-diketonate thenoyltrifluoroacetonate and the bpy is 2,2-bipyridine, was formed on TiO₂ surface by physical adsorption of europium ions and successive formation of stable chelate complexes with TTA and bpy ligands. In this way a well dispersion of the complex has been achieved giving significant emission intensity. The excitation of the material in the UV region (374 nm) results in the emission of a nearly monochromatic light at 613 nm (⁵D₀→⁷F₂) via energy transfer from the excited organic ligands to europium emissive state.