An approach for obtaining the intensity of the radiation transmitted through a scatterer

Monte Carlo simulation was applied to the investigation of intensity of the radiation transmitted through a scatterer. Simulations consisted of a pencil beam of monoenergetic photons with energies from 50 keV to 10 meV incident on water, aluminium, iron, copper, tin and lead slabs. We determined the scattered radiation and the scatter fractions recorded in the detector plane. An empirical formula, which is a function of the physical parameters scatterer thickness, the linear attenuation coefficient, and the atomic number was obtained for intensity of radiation transmitted through a scatterer. This work was supported by the Scientific and Technical Research Council of Turkey (TUBITAK) [TBAG-2032 (101T053)].     

Finite-dimensional quantum systems: Complementarity,phase space,and all that

A complete set of d + 1 mutually unbiased bases exists in a Hilbert space of dimension d whenever d is power of a prime. We discuss a simple construction of d + 1 disjoint classes (each one having d ? 1 commuting operators) such that the corresponding eigenstates form sets of unbiased bases. One of these classes is diagonal and can be mapped to “ladder” operators by means of the finite Fourier transform. Using this idea, we naturally introduce the notion of quantum phase as complementary to the inversion. Relevant examples involving qubits and qutrits are discussed.     

Generalized parton distributions from nucleon form factor data

We present a simple empirical parameterization of the x- and t-dependence of generalized parton distributions at zero skewness, using forward parton distributions as input. A fit to experimental data for the Dirac, Pauli and axial form factors of the nucleon allows us to discuss quantitatively the interplay between longitudinal and transverse partonic degrees of freedom in the nucleon (nucleon tomography). In particular we obtain the transverse distribution of valence quarks at given momentum fraction x. We calculate various moments of the distributions, including the form factors that appear in the handbag approximation to wide-angle Compton scattering. This allows us to estimate the minimal momentum transfer required for reliable predictions in that approach to be around . We also evaluate the valence contributions to the energy-momentum form factors entering Jis sum rule.Received: 16 September 2004, Revised: 15 October 2004, Published online: 17 December 2004     

Ab initio modeling of the electronic and spin properties of the [NV]<Superscript>−</Superscript> centers in diamond nanocrystals

The electronic and spin properties of different nanocrystals of carbon are studied. The properties of these cluster systems are modeled in terms of the ab initio (Hartree-Fock) and semiempirical (PM3, AM1) quantum-chemical methods. The calculations are performed for different carbon nanocluster systems: defect-free and with [NV]^? centers, hydrogen passivated (C₃₈H₄₂, C₇₁H₈₄, C₈₆H₇₈), and with a free (unpassivated) surface (C₃₈, C₇₁, C₈₆). The spin properties of unhydrated nanoclusters were studied for the first time. The structure of all the clusters under study was optimized using the total energy minimization principle. It is shown that, in the case of hydrated carbon nanocrystals passivated by hydrogen atoms, diamond-like clusters are formed. The atomic structure of an unpassivated nanocrystal depends on the number of atoms in the cluster, as well as on its initial geometrical parameters. In some cases, clusters with a fullerene-like surface are formed. In hydrogenpassivated diamond nanocrystals with [NV]^? centers, the spin density is localized at the nuclei of C atoms nearest to the center vacancies. For the unpassivated counterparts, the spin density is localized at the nuclei of C atoms forming the surface of the corresponding nanocrystal.     

Monolayer assembly and striped architecture of Co nanoparticles on organic functionalized Si surfaces

We present a new strategy to fabricate a monolayer assembly of Br-terminated Co nanoparticles on functionalized Si surfaces by using chemical covalent bonding and microcontact printing method. Self-assembled monolayers (SAMs) of the Co nanoparticles formed on the hydroxyl-terminated Si surface exhibit two-dimensional island networks with locally ordered arrays via covalent linkage between nanoparticles and surface. On the other hand, SAMs of the nanoparticles on the aminopropyl-terminated Si surface show an individual and random distribution over an entire surface. Furthermore, we have fabricated striped architectures of Co nanoparticles using a combination of microcontact printing and covalent linkage. Microcontact printing of octadecyltrichlorosilane and selective covalent linkage between nanoparticles and functionalized Si surfaces lead to a hybrid nanostructure with selectively assembled nanoparticles stripes on the patterned functionalized Si surfaces. PACS 81.07.Ta; 61.46.+w; 81.16.Dn; 81.16.Be; 68.37.Hk; 82.80.Pv     

Molecular dynamics simulation study of the nano-wear characteristics of alkanethiol self-assembled monolayers

A molecular dynamics (MD) simulation study of the probe-based nano-lithography of an alkanethiol self-assembled monolayer (SAM) on a metal surface was performed. The motivation of this work was to understand the nano-tribological phenomena of the nano-metric scribing process of alkanethiol molecules and gain insight into the interaction between the probe tip and the SAM-coated surface during the scribing process. The simulation results revealed that the organothiol molecules were displaced and dragged by the probe tip during scribing due to the strong interchain interactions. It was also found that the scribed pattern width was largely dependent on the tip–surface interaction induced by the probe shape rather than the tip–surface contact size. Also, the minimum load for tip–substrate contact changed with the number of molecules that interact with the probe tip. Furthermore, from the investigation of the effect of the scribing speed on the surface-damage characteristics of the chain molecules, it was found that relatively high-speed scribing could induce excessive removal of the SAM molecules from the surface. PACS 02.70.Ns; 31.15.Qg; 81.16.Nd; 68.35.Af     

Tapered fiber optic surface plasmon resonance sensor for analyses of vapor and liquid phases

We report a new approach to analyze both vapor and liquid phases by utilizing a tapered fiber optic surface plasmon resonance (SPR) probe. This technique employs a fiber optic SPR probe with a modified geometry to tune the SPR coupling wavelength-angle pair. The observed composite spectrum included two distinct SPR dips associated with surface plasmons excited in the gas and liquid active regions. This sensor is able to detect refractive index changes in both vapor and liquid phases individually by simultaneous monitoring SPR coupling wavelengths from the two sensing surfaces.     

The emission wavelength tuning of InAs/InP quantum dots with thin GaAs, InGaAs, InP capping layers by MOCVD

InAs quantum dots (QDs) were grown on InP substrates by metalorganic chemical vapor deposition. The width and height of the dots were 50 and 5.8 nm, respectively on the average and an areal density of 3.0×10¹⁰ cm⁻² was observed by atomic force microscopy before the capping process. The influences of GaAs, In_0.53Ga_0.47As, and InP capping layers (5–10 ML thickness) on the InAs/InP QDs were studied. Insertion of a thin GaAs capping layer on the QDs led to a blue shift of up to 146 meV of the photoluminescence (PL) peak and an InGaAs capping layer on the QDs led to a red shift of 64 meV relative to the case when a conventional InP capping layer was used. We were able to tune the emission wavelength of the InAs QDs from 1.43 to 1.89 μm by using the GaAs and InGaAs capping layers. In addition, the full-width at half-maximum of the PL peak decreased from 79 to 26 meV by inserting a 7.5 ML GaAs layer. It is believed that this technique is useful in tailoring the optical properties of the InAs QDs at mid-infrared regime.     

Mobility of electrons and holes in an n-type organic semiconductor perylene diimide thin film

N,N′-diphenylbutyl-3,4,9,10-perylenebiscarboximide (PTCDI-C4Ph) were characterized by optical and electrochemical methods. A device with an ITO/PTCDI-C4Ph (≈2 μm)/Al structure was fabricated to measure mobility by time-of-flight techniques. This vacuum deposited organic layer was an amorphous state. Electrons were observed faster than holes. The electron and hole mobilities were 1.8 × 10⁻⁴ cm²/V s and 1.1 × 10⁻⁴ cm²/V s under the electric field of 500 (V/cm)^1/2, respectively. This result shows that this organic compound is a good candidate for an n-type conduction.     

Three-dimensional modelling and dynamic analysis of an automatic ball balancer in an optical disk drive

Dynamic behaviours and stability of an automatic ball balancer (ABB) in an optical disk drive are analyzed based on the proposed three-dimensional dynamic model. For dynamic analysis, the feeding deck with the ball balancer and a spindle motor is modelled as a rigid body with six degrees of freedom. The nonlinear equations of motion are derived using Lagrange's equation in order to describe the translational and rotational motions of the system. From the derived nonlinear equations, the linearized equations of motion in the neighbourhood of a balanced equilibrium position are obtained by the perturbation method. These equations are coupled, linear, differential equations with time-dependent periodic coefficients, from which the stability of the system is analyzed by using the Floquet theory. Finally, the time responses are computed to verify the results of the stability analysis, and to investigate the balancing performance of the ABB.