Wave scattering in layers with rigid boundaries and lateral coupling through small windows

Wave propagation in weakly coupled acoustic layers with rigid boundaries is studied. The method of matching the asymptotic expansions of the solutions to boundary-value problems is used. Directional patterns are constructed for different positions of the coupling windows and different directions of wave incidence.     

Diffusion of flux lines in a random potential

A liquid of flux lines in a random superconductor is considered. We find spontaneous breaking of a continous internal symmetry from averaging over the random potential. A corresponding Goldstone mode describes diffusion of the flux lines. The diffusion coefficient vanishes if we exceed a critical strength of randomness. This is accompanied by a divergent correlation length. An interpretation of the behavior is a freezing transition of the flux liquid into a glass-like state.     

Diffusion and relaxation dynamics in cluster crystals

For a large class of fluids exhibiting ultrasoft bounded pair potentials, particles form crystals consisting of clusters located in the lattice sites, with a density-independent lattice constant. Here we present an investigation on the dynamic features of a representative example of this class. It is found that particles can diffuse between lattice sites, maintaining the lattice structure, through an activated hopping mechanism. This feature yields finite values for the diffusivity and full relaxation of density correlation functions. Simulations suggest the existence of a localization transition which is avoided by hopping and a dynamic decoupling between self- and collective correlations.     

Diffusion through a polycrystalline thin film

Analytical solutions to Fick’s second law of diffusion have been simultaneously derived without the restrictions of parabolic profiles along the x-axis in grain boundaries and expressed in a series for both grain interior and grain boundary diffusing through a polycrystalline thin film. The analysis takes segregation of diffusion species at grain boundaries into account. The analytic solutions lead to the concentration profiles in the grain interior and in the grain boundary, to the average-integrated amount of diffusion species at the exit surface, and to the time lag, which can be technologically used for depth profile studies and kinetic accumulation measurements.     

The frequency and width of a surface plasmon in a small cluster

The dependence of the frequency and width of a surface plasmon ω_s on the cluster size is analyzed. In the process of the investigation, a large numerical parameter appears that determines the shape of the potential of the electromagnetic field inside the cluster and leads to a wide plateau on the dependence of ω_s on the cluster radius.     

Creation of spectral bands for a periodic domain with small windows

We consider a Schrödinger operator in a periodic system of strip-like domains coupled by small windows. As the windows close, the domain decouples into an infinite series of identical domains. The operator similar to the original one, and defined on one copy of these identical domains, has an essential spectrum. We show that once there is a virtual level at the threshold of this essential spectrum, the windows turn this virtual level into the spectral bands for the original operator. We study the structure and the asymptotic behavior of these bands.     

Producing cluster states in charge qubits and flux qubits

We propose a method to efficiently generate cluster states in charge qubits, both semiconducting and superconducting, as well as flux qubits. We show that highly entangled cluster states can be realized by a "one-touch" entanglement operation by tuning gate bias voltages for charge qubits. We also investigate the robustness of these cluster states for nonuniform qubits, which are unavoidable in solid-state systems. We find that quantum computation based on cluster states is a promising approach for solid-state qubits.     

Diffusion of small molecules in polymers

A very large amount of information is available in the open literature on the diffusion, or transport, of small molecules in polymers. A comprehensive discussion of this field would require an amount of space exceeding by far the confines of the present monograph. It is to be hoped that this herculean but extremely useful task will be undertaken by some enterprising investigator, or investigators, in the not-too-distant future. In the meantime, a review is presented here of some of the important and more recent work on the diffusion of small molecules in both rubbery and glassy polymers. The main purpose of the review is to outline the thrust of this work and thus provide a perception of possible new developments in the field.     

Diffusion of small penetrant molecules in polybutadienes

The diffusion coefficient D in the dilute limit for three different penetrants – oxygen, water, and methanol – in three different conformations of polybutadiene (all cis-1,4, all trans-1,4, and a random copolymer containing 50% trans-1,4, 40% cis-1,4, and 10% vinyl-1,2 repeat units) has been computed using molecular dynamics simulations for temperatures in the range T?=?300–400?K. Simulation runs of 25 and 50?ns were made for each of the 45 combinations of penetrant, conformation, and temperature studied. Over this temperature range the density of the all-cis-1,4 conformation is higher than that of the all-trans-1,4 and random copolymer conformations, which are approximately equal. For all three conformations, D for oxygen and water are comparable and larger than that of methanol. However for a given penetrant, strong differences were observed in the rate of increase of D for the three conformations. We find that the activation barriers for the three penetrants are generally between 20 and 30?kJ?mol^?1, in agreement with experimental results. The magnitude of the activation energy is directly proportional to the size, rather than the mass, of the penetrant molecule.     

Collective excitations in small carbon cluster

We apply the time-dependent local density approximation to the small carbon clusters. The carbon clusters of chain and ring shapes are found to show strong π-electron transitions. An interpretation is given for them as the one-dimensional plasmon excitation. Presented by K. Yabana at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997.     

Resonant tunneling of a few-body cluster through repulsive barriers

A model for quantum tunnelling of a cluster comprised of A identical particles, interacting via oscillator-type potential, through short-range repulsive barrier potentials is introduced for the first time in symmetrized-coordinate representation and numerically studied in the s-wave approximation. A constructive method for symmetrizing or antisymmetrizing the (A ? 1)-dimensional harmonic oscillator basis functions in the new symmetrized coordinates with respect to permutations of coordinates of A identical particles is described. The effect of quantum transparency, manifesting itself in nonmonotonic resonance-type dependence of the transmission coefficient upon the energy of the particles, their number A = 2, 3, 4 and the type of their symmetry, is analyzed. It is shown that the total transmission coefficient demonstrates the resonance behavior due to the existence of barrier quasi-stationary states, embedded in the continuum.     

Plume propagation through a buffer gas and cluster size prediction

A mixed-propagation picture for the expansion of an ablation plume through a buffer gas at different pressures is proposed after a brief discussion of the available analytical expansion models. Using parameter values deduced by the model the size of carbon clusters formed during plume propagation in helium is evaluated and compared with experimental data. Starting from experiments on silver plume propagation through molecular oxygen the size of silver clusters is predicted.     

Deuteron flux production in a small high-voltage high-current diode with pulsed magnetic insulation

Technical Physics - The results of new studies on the production of accelerated deuteron fluxes in a small ion diode with pulsed magnetic insulation of electrons have been presented. A plasma anode...     

AunPt-阴离子小团簇的量子化学研究

在相对论有效势(RECP)近似下,用基于密度泛函理论(B3LYP)的从头计算方法和LANL2DZ基组,系统研究了阴离子团簇AunPt-(n=1-5)g能的几何构型和电子态,确定了低能量异构体.结果表明.AunPt-团簇的剩余电子改变了中性团簇AunPt的稳定结构,这种影响随团簇体积的增大而减小.     

分子动力学方法研究金属Ti中He小团簇的迁移

采用分子动力学方法模拟了不同温度下He原子及He团簇在金属Ti中的迁移特性,并计算了扩散前系数和激活能. 研究发现这种扩散的各向异性非常显著,具体表现在He原子及He团簇在不同方向上扩散系数的前系数完全不同,但它们的激活能却相同. 研究表明:在预测金属中He的扩散行为时,必须采用动态模拟方法才能得到准确的前系数,仅仅考虑势垒的静态模拟方法是不行的. 另外,还发现一个不同于直觉的现象,即较低温度下He二聚物的扩散系数比单个He原子的扩散系数大;此外,在所模拟的温度范围内Arrhenius方程能够很好地描述它们的扩散. 这说明动力学模拟对预测金属中He的扩散行为具有重要的意义. 关键词： 分子动力学 扩散系数 各向异性 氦     

Strong electron tunneling through a small metallic grain

Electron tunneling through mesoscopic metallic grains can be treated perturbatively only under the conduction that the tunnel junction conductances are sufficiently small. If that is not the case, fluctuations of the grain charge become strong. As a result (i) the contributions of all—including high energy—charge states become important, and (ii) the excited charge states become broadened and essentially overlap. At the same time, the grain charge remains discrete and the system conductance e-periodically depends on the gate charge. We develop a non-perturbative approach which accounts for all these features and calculate the temperature-dependent conductance of the system in the strong tunneling regime at different values of the gate charge. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 953–958 (25 June 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Mesoscopic transport through toroidal carbon nanotubes threaded with a THz magnetic flux

We have investigated the quantum transport through mesoscopic systems with a toroidal carbon nanotube coupled with two metal leads (N-TCN-N) threaded with an ac magnetic flux. The energy shifting takes place by applying the magnetic flux, and this shifting arises from both the dc and ac components of magnetic flux. The dc magnetic flux induces the periodic variation of energy gap E _g of the TCN, and the ac magnetic flux component always increases the energy gap. As the photon energy is larger than the energy gap , the electrons in the valence band can jump to the conductance band at zero temperature, and the tunneling current appears for , ( ). The differential conductance and tunneling current display clear effect of ac flux by modifying the current oscillation structures. The photon-assisted tunneling current exhibits stair-like I-V characteristics, and it shows different behaviors for different TCN systems. The magnitude of the current is suppressed by the applied ac flux. We also present the time-dependent current evolution, which is contributed by the oscillating current components.Received: 31 May 2004, Published online: 3 August 2004PACS: 73.40.-c Electronic transport in interface structures - 73.63.Fg Nanotubes - 73.61.Wp Fullerenes and related materials - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals