Optimization of two parallel wires of laser plasma filaments for channeling electromagnetic energy

We theoretically investigated the electromagnetic wave (EMW) transmission along two parallel wires of laser plasma filaments produced by the filamentation of ultrafast laser pulses in air. Many factors, such as wire diameter and separation, electron density, and operating frequency are shown to influence the propagation loss. By taking into consideration the radiation and transmission effects of the wires, the calculations of the two parallel filament wires reasonably agrees with that of the standard commercial twin-lead wire. Specifically, the optimum separation of the two wires is determined for a given frequency and an effective electron density of the wires. When compared with free-space propagation, transmission enhancement of tens dB is obtained using optimized wire configurations. Thus, the two plasma wires may be a potential channel for point to point directed delivery of EM energy or communication of pulsed-modulated EM radiation.     

Saturation of optical transitions in quantum dots and wires of porous silicon

The bleaching bands have been observed in the time-resolved nonlinear transmission spectra of porous silicon. The increase of transmission at discrete frequencies has been attributed to a saturation of optical transitions between the energy levels of electrons and holes spatially confined within quasi-zero-dimensional (quantum dots) and quasi-one-dimensional (quantum wires) nanostructures. The results of independent measurements using transmission electron microscopy have confirmed the existence of quantum dots and wires of corresponding size. The slowed-down energy relaxation from upper to lower levels of size quantization compared with intraband relaxation in the bulk have been observed in the cooled (80K) platelets of porous silicon.     

Influence of conformation on electron transport properties of oligophenyleneimine molecular wires

We propose molecular wires based on oligophenyleneimine (OPI) sandwiched between two gold electrodes. The electron transport properties of molecular wires attached to side groups are investigated using steady-state theoretical model and density functional theory by using GAUSSIAN 09 software. We investigate the influence of the side group and torsion angle on the electronic properties of molecular wires. We calculate the spatial distribution of the frontier orbitals, energy gap, transmission probability and the current rectifying ratio for OPI, OPI-pyridine, OPI-pyrazine, OPI-thiophene and OPI-thiazole. The transmission spectra change remarkably depending on the type of side group and torsion angle. The current rectifying ratio will increase by increasing the difference between torsion angles depending on the type of side group. That means the OPI-side group can be employed in molecular electronics.     

Bi-2212/Ag-Mn多芯线材的制备

Bi-2212(Bi2Sr2CaCu2Ox)线材具有优异的低温高场性能,线材的各向同性为磁体绕制提供方便,使其成为超高磁场磁体内插线圈的首选材料.本研究采用PIT技术与部分熔化工艺相结合的工艺制备Bi-2212/AgMn线材,系统研究熔化温度、降温速率以及低温保温时间后处理气氛等参数对线材相成分、微观结构及超导电性的影响,最终制备出临界电流密度Jc达到14.7kA/cm2(77K,self)的Bi-2212多芯线材.     

Density of states and Friedel sum rule in one- and quasi-one-dimensional wires

We analyze the relation between the density of states obtained from the energy derivative of the Friedel phase and that obtained from the Green's function of one- and quasi-one-dimensional wires with a double δ-potential. In the case of repulsive δ-potentials (in both one- and quasi-one-dimension), we show that the local Friedel sum rule is valid when a correction term is included. Various properties of the one-dimensional local density of states are also discussed. In the case of attractive δ-potentials in a quasi-one-dimensional wire, it is well known that the transmission probability may exhibit a Fano resonance (due to a zero-pole pair). In this case, we show that the local Friedel sum rule is valid provided that the tail of the quasibound state is taken into account by the integrated local density of states. In addition, we show that the density of states in a Fano resonance always has a Lorentz shape with peak position at the resonance energy regardless of the (Fano) asymmetry parameter.     

The effect of semi-infinite crystalline electrodes on transmission of gold atomic wires using DFT

First principle calculations of the conductance of gold atomic wires containing chain of 3–8 atoms each with 2.39 Å bond lengths are presented using density functional theory. Three different configurations of wire/electrodes were used. For zigzag wire with semi-infinite crystalline electrodes, even–odd oscillation is observed which is consistent with the previously reported results. A lower conductance is observed for the chain in semi-infinite crystalline electrodes compared to the chains suspended in wire-like electrode. The calculated transmission spectrum for the straight and zig-zag wires suspended between semi-infinite crystalline electrodes showed suppression of transmission channels due to electron scattering occurring at the electrode-wire interface.     

Time-resolved screening of the piezoelectric field in InGaAs/GaAs V-shaped quantum wires of variable profile

InGaAs/GaAs V-shaped quantum wires grown in grooves with either (111) or (411) sidewalls have been studied by ps-transient photoluminescence as a function of the excitation intensity. The optical nonlinearity associated with the screening of the internal piezoelectric field is temporally monitored by the blue shift of the spectrally resolved photoluminescence, occurring in the first 150 ps after the laser pulse, followed by a red shift at longer delays. Such an energy shift strongly depends on the photoexcited carrier density and reaches a maximum value of about 14 meV in the (411) wires. Despite their larger piezoelectric field, we observe a smaller energy shift in wires with (111) sidewalls, due to the enhanced confinement which localizes the wire wavefunction at the bottom of the groove. The observed energy shifts are consistent with the theoretical calculation of the polarization charge density induced by the strain via the piezoelectric effect.     

电爆炸丝1维磁流体模型数值模拟

 给出了电爆炸丝1维磁流体模型的具体描述和基本方程,并利用数值方法求解。通过对含电爆炸丝断路开关的电感储能脉冲功率电路模型的计算,给出了电爆炸丝电流、电压随时间的变化,以及电爆炸过程中丝的密度空间分布,膨胀半径随时间的变化,讨论了电爆炸丝长度和初始电压对电爆炸丝电流、电压的影响。     

Electronic Transport Calculations Using Maximally-Localized Wannier Functions

I present a method to calculate the ballistic transport properties of atomic-scale structures under bias. The electronic structure of the system is calculated using the Kohn-Sham scheme of density functional theory (DFT). The DFT eigenvectors are then transformed into a set of maximally localized Wannier functions (MLWFs) [N. Marzari and D. Vanderbilt, Phys. Rev. B 56 (1997) 12847]. The MLWFs are used as a minimal basis set to obtain the Hamitonian matrices of the scattering region and the adjacent leads,which are needed for transport calculation using the nonequilibrium Green's function formalism. The coupling of the scattering region to the semi-infinite leads is described by the self-energies of the leads. Using the nonequilibrium Green's function method, one calculates self-consistently the charge distribution of the system under bias and evaluates the transmission and current through the system. To solve the Poisson equation within the scheme of MLWFs I introduce a computationally efficient method. The method is applied to a molecular hydrogen contact in two transition metal monatomic wires (Cu and Pt). It is found that for Pt the I-V characteristics is approximately linear dependence, however, for Cu the I-V characteristics manifests a linear dependence at low bias voltages and exhibits apparent nonlinearity at higher bias voltages. I have also calculated the transmission in the zero bias voltage limit for a single CO molecule adsorbed on Cu and Pt monatomic wires. While a chemical scissor effect occurs for the Cu monatomic wire with an adsorbed CO molecule, it is absent for the Pt monatomic wire due to the contribution of d-orbitals at the Fermi energy.     

Quantum transmission across a low-dimensional nanoscale diffuse junction

The transmission across a nano-scale diffuse junction between two metals is studied numerically for 1D wires and 2D ribbons within a tight-binding model. In such a situation, a junction electric potential must appear to equilibrate the electrochemical potential on each side of the junction. We compute the transmission under two very different potential distributions and we find that the typical transmission does not depend on the details of this distribution. The transmission is found to follow a scaling law. The variations of the 1D transmission are explained in a semi-classical way by considering the multiple reflections caused by the barriers due to the on-site energy mismatch between the two metals. Both the 1D and 2D results can also be partly interpreted in the frame of the Anderson theory of localisation.     

Inelastic Electron Transport in Monoatomic Wires

Based on non-equilibrium Green＇s function theory and density functional theory, we investigate the vibrational property and electron-phonon （el-ph） interaction induced inelastic scattering in electron transport through metallic monoatomic wires.     

Direct imaging of stochastic domain-wall motion driven by nanosecond current pulses

Magnetic transmission x-ray microscopy is used to directly visualize the influence of a spin-polarized current on domain walls in curved permalloy wires. Pulses of nanosecond duration and of high current density up to 1.0x10(12) A/m(2) are used to move and to deform the domain wall. The current pulse drives the wall either undisturbed, i.e., as composite particle through the wire, or causes structural changes of the magnetization. Repetitive pulse measurements reveal the stochastic nature of current-induced domain-wall motion.     

Shielding failure rate calculation by means of downward and upward lightning leader movement models: Effect of environmental conditions

In this paper the effects of environmental conditions on shielding failure rate (SFR) of transmission lines are investigated. The study utilizes a previously published work in which leader progression model for lightning upward and downward leaders are used to calculate the SFR. Taking into account the effects of reduced air density and humidity on the parameters of upward leader model and wind pressure on the movement of lightning leaders and wires, SFR and maximum lightning stroke current causing shielding failure are computed. The electric field in all simulations is calculated by means of charge simulation method. The results of simulation show that the effects of relative air density and height of installation are quite higher than that of the wind pressure and humidity while the humidity has the lowest impact on the SFR of investigated transmission line.     

Effective medium theory of left-handed materials

We analyze the transmission and reflection data obtained through transfer matrix calculations on metamaterials of finite lengths, to determine their effective permittivity epsilon and permeability micro. Our study concerns metamaterial structures composed of periodic arrangements of wires, cut wires, split ring resonators (SRRs), closed SRRs, and both wires and SRRs. We find that the SRRs have a strong electric response, equivalent to that of cut wires, which dominates the behavior of left-handed materials (LHM). Analytical expressions for the effective parameters of the different structures are given, which can be used to explain the transmission characteristics of LHMs. Of particular relevance is the criterion introduced by our studies to identify if an experimental transmission peak is left or right handed.     

Waveguide atom beam splitter for laser-cooled neutral atoms

A laser-cooled neutral-atom beam from a low-velocity intense source is split into two beams while it is guided by a magnetic-field potential. We generate our multimode beam-splitter potential with two current-carrying wires upon a glass substrate combined with an external transverse bias field. The atoms are guided around curves and a beam-splitter region within a 10-cm guide length. We achieve a maximum integrated flux of 1.5x10(5)atoms/s with a current density of 5x10(4)amp/cm (2) in the 100-microm -diameter wires. The initial beam can be split into two beams with a 50/50 splitting ratio.     

Electronic structure effects on stability and quantum conductance in 2D gold nanowires

In this study, we have investigated the stability and conductivity of unsupported, two-dimensional infinite gold nanowires using ab initio density functional theory (DFT). Two-dimensional ribbon-like nanowires with 1–5 rows of gold atoms in the non-periodic direction and with different possible structures have been considered. The nanowires with >2 rows of atoms exhibit dimerization, similar to finite wires, along the non-periodic direction. Our results show that in these zero thickness nanowires, the parallelogram motif is the most stable. A comparison between parallelogram- and rectangular-shaped nanowires of increasing width indicates that zero thickness (111) oriented wires have a higher stability over (100). A detailed analysis of the electronic structure, reveals that the (111) oriented structures show increased delocalization of s and p electrons in addition to a stronger delocalization of the d electrons and hence are the most stable. The density of states show that the nanowires are metallic and conducting except for the double zigzag structure, which is semiconducting. Conductance calculations show transmission for a wide range of energies in all the stable nanowires with more than two rows of atoms. The conductance channels are not purely s and have strong contributions from the d levels, and weak contributions from the p levels.     

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.     

Numerical Study on Left-Handed Materials Made of Ferrite and Metallic Wires

Due to coupling effect, we show that it is difficult to realize the left-handed material by placing metallic wires directly into a ferrite matrix. However by introducing an insulating material round the metallic wires to decouple the direct interaction between the metallic wire and ferrite matrix, we have proposed two microstructures, which are shown by numerical simulation to have negative refractive indexes. The influence of microstructure on the transmission property is also examined.     

Electron transport in gated InGaAs and InAsP quantum well wires in selectively grown InP ridge structures

The purpose of this work is to fabricate ribbon-like InGaAs and InAsP wires embedded in InP ridge structures and investigate their transport properties. The InP ridge structures that contain the wires are selectively grown by chemical beam epitaxy (CBE) on pre-patterned InP substrates. To optimize the growth and micro-fabrication processes for electronic transport, we explore the Ohmic contact resistance, the electron density, and the mobility as a function of the wire width using standard transport and Shubnikov–de Haas measurements. At low temperatures the ridge structures reveal reproducible mesoscopic conductance fluctuations. We also fabricate ridge structures with submicron gate electrodes that exhibit non-leaky gating and good pinch-off characteristics acceptable for device operation. Using such wrap gate electrodes, we demonstrate that the wires can be split to form quantum dots evidenced by Coulomb blockade oscillations in transport measurements.     

非均匀缺陷环对微波左手材料的影响

以金属铜六边形开口谐振环(SRRs)与金属铜线的组合为结构单元，研究了三维左手材料中的 缺陷效应. 利用电路板刻蚀技术制备了左手材料样品，采用波导法测量了SRRs构成的点缺陷 和线缺陷对左手材料X波段（8—12 GHz）微波透射行为的影响. 实验结果表明，引入不同尺 寸SRRs构成的点缺陷，材料谐振峰强度下降，最多达6 dB，相当于原来的186%，谐振频率 移动，通频带宽在630—720 MHz范围变化；引入不同尺寸SRRs构成三种取向的线缺陷时，材 料谐振峰强度下降，最多达11 dB，相当于原来的34 关键词： 左手材料 缺陷效应