Statistical mechanics of collisionless relaxation in a non-interacting system

We introduce a model of uncoupled pendula, which mimics the dynamical behaviour of the Hamiltonian mean-field (HMF) model. This model has become a paradigm for long-range interactions, such as Coulomb or dipolar forces. As in the HMF model, this simplified integrable model is found to obey the Vlasov equation and to exhibit quasi-stationary states (QSSs), which arise after a 'collisionless' relaxation process. Both the magnetization and the single-particle distribution function in these QSSs can be predicted using Lynden-Bell's theory. The existence of an extra conserved quantity for this model, the energy distribution function, allows us to understand the origin of some discrepancies of the theory with numerical experiments. It also suggests an improvement of Lynden-Bell's theory, which we fully implement for the zero-field case.     

Energy density,heat capacity,and diffusion coefficient of nonideal dusty plasma

New semiempirical approximations for the energy density and heat capacity coefficient of three-dimensional dissipative systems of macroparticles interacting with screened Coulomb potential are proposed. The relation between the energy density and diffusion coefficient in such systems is considered. The analytical and numerical models are compared. Simulation is performed in a wide range of parameters typical of laboratory experiments with dusty plasma.     

Process of the Deposition of Charged Polydisperse Gas Suspension on the Plate Surface in an Electrical Field

The process of powder sputtering onto a flat surface in an electric field is described on the basis of the numerical solution of a system of equations of the dynamics of a polydisperse gas suspension. The model includes equations for the motion of the carrier and disperse phases under the action of the aerodynamic friction force and the Coulomb force, taking into account the interphase exchange of momentum and energy. The system is solved by the explicit predictor–corrector method with splitting over the spatial directions and the nonlinear correction scheme. The numerical model is used to obtain the velocity and density fields of the gas suspension in the interelectrode space and on the surface of the target electrode.     

Multiexciton generation by a single photon in nanocrystals

We have theoretically shown that efficient generation of multi-electron-hole pairs by a single photon observed recently in semiconductor nanocrystals1-4 is caused by breaking the single electron approximation for carriers with kinetic energy above the effective energy gap. Due to strong Coulomb interaction, these states form a coherent superposition with charged excitons of the same energy. This concept allows us to define the conditions for dominant two-exciton generations by a single photon: the thermalization rate of a single exciton, initiated by light, should be lower than both the two-exciton state thermalization rate and the rate of Coulomb coupling between single and two exciton states. Possible experimental manifestations of our model are discussed.     

单轴压缩载荷下闭孔泡沫铝的变形机制

基于X射线计算机断层扫描技术,重构了能够反映闭孔泡沫铝真实细观结构的三维有限元模型。采用数值模拟与试验测试相结合的方法,研究了泡沫铝在准静态单轴压缩载荷作用下的力学响应及其变形机制,重点关注了平台阶段及致密化阶段的变形模式。结果表明:试件中变形带的出现是压缩过程进入平台阶段的一个标志,此时棱杆和孔壁的变形以塑性弯曲为主;平台阶段,棱杆及孔壁的变形逐渐向塑性起皱与塑性屈曲转变;伴随致密化阶段的发生,变形带内部的胞孔严重坍塌,呈‘双凹圆盘’状。闭孔泡沫铝细观结构变形模式的数值模拟与试验结果相符,验证了该模型的有效性,为进一步研究各相关物理量(相对密度、加载速率等)及变形机制对其宏观吸能性能的影响奠定了基础。     

Carrier multiplication in semiconductor nanocrystals: theoretical screening of candidate materials based on band-structure effects

Direct carrier multiplication (DCM) occurs when a highly excited electron-hole pair decays by transferring its excess energy to the electrons rather than to the lattice, possibly exciting additional electron-hole pairs. Atomistic electronic structure calculations have shown that DCM can be induced by electron-hole Coulomb interactions, in an impact-ionization-like process whose rate is proportional to the density of biexciton states rho XX. Here we introduce a DCM "figure of merit" R2(E) which is proportional to the ratio between the biexciton density of states rhoXX and the single-exciton density of states rhoX, restricted to single-exciton and biexciton states that are coupled by Coulomb interactions. Using R2(E), we consider GaAs, InAs, InP, GaSb, InSb, CdSe, Ge, Si, and PbSe nanocrystals of different sizes. Although DCM can be affected by both quantum-confinement effects (reflecting the underly electronic structure of the confined dot-interior states) and surface effects, here we are interested to isolate the former. To this end the nanocrystal energy levels are obtained from the corresponding bulk band structure via the truncated crystal approximation. We find that PbSe, Si, GaAs, CdSe, and InP nanocrystals have larger DCM figure of merit than the other nanocrystals. Our calculations suggest that high DCM efficiency requires high degeneracy of the corresponding bulk band-edge states. Interestingly, by considering band structure effects we find that as the dot size increases the DCM critical energy E0 (the energy at which R2(E) becomes >or=1) is reduced, suggesting improved DCM. However, whether the normalized E0/epsilong increases or decreases as the dot size increases depends on dot material.     

Status of the Electrostatic Levitation Furnace (ELF) in the ISS-KIBO

The electrostatic levitation method is a containerless processing technique that utilizes Coulomb force between a charged sample and the surrounding electrodes. The Japan Aerospace Exploration Agency (JAXA) has been developing this technique for more than 20 years. In 2016, JAXA completed the flight model assembly, and the Electrostatic Levitation Furnace (ELF) for the International Space Station (ISS) was launched to the ISS. The ELF is mainly intended to handle oxide melts that are difficult to levitate on the ground based electrostatic levitator due to gravity and due to insufficient charging. ISS-ELF can measure the thermophysical properties (density, surface tension and viscosity) of high temperature melts above 2000 ^°C. The thermophysical properties data of materials at high temperature is useful for the study of liquid states and improvement of numerical simulation by modeling the manufacturing processes using the liquid state. Moreover, the interfacial energy of immiscible melts will be measured by creating a core-shell droplet configuration which otherwise cannot be obtained on the ground due to sedimentation. This paper briefly describes the ELF facility and presents the results of a functional checkout that includes the density measurement of molten alumina.     

The generalized Hartree-Fock theory of nonmagnetic high-temperature superconductors

The Eliashberg equations are generalized to apply to high-temperature superconductors without spin correlations. The generalization assumes any general electronic density of states. Consequently, it treats the Coulomb interaction dynamically, and takes into account the averaged momentum dependence of the self-energy and of the interactions. Unlike in the conventional Eliashberg equation, the bare Coulomb interaction yields a frequency-independent term in the renormalization function. This term breaks the symmetry of the self-energy, and changes the renormalization.     

A study of a phase formalism for calculating the cumulative density of states of one-dimensional photonic crystals

We explore a phase formalism that underpins a method of calculation of the cumulative density of states of one-dimensional photonic crystals based on the node counting theorem. Node counting is achieved by considering the spatial dependence of a phase variable proportional to the logarithmic derivative of the electric field in the structure. The properties of the phase variable are considered for photonic crystals in general, and illustrative algebraic and numerical results are presented for the phase variable and cumulative density of states of a model crystal. It is also shown how a simple extension of the theory can facilitate the calculation of the reflectivity of finite samples. For a disordered model crystal, a differential equation for the distribution function of the phase variable is derived and then used to obtain a closed-form expression for the ensemble-averaged cumulative density of states and numerical results to illustrate band tailing in the photonic bandgap.     

Tunable graphene single electron transistor

We report electronic transport experiments on a graphene single electron transistor. The device consists of a graphene island connected to source and drain electrodes via two narrow graphene constrictions. It is electrostatically tunable by three lateral graphene gates and an additional back gate. The tunneling coupling is a strongly nonmonotonic function of gate voltage indicating the presence of localized states in the barriers. We investigate energy scales for the tunneling gap, the resonances in the constrictions, and for the Coulomb blockade resonances. From Coulomb diamond measurements in different device configurations (i.e., barrier configurations) we extract a charging energy of approximately 3.4 meV and estimate a characteristic energy scale for the constriction resonances of approximately 10 meV.     

Numerical solutions of Fokker-Planck equation of nonlinear systems subjected to random and harmonic excitations

A new approach for an efficient numerical implementation of the path integral (PI) method based on non-Gaussian transition probability density function (PDF) and the Gauss-Legendre integration scheme is developed. This modified PI method is used to solve the Fokker-Planck (FP) equation and to study the nature of the stochastic and chaotic response of the nonlinear systems. The steady state PDF, periodicity, jump phenomenon, noise induced changes in joint PDF of the states are studied by the modified PI method. A computationally efficient higher order, finite difference (FD) technique is derived for the solution of higher-dimensional FP equation. A two degree of freedom nonlinear system having Coulomb damping with a variable friction coefficient subjected to Gaussian white noise excitation is considered as an example which can represent a bladed disk assembly of turbo-machinery blades. Effects of normal force and viscous damping on the mean square response are investigated.     

Two carriers in vertically coupled quantum dots: magnetic field effect

We study a two-charge-carrier (two holes or two electrons) quantum dot molecule in a magnetic field. In comparison with the electron states in the double quantum dot, the switching between the hole states is achieved by changing both the inter-dot distance and magnetic field. We use harmonic potentials to model the confining of two charge carriers and calculate the energy difference delta E between the two lowest energy states with the Hund-Mulliken technique, including the Coulomb interaction. Introducing the Zeeman effect, we note a ground-state crossing, which can be observed as a pronounced jump in the magnetization at a perpendicular magnetic field of a few Tesla. The ground states of the molecule provide a possible realization for a quantum gate.     

On a matrix cracking model using Coulomb’s friction law

A matrix cracking model is developed based upon Coulomb friction law instead of a constant frictional shear stress usually assumed in the matrix cracking analyses. A Lamé formulation incorporated with Coulomb friction law is adopted to solve the elastic states of fiber/matrix stress-transfer through a frictionally constrained interface in the slipping region and a modified shear lag model is applied to evaluate the elastic responses in the intact region. By using an energy balance approach, the critical stress for propagating a semi-infinite fiber-bridged crack in a unidirectional fiber reinforced composite is formulated in terms of the frictional coefficient rather than the frictional shear stress usually equated in the matrix cracking stress formulations. The critical stress for matrix cracking and the corresponding stress distributions calculated by the present Coulomb friction model will be compared with those predicted by the constant frictional shear stress models. The effect of Poisson contraction caused by stress redistribution between the fiber and matrix on the matrix cracking mechanics will be shown and discussed in the present analysis.     

Excitations in Quantum Hall Ferromagnet with Strong Coulomb Interaction

The limit of the large Coulomb energy compare the cyclotron energy is considered. It is possible to relate the electron density with the analytical properties of one electron Green function and show that one electron gap exists at integer fillings of Landau level. The energy of collective excitations of plasmon type is calculated at small momenta. It is shown that the activation energy of Skyrmion-antiskyrmion pairs and the energy of spin waves is proportional to the cyclotron energy in this limit.     

On crushing response of the three-dimensional closed-cell foam based on Voronoi model

The crushing response of the three dimensional closed-cell foams is investigated using mesoscale numerical models based on Voronoi tessellation. The crushing stress at the impact and stationary sides of the Voronoi structures are obtained. The effects of the impact velocity, the cell shape irregularity degree, the relative density, inertia of cell walls and the dependence of the base material on the crushing stress are discussed. Meanwhile, the contention of the rate dependency of cellular materials are expounded by the comparison of numerical results of the Voronoi model and solid continuum model as well as the shock wave theory, in which the densification strain and plateau stress are calculated using the energy absorption efficiency approach.     

Compact analytical model for room-temperature-operating silicon single-electron transistors with discrete quantum energy levels

A compact and analytical model for silicon single-electron transistors (SETs) considering the discrete quantum energy levels and the parabolic tunneling barriers is proposed. The model is based on a steady-state master equation that considers only the three most probable states derived from ground level and the first excited level for each number of electrons in the dot to reduce the complexity while accounting for the quantum-level spacing and multiple peaks in Coulomb oscillation. Negative differential conductance (NDC) characteristics and aperiodic Coulomb oscillations due to nonuniform quantum-level spacings can be reproduced in this model. The model was compared with measurements, and good agreement was obtained. Simulations of some basic circuits that utilize NDC are successfully carried out by applying our model to the HSPICE circuit simulation. Our model can provide suitable environments for designing CMOS-combined room-temperature-operating highly functional SET circuits.     

六角形纸蜂窝夹层板能量吸收研究进展

分析了纸蜂窝夹层板动静态压缩试验方法及不同结构参数的纸蜂窝夹层板动静态缓冲吸能特性。试验结果表明,平台应力是蜂窝胞壁厚跨比的幂指数函数。引入压缩密实化应变概念,构建了纸蜂窝材料压缩密实化应变评估方程。将纸蜂窝夹层板压缩应力应变曲线简化为线弹性区、平台区和密实化区,构建了纸蜂窝夹层板能量吸收曲线理论模型。基于纸蜂窝夹层板动静态压缩试验,可构建纸蜂窝夹层板二维能量吸收图,以便更好地袁征纸蜂窝夹层板的缓冲性能,并指出了该研究有待进一步完善之处。     

Eliashberg theory of the critical temperature and isotope effect. Dependence on bandwidth,band-filling,and direct Coulomb repulsion

We investigate the dependence of the superconducting critical temperature and the isotope coefficient on bandwidth, band-filling, and the direct Coulomb repulsion, within Eliashberg theory. The Migdal approximation is assumed throughout, and the Coulomb repulsion is modelled by the Hubbard U and treated in the simplest approximation. We assume a constant density of states with a finite bandwidth. We find that while, in principle, small isotope coefficients are possible, it is unlikely that the isotope coefficient can ever be negative within this model. Furthermore, it is difficult to achieve small isotope coefficients for realistic parameters. Finally, we discuss a possible means by which large isotope coefficients can occur at low filling.     

Observation of negative differential resistance and single-electron tunneling in electromigrated break junctions

We observed a negative differential resistance (NDR) along with single-electron tunneling (SET) in the electron transport of electromigrated break junctions with metal-free tetraphenylporphyrin (H₂BSTBPP) at a temperature of 11 K. The NDR strongly depended on the applied gate voltages, and appeared only in the electron tunneling region of the Coulomb diamond. We could explain the mechanism of this new type of electron transport by a model assuming a molecular Coulomb island and local density of states of the source and the drain electrodes.     

A Preliminary Analysis of the Superconductivity of Ba1−x K x BiO by the Generalized Hartree–Fock Theory

The gap and the renormalization functions of Ba_1?x K _x BiO have been numerically analyzed by means of the simplified equations of the generalized Hartree–Fock (GHF) theory. Measured functions and parameters have been used as inputs to the GHF integral equations, and all the relevant functions have been iterated self-consistently over the large energy range of 9 eV. The results show a reasonably large gap function which does not reverse its sign below 2 eV, despite static Coulomb repulsion and despite the low density of states at the Fermi level. It is also shown that these results are inconsistent with the conventional Eliashberg equations.