Transport Properties of a Modified Lorentz Gas

We present a detailed study of the first simple mechanical system that shows fully realistic transport behavior while still being exactly solvable at the level of equilibrium statistical mechanics. The system under consideration is a Lorentz gas with fixed freely-rotating circular scatterers interacting with point particles via perfectly rough collisions. Upon imposing a temperature and/or a chemical potential gradient, a stationary state is attained for which local thermal equilibrium holds for low values of the imposed gradients. Transport in this system is normal, in the sense that the transport coefficients which characterize the flow of heat and matter are finite in the thermodynamic limit. Moreover, the two flows are non-trivially coupled, satisfying Onsager's reciprocity relations to within numerical accuracy as well as the Green–Kubo relations. We further show numerically that an applied electric field causes the same currents as the corresponding chemical potential gradient in first order of the applied field. Puzzling discrepancies in higher order effects (Joule heating) are also observed. Finally, the role of entropy production in this purely Hamiltonian system is shortly discussed.     

Negative Differential Resistance in Atomic Carbon Chain-Graphene Junctions

We investigate the electronic transport properties of atomic carbon chain-graphene junctions by using the density-functional theory combining with the non-equilibrium Green's functions. The results show that the transport properties are sensitively dependent on the contact geometry of carbon chain. From the calculated I-V curve we find negative differential resistance (NDR) in the two types of junctions. The NDR can be considered as a result of molecular orbitals moving related to the bias window.     

Negative Differential Resistance and Shapiro Steps in a Superconducting MoN Strip with a Cut

JETP Letters - Regions with a negative differential resistance have been revealed in the voltage–current characteristics of superconducting MoN strips with a side cut under high-power...     

Exchange Fluctuations in a Nonequilibrium Lorentz Gas

It has been shown that a correlation mechanism that is based on the exchange interaction and destroys the relation between distribution functions and response (Price relation) occurs in a nonequilibrium Lorentz gas (particles interact only with the thermostat). The physical nature of this phenomenon is that the scattering of particles of the gas in the same state on a single particle of the thermostat creates a flux of correlated pairs, which depends on the form of a nonequilibrium distribution function, making impossible the existence of a universal relation between distribution functions and response.     

Decay of a Linear Pendulum in a Free-Molecular Gas and in a Special Lorentz Gas

A circular disk without thickness is placed in a gas, and an external force, obeying Hooke’s law, is acting perpendicularly on the disk. If the disk is displaced perpendicularly from its equilibrium position and released, then it starts an oscillatory or non-oscillatory unsteady motion, which decays as time goes on because of the drag exerted by the gas molecules. This unsteady motion, i.e., the decay of this linear pendulum, is investigated numerically, under the diffuse reflection condition on the surface of the disk, with special interest in the manner of its decay, for two kinds of gases: one is a collisionless gas (or Knudsen gas) and the other is a special Lorentz gas interacting with a background. It is shown that the decay of the displacement of the disk is slow and is in proportion to an inverse power of time for the collisionless gas. The result complements the existing mathematical study of a similar problem (Caprino et al. in Math. Models Methods Appl. Sci. 17:1369–1403, 2007) in the case of non-oscillatory decay. It is also shown that the manner of the decay changes significantly for the special Lorentz gas.     

Equilibration and Diffusion for a Dynamical Lorentz Gas

We consider a model of a dynamical Lorentz gaz: a single particle is moving in \({\mathbb {R}}^d\) through an array of fixed and soft scatterers each possessing an internal degree of freedom coupled to the particle. Assuming the initial velocity is sufficiently high and modelling the parameters of the scatterers as random variables, we describe the evolution of the kinetic energy of the particle by a Markov chain for which each step corresponds to a collision. We show that the momentum distribution of the particle approaches a Maxwell–Boltzmann distribution with effective temperature T such that \(k_BT\) corresponds to an average of the scatterers’ kinetic energy.     

Motion of a Rigid Body in a Special Lorentz Gas: Loss of Memory Effect

Linear motion of a rigid body in a special kind of Lorentz gas is mathematically analyzed. The rigid body moves against gas drag according to Newton’s equation. The gas model is a special Lorentz gas consisting of gas molecules and background obstacles, which was introduced in Tsuji and Aoki (J Stat Phys 146:620–645, 2012). The specular boundary condition is imposed on the resulting kinetic equation. This study complements the numerical study by Tsuji and Aoki cited above—although the setting in this paper is slightly different from theirs, qualitatively the same asymptotic behavior is proved: The velocity V(t) of the rigid body decays exponentially if the obstacles undergo thermal motion; if the obstacles are motionless, then the velocity V(t) decays algebraically with a rate \(t^{-\,5}\) independent of the spatial dimension. This demonstrates the idea that interaction of the molecules with the background obstacles destroys the memory effect due to recollision.     

含氮-空位缺陷锯齿状石墨烯纳米条带中负微分电阻和自旋过滤效应

采用第一性原理和非平衡格林函数方法,系统研究了含氮空位缺陷锯齿状石墨烯纳米条带的自旋极化输运特性．理论计算结果表明边界非对称的这类石墨纳米条带的基态具有铁磁性,由其构建的分子结中负微分电阻效应具有鲁棒性,是电极局域的态密度及依赖偏压的散射区-电极耦合作用结果．此外,在特定偏压区域还观察到几乎完美的自旋过滤效应．     

对爱因斯坦模型的修正及考虑热效应的三个通用状态方程

 对固体考虑热效应的爱因斯坦模型进行了修正。指出考虑热效应的通用状态方程中不应该包含零点振动项，方程参数不应该直接取为参考温度下的实验值V_R，B_R，B′_R和γ^G_R，而应该取为零温下的数值V₀，B₀，B′₀和γ^G₀；提出了一种从V_R，B_R，B′_R和γ^G_R求解V₀，B₀，B′₀和γ^G₀的方法。将提出的方法应用于三个典型的通用状态方程，包括Baonza、mMNH和Vinet方程。数值结果表明，利用相同的实验参数对三个方程解出的参数，以及预测的零压和低压下的热物理性质差异很小，而且都与实验数据符合很好。这些结果表明，在零压和低压下预测热物理性质的精确度不足以用来判断各种通用状态方程的适用性。     

Universal Low-energy Behavior in a Quantum Lorentz Gas with Gross-Pitaevskii Potentials

We consider a quantum particle interacting with N obstacles, whose positions are independently chosen according to a given probability density, through a two-body potential of the form N²V (Nx) (Gross-Pitaevskii potential). We show convergence of the N dependent one-particle Hamiltonian to a limiting Hamiltonian where the quantum particle experiences an effective potential depending only on the scattering length of the unscaled potential and the density of the obstacles. In this sense our Lorentz gas model exhibits a universal behavior for N large. Moreover we explicitely characterize the fluctuations around the limit operator. Our model can be considered as a simplified model for scattering of slow neutrons from condensed matter.     

FeN3掺杂扶手椅型石墨烯纳米条带的极强电流极化和微分负导效应

本文运用第一性原理研究了FeN₃掺杂扶手椅型和锯齿型石墨烯纳米条带的电子结构和输运性质. 结果表明,FeN₃掺杂可导致两种类型的条带的能带结构发生显著变化,导致体系具有稳定的室温铁磁基态. 但是,只有扶手椅型条带具有明显的负微分电导和极强的电流极化效应(接近100%). 这是由于FeN₃掺杂引入孤立的两条自旋向下能级,导致极强的电流极化. 同时,它们与自旋向下的不同子能带的耦合强度完全不同,导致体系呈现出负微分电导行为. 结果说明,通过FeN₃掺杂扶手椅型石墨烯纳米条带也可用于制备自旋电子学器件.     

Characterization of Powder Beds by Thermal Conductivity: Effect of Gas Pressure on the Thermal Resistance of Particle Contact Points

The thermal conductivity of ceramic powder packed beds was measured at temperatures below 100 °C for various powder sizes and compositions and under different gas atmospheres. Measurements at low pressures (down to 10 Pa) combined with a theoretical model allowed the elucidation of geometrical and thermal resistance parameters for the contact points between granules. The gap thickness and contact point size were found to be well correlated with the mean particle size. The thermal conductivities of all powders at low pressure were found to differ at most by a factor of two, whereas the solid‐phase conductivities of the powder materials differed by more than one order of magnitude. A theoretical model accounting for the size‐dependence of contact point conductivity is incorporated to rationalize this trend.     

FRW Cosmological Model with Modified Chaplygin Gas and Dynamical System

The Friedmann-Robertson-Walker (FRW) model with dynamical Dark Energy (DE) in the form of modified Chaplygin gas (MCG) has been investigated. The evolution equations are reduced to an autonomous system on the two dimensional phase plane and it can be interpreted as the motion of the particle in an one dimensional potential. Also the dynamical system analysis has been extended to examine the critical points at infinity with will exist provided the equation of state parameter $\omega<-\frac{1}{3}$ . Finally, theoretical points are asymptotically stable or unstable.     

Microscopic Chaos and Reaction-Diffusion Processes in the Periodic Lorentz Gas

We apply the hypothesis of microscopic chaos to diffusion-controlled reaction which we study in a reactive periodic Lorentz gas. The relaxation rate of the reactive eigenmodes is obtained as eigenvalue of the Frobenius–Perron operator, which determines the reaction rate. The cumulative functions of the eigenstates of the Frobenius–Perron operator are shown to be generalizations of Lebesgue's singular continuous functions. For small enough densities of catalysts, the reaction is controlled by the diffusion. A random-walk model of this diffusion-controlled reaction process is presented, which is used to study the dependence of the reaction rate on the density of catalysts.Aspirant FNRS     

基于CMY模型改进的界面热阻模型及其应用

导热膏填充的接触界面热阻由接触热阻和间隙热阻两部分组成。本文采用截锥体接触的单热流通道模型代替CMY模型中的圆盘接触的单热流通道模型,推导出了改进的接触热阻计算公式。本文还结合间隙热阻的计算公式,得到了一种改进的导热膏填充的接触界面热阻模型。通过分析得出了如下结论:对于使用导热膏填充的接触界面的热阻而言,其主要影响因素为接触表面的粗糙度和导热膏的导热系数,而接触界面间压力对其的影响则相对较小。     

定态双向环形气体激光中的饱和效应

从理论上分析了双向环形气体激光中的全饱和效应，导出了定态激光强度分布函数的解析形式，算出了定态双模强度的自相关和相相关函数。与激光的三次模型 实验测相比较，激光的全饱和模型与实验测量符合更，而三次模型的结果偏差较大。     

Evolution of Holographic Dark Energy in Interacting Modified Chaplygin Gas Model

We investigate the modified Chaplygin gas （MCG） with interaction between holographic dark energy proposed by Li and dark matter. In this model, evolution of the universe is described in detail, which is from deceleration to acceleration. Specifically, the evolutions of related cosmological quantities such as density parameter, the equation of state of holographic dark energy, deceleration parameter and transition redshift are discussed. Moreover, we also give their present values which are consistent with the lately observations. Furthermore, the results given by us show such a model can accommodate a transition of the dark energy from a normal state wx 〉 -1 to ωx 〈 -1 phantom regimes.     

Negative Thermal Transport in Conduction and Advection

Negative refractive index has drawn a great deal of attention due to its unique properties and practical applications in wave systems. To promote the related physics in thermotics, here we manage to coin a complex thermal conductivity whose imaginary part corresponds to the real part of complex refractive index. Therefore, the thermal counterpart of negative refractive index is just negative imaginary thermal conductivity, which is featured by the opposite directions of energy flow and wave vector in thermal conduction and advection, thus called negative thermal transport herein. To avoid violating causality, we design an open system with energy exchange and explore three different cases to reveal negative thermal transport. We further provide experimental suggestions with a solid ring structure. All finite-element simulations agree with theoretical analyses, indicating that negative thermal transport is physically feasible. These results have potential applications such as designing the inverse Doppler effect in thermal conduction and advection.     

基于热峰模型的单粒子效应模拟研究

单粒子效应是航天电子器件失效的重要原因,研究其物理过程对航天电子器件寿命预测、器件抗辐照加固有着重要的意义。现有的模型多从线性能量沉积的角度来解释单粒子翻转,因此无法解释单粒子效应地面实验模拟过程中的温度效应。建立了一个新模型,从高能带电离子与材料相互作用的物理过程出发,通过解三维的热扩散方程,计算出能量在材料中沉积、交换、扩散,得到电子和晶格温度的空间分布以及时间演化过程。推断出离子辐照过程中导致的自由电子浓度和收集电荷随LET的变化关系。此模型解释了单粒子效应中随着器件温度升高,单粒子效应截面增加的现象。     

Effect of a magnetic field on Lorentz collisions in acetylene-air flames

The damping constants (α-parameter) for Li, Na, K, Mg, and Ca were determined from curves of growth prepared from emission measurements at 0, 1.3, 2.2, and 3 kG. The collisional (Lorentz) half-intensity width Δλ_L, the absolute atom concentrations N in the flame, and the effective collisional cross sections Q_L have been calculated. They show appreciable increases with magnetic field strength. From the line profiles, it is found that the half-intensity width $\text{Δλ}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ did not change, while the width at the extreme wings Δλ_w has broadened slightly. The kind of perturbers which are thought to play an important role in collisions and the preponderant type of interaction are discussed.