Current partition in multiprobe conductors in the presence of slowly oscillating external potentials

In the presence of a static potential drop a carrier stream incident at a contact of the sample is partitioned into the other contacts according to the transmission probabilities of the sample. The bare response to oscillating potentials, on the other hand, violates current conservation due to the piling up of unscreened charges in the sample, and has to be modified by taking the induced screening potential into account. We present a novel derivation of the conductance response to oscillating external chemical potentials, find the response to an arbitrary internal potential in terms of functional derivatives with respect to the local potential of the scattering matrix of the conductor, and determine the screening potential for slowly oscillating potentials from the condition of local charge neutrality. We find that the current partitioning depends on ratios of local densities of states which reflect the injection and emission properties of the contacts of the sample.     

Contact effect in the dynamic electron transport of a two-probe mesoscopic conductor

Based on the self-consistent electron dynamic transport theory for multi-probe mesoscopic systems, we calculate the distribution of internal potential, charge density, and ac conductance of a two-probe mesoscopic conductor with wide trapezoid reservoirs, and study the contact effect. The results show that including the contact effect can make a significant difference to the frequency-dependent electron transport properties. In the nonzero frequency case, the internal potential and the charge density are complex with extremely small imaginary parts. Importantly, the imaginary part of the charge density gives rise to a real ac conductance (admittance), which corresponds to the charge-relaxation resistance.     

Dynamic conductance of a ballistic quantum wire

Within the framework of exact linear response theory, we derive a general formula, with which the dynamic conductance of mesoscopic system can be determined in the absence of Coulomb interaction. In addition, we present a solution to the problem of current partition in the system. These allow the derivation of dynamic conductance in time-dependent case. As a natural consequence, the current (charge) conservation and gauge invariance conditions are fulfilled. To give an example, we discuss the dynamic conductance of a ballistic quantum wire, and the effect of contacts on the conductance is also discussed.     

微通道板的饱和效应对条纹相机动态范围的影响分析

条纹相机通常采用微通道板的内增强或外增强这两种方式来提高信号探测阈值, 也由此引入了微通道板饱和效应对系统动态范围的限定. 通过一个非连续电阻电容打拿极链的通道模型, 对微通道板的饱和效应进行了描述,说明了在条纹相机中, 微通道板输入输出的线性范围限定于通道的贮存电荷, 即使是在外增强方式中采用低阻抗微通道板,传导电流的补偿作用也极其有限, 微通道板内增强和外增强条纹相机应具有相近的动态范围, 低阻抗微通道板仅在高重复率的连续拍摄时方可发挥功效, 同时还说明了微通道板增益的正确设置对条纹相机动态范围的重要影响.     

Ramo-Shockley relation for a series <Emphasis Type="Italic">RCL</Emphasis> circuit

The current induced by the passage of an external point charge through a plane vacuum capacitor in an RCL circuit free of current (voltage) sources is calculated. The case is also analyzed when an internal point charge is emitted by one of the capacitor plates, moves to the other plate, and is absorbed by it. A technique is proposed to measure the internal charge and its velocity component perpendicular to the capacitor plates in a passive RCL circuit.     

Pure spin-current diode based on interacting quantum dot tunneling junction

A magnetic field-controlled spin-current diode is theoretically proposed, which consists of a junction with an interacting quantum dot sandwiched between a pair of nonmagnetic electrodes. By applying a spin bias V_S across the junction, a pure spin current can be obtained in a certain gate voltage regime,regardless of whether the Coulomb repulsion energy exists. More interestingly, if we applied an external magnetic field on the quantum dot, we observed a clear asymmetry in the spectrum of spin current I_S as a function of spin bias, while the charge current always decays to zero in the Coulomb blockade regime. Such asymmetry in the current profile suggests a spin diode-like behavior with respect to the spin bias, while the net charge through the device is almost zero. Different from the traditional charge current diode, this design can change the polarity direction and rectifying ability by adjusting the external magnetic field, which is very convenient. This device scheme can be compatible with current technologies and has potential applications in spintronics or quantum processing.     

Spin waves in quasiequilibrium spin systems

Using the Landau Fermi liquid theory we discovered a new propagating transverse spin wave in a paramagnetic system which is driven slightly out of equilibrium without applying an external magnetic field. We find a gapless mode which describes the uniform precession of the magnetization in the absence of a magnetic field. We also find a gapped mode associated with the precession of the spin current around the internal field. The gapless mode has a quadratic dispersion leading to a T3/2 contribution to the specific heat. These modes significantly contribute to the dynamic structure function.     

旋转内接悬臂梁的刚柔耦合动力学特性分析

对固结于旋转刚环上内接柔性梁的刚柔耦合动力学特性进行了研究. 在精确描述柔性梁非线性变形基础上, 利用Hamilton变分原理和假设模态法, 在计入柔性梁由于横向变形而引起的轴向变形二阶耦合量的条件下, 推导出一次近似耦合模型. 忽略柔性梁纵向变形的影响,给出一次近似简化模型,引入无量纲变量, 对简化模型做无量纲化处理. 首先分析在非惯性系下内接悬臂梁的动力学响应, 并与外接悬臂梁进行比较; 其次研究内接悬臂梁的稳定性;最后分析内接悬臂梁失稳临界转速的收敛性. 研究发现, 与外接悬臂梁存在动力刚化效应不同,内接悬臂梁存在着动力柔化效应; 给出了内接悬臂梁无条件稳定的临界径长比以及失稳的临界转速的计算方法; 若第一阶固有频率随转速增大而减小,则该内接悬臂梁处于有条件稳定; 随着模态截断数的增加,内接悬臂梁失稳的临界转速减小且有收敛值. 关键词： 内接悬臂梁 一次近似简化模型 动力柔化 临界转速     

Quantum charge fluctuations and the polarizability of the single-electron box

We measure the average charge on the island of a single-electron box, with an accuracy of two thousandths of an electron. Thermal fluctuations alone cannot account for the dependence of the average charge on temperature, on external potential, or on the quasiparticle density of states in the metal from which the box is formed. In contrast, we find excellent agreement between these measurements and a theory that treats the quantum fluctuations of charge perturbatively.     

External forces outside of a layered electron gas

We calculate the external forces outside of the surface of a layered electron gas (LEG). The LEG is a model of a metal where the electrical current is carried in parallel layers, and there is no current between layers. It describes the high-temperature cuprate superconductors and many other layered solids. We calculate the image potential from an external charge, the van der Waals potential from a neutral atom and the Casimir force between the parallel surfaces of two LEGs. Our theory does not use dielectric functions. We write down the quantum mechanical Hamiltonian, calculate the exact ground state energy and deduce the forces from the energy. We also show that the LEG has no surface plasmon.     

尾流效应对快速双原子分子离子在固体中电荷态及分子轴取向的影响

研究了快速双原子分子离子在固体中穿行时,尾流效应对各离子电荷态以及库仑爆炸过程的影响.借助于线性介电响应理论和局域介电函数,离子之间的动力学相互作用势可以表示成对称的屏蔽库仑势和非对称的尾势.通过对分子离子上所有束缚电子的总能量进行变分和求解单个离子的运动方程,自洽地确定出分子离子中每个离子的电荷态.数值结果表明,由于尾流效应的影响,在初始穿行阶段,分子离子中导航离子的电荷数随穿行深度的增加而单调递增,而尾随离子的电荷数则随穿行深度的增加而振荡.但当穿行深度很大时,两个离子的电荷数都趋于具有相同速度的孤立离子的电荷数.此外,还发现分子轴的取向朝入射速度方向偏转     

Surface plasmon-polaritons on ultrathin metal films

We discuss the surface plasmon-polaritons used for ultrathin metal films with the aid of linear response theory and make comparisons with the known result given by Economou E N. In this paper we consider transverse electromagnetic fields and assume that the electromagnetic field in the linear response formula is the induced field due to the current of the electrons. It satisfies the Maxwell equation and thus we replace the current (charge) term in the Maxwell equation with the linear response expectation value. Finally,taking the external field to be zero,we obtain the dispersion relation of the surface plasmons from the eigenvalue equation. In addition,the charge-density and current-density in the z direction on the surface of ultrathin metal films are also calculated. The results may be helpful to the fundamental understanding of the complex phenomenon of surface plasmon-polaritons.     

The self-consistent response of the electron density of a crystal copper film to an external electrostatic field

The response of the electrons of an fcc copper (001) film to an external electrostatic field is calculated. In order to determine the distribution of the screening charge, the equations of the electron density-functional theory are solved self-consistently by an original method. The position of the “image plane,” which is involved in a correct asymptotic expression for the exchange-correlation potential in the vacuum region, is first determined when performing a quantum-mechanical calculation for an anisotropic crystal film. The nonlinear electron response is characterized by the evolution of the “center of gravity” of the induced charge, which is also investigated. The calculations take into account the crystal structure of the film, and the results differ essentially from the predictions of the “jelly” model.     

Thermo-Magneto-Electric Currents with Dynamical Magnetization Inhomogeneities

We study the effect of potential and thermal gradient induced non-equilibrium magnetization in quasi 1-d itinerant magnets. A semi-phenomenological theory is employed in conjunction with the drift-diffusion model for this study. Using the methods of non-equilibrium thermodynamics, we derive the transport currents corresponding to charge, heat, and magnetization flows in the presence of non-equilibrium magnetization textures.It is shown how time-dependent magnetic textures give rise to charge and thermalcurrents even in the absence of external potential and thermal gradients through spinpumping.The presence of dynamical textures also affect the thermodynamic parameters of the system. As an application, we consider the case of a helimagnet.     

Thermal convection in a rotating layer of a magnetic fluid

We consider Brownian particles with the ability to take up energy from the environment, to store it in an internal depot, and to convert internal energy into kinetic energy of motion. Provided a supercritical supply of energy, these particles are able to move in a “high velocity” or active mode, which allows them to move also against the gradient of an external potential. We investigate the critical energetic conditions of this self-driven motion for the case of a linear potential and a ratchet potential. In the latter case, we are able to find two different critical conversion rates for the internal energy, which describe the onset of a directed net current into the two different directions. The results of computer simulations are confirmed by analytical expressions for the critical parameters and the average velocity of the net current. Further, we investigate the influence of the asymmetry of the ratchet potential on the net current and estimate a critical value for the asymmetry in order to obtain a positive or negative net current. Received 20 September 1999     

深能级杂质对InGaAsP/InP发光管输出功率饱和性的影响

本文从文献测得InGaAsP材料中存在着较大的俘获截面、较高的深能级杂质(或缺陷)浓度的事实出发,应用了文献(13)所提出的深能级引起的多声子非辐射复合模型,在此模型基础上,使用了一系列与多声子复合有关的关系式,从理论和实验上证实了考虑多声子复合对LED输出功率饱和性影响的必要性。讨论了深能缀杂质(或缺陷)的能级及其浓度对注入电流I与载流子寿命τ、内量子效率qηi,输出功率间关系的影响。也讨论了温度的影响。研究了各参数闯相互关系,并找出了影晌饱和性的原因所在。     

Graphene based single molecule nanojunction

We introduce the ab-initio framework for zigzag-edged graphene fragment based single-electron transistor (SET) operating in the Coulomb blockade regime. Graphene is modeled using the density-functional theory and the environment is described by a continuum model. The interaction between graphene and the SET environment is treated self-consistently through the Poisson equation. We calculate the charging energy as a function of an external gate potential, and from this we obtain the charge stability diagram. Specifically, the importance of including re-normalization of the charge states due to the polarization of the environment has been demonstrated.     

External and internal coupling effects of rotor's bending and torsional vibrations under unbalances

External and internal bending–torsion coupling effects of a rotor system with comprehensive unbalances are studied by analytical analysis and numerical simulations. Based on Lagrangian approach, a full-degree-of-freedom dynamic model of a Jeffcott rotor is developed. The harmonic balance method and the Floquet theory are combined to analyze the stability of the system equations. Numerical simulations are conducted to observe the bending–torsion coupling effects. In the formulation of rotordynamic model, two bending–torsion coupling patterns, external coupling and internal coupling, are suggested. By analytical analysis, it is concluded that the periodic solution of the system is asymptotically stable. From numerical simulations, three bending–torsion coupling effects are observed in three cases. Under static unbalance, synchronous torsional response is observed, which is the result of external coupling under unbalanced force. Under dynamic unbalance, two-time synchronous frequency torsional response is observed, which is the result of internal coupling under unbalanced moment. Under comprehensive unbalance, synchronous and two-time synchronous frequency torsional components are observed, which are the results of both external and internal couplings under unbalanced force and moment. These observations agree with the analytical analysis. It is believed that these observed phenomena should make sense in the dynamical design and fault diagnostics of a rotor system.     

Charge shuttle as a nanomechanical rectifier

We consider the charge shuttle proposed by Gorelik et al. driven by a time-dependent voltage bias. In the case of asymmetric setup, the system behaves as a rectifier. For pure ac drive, the rectified current shows a rather rich frequency dependent response characterized by frequency locking at fractional values of the external frequency. Because of the nonlinear dynamics of the shuttle, rectification is present also for very low frequencies. These effects could be useful to unveil the internal dynamics of nanomechanical devices.     

Leading Order Response of Statistical Averages of a Dynamical System to Small Stochastic Perturbations

The classical fluctuation-dissipation theorem predicts the average response of a dynamical system to an external deterministic perturbation via time-lagged statistical correlation functions of the corresponding unperturbed system. In this work we develop a fluctuation-response theory and test a computational framework for the leading order response of statistical averages of a deterministic or stochastic dynamical system to an external stochastic perturbation. In the case of a stochastic unperturbed dynamical system, we compute the leading order fluctuation-response formulas for two different cases: when the existing stochastic term is perturbed, and when a new, statistically independent, stochastic perturbation is introduced. We numerically investigate the effectiveness of the new response formulas for an appropriately rescaled Lorenz 96 system, in both the deterministic and stochastic unperturbed dynamical regimes.