Coherently controllable non-equilibrium charge accumulation inside a magnetic quantum dot: a non-equilibrium Green’s function approach

We have performed a non-equilibrium quantum transport calculations for a two-terminal mesoscopic system including a magnetic quantum dot. Using the non-equilibrium Green’s function technique, we have obtained electric current and charge distribution in the temperature range from 1 to 10 K as a function of magnetic field. Results indicate that the density of carriers essentially can be controlled by temperature and bias voltage.     

T型双量子点分子Aharonov-Bohm干涉仪的电输运

利用非平衡格林函数方法, 理论研究T型双量子点分子Aharonov-Bohm (A-B)干涉仪的电荷及其自旋输运性质. 通过控制T型双量子点分子内量子点间有无耦合, 能够实现在同一电子能级位置处分别出现共振和反共振状态, 根据此性质, 能将体系设计成量子开关器件. 当将两个完全相同的T型双量子点分子分别嵌入A-B干涉仪两臂中时, 磁通取适当数值, 能够出现完全的量子相消干涉. 通过调节量子点能级、左右两电极间的偏压和Rashba自旋轨道相互作用强度, 可对体系自旋流进行调控. 关键词： 非平衡格林函数 T型双量子点分子 Aharonov-Bohm干涉仪 自旋输运     

Quark-Gluon Transport Equations in Lee Model

The non-equilibrium processes of quark-gluon-plasma (QGP) in the coexistent phase of first order phase transition are studied under Lee's model. Both the classical and the quantum transport equations of quark as well as the corresponding hydrodynamical equations are obtained. The classical transport equations are deduced from the quantum ones in the semiclassical limit, showing that the theory is self-consistent. The transport equations of gluon in the semi-classical limit and the equation for the fluctuation of gluon distribution function under the condition of near-equilibrium are also derived.     

Off-Shell Propagat ion in a Quark-Gluon Plasma

The off-shell propagation in a quark-gluon plasma is estimated from a quantum transport model beyond the quasi-particle approach. While our ansatz is crude concerning the properties of real quarks and gluons, it nevertheless takes very serious the basic principles of quantum field theory for non-equilibrium states.     

Anyon Quantum Transport and Noise Away from Equilibrium

The quantum transport of anyons in one space dimension is investigated. After establishing some universal features of non-equilibrium systems in contact with two heat reservoirs in a generalized Gibbs state, the abelian anyon solution of the Tomonaga–Luttinger model possessing axial-vector duality is focused upon. In this context a non-equilibrium representation of the physical observables is constructed, which is the basic tool for a systematic study of the anyon particle and heat transport. The associated Lorenz number is determined and the deviation from the standard Wiedemann–Franz law induced by the interaction and the anyon statistics is explicitly described. The quantum fluctuations generated by the electric and helical currents are investigated and the dependence of the relative noise power on the statistical parameter is established.     

Inter-dot coupling effects on transport through correlated parallel coupled quantum dots

Transport through symmetric parallel coupled quantum dot system has been studied, using non-equilibrium Green function formalism. The inter-dot tunnelling with on-dot and inter-dot Coulomb repulsion is included. The transmission coefficient and Landaur-Buttiker like current formula are shown in terms of internal states of quantum dots. The effect of inter-dot tunnelling on transport properties has been explored. Results, in intermediate inter-dot coupling regime show signatures of merger of two dots to form a single composite dot and in strong coupling regime the behaviour of the system resembles the two decoupled dots.      

DC voltage profile of a 1D pumped wire with two dynamical and one static impurities

In this work we study the behavior of the voltage profile of a 1D quantum wire with an impurity when transport is induced by two ac voltages that oscillating with a phase lag define a quantum pump. The voltage profile sensed along the wire by the voltage probe, that we assume weakly coupled to the system, exhibits a Friedel's oscillations structure inside the region delimited by the position of the two ac voltages that induce transport. On the other hand, outside this region the oscillations are suppressed. Using perturbation theory in the coupling constant of the voltage probe we derived analytical expressions for the DC current valid for the adiabatic regime. We also compare our analytical results with the exact numerical calculations using Keldysh non-equilibrium Green's functions formalism.     

High thermoelectric performance of one graphene nanoribbon with line defects

We investigate the quantum transport through zigzag graphene nanoribbons with embedded “5-7-5”-edge line defects, by means of the non-equilibrium Green's function technique. It is found that when two semi-infinite line defects exist in the nanoribbon, notable Fano antiresonance takes place in the quantum transport process, which enables to drive the apparent thermoelectric effect. We propose this structure to be a promising candidate for improving the thermoelectric efficiency based on graphene nanoribbons.     

A functional renormalization group approach to non-equilibrium properties of mesoscopic interacting quantum systems

We present an extension of the concepts of the functional renormalization group approach to quantum many-body problems in non-equilibrium situations. The approach is completely general and allows calculations for both stationary and time-dependent situations. As a specific example we study the stationary state transport through a quantum dot with local Coulomb correlations. We discuss the influence of finite bias voltage and temperature on the current and conductance.     

Effect of boundary treatments on quantum transport current in the Green’s function and Wigner distribution methods for a nano-scale DG-MOSFET

In this paper, we conduct a study of quantum transport models for a two-dimensional nano-size double gate (DG) MOSFET using two approaches: non-equilibrium Green’s function (NEGF) and Wigner distribution. Both methods are implemented in the framework of the mode space methodology where the electron confinements below the gates are pre-calculated to produce subbands along the vertical direction of the device while the transport along the horizontal channel direction is described by either approach. Each approach handles the open quantum system along the transport direction in a different manner. The NEGF treats the open boundaries with boundary self-energy defined by a Dirichlet to Neumann mapping, which ensures non-reflection at the device boundaries for electron waves leaving the quantum device active region. On the other hand, the Wigner equation method imposes an inflow boundary treatment for the Wigner distribution, which in contrast ensures non-reflection at the boundaries for free electron waves entering the device active region. In both cases the space-charge effect is accounted for by a self-consistent coupling with a Poisson equation. Our goals are to study how the device boundaries are treated in both transport models affects the current calculations, and to investigate the performance of both approaches in modeling the DG-MOSFET. Numerical results show mostly consistent quantum transport characteristics of the DG-MOSFET using both methods, though with higher transport current for the Wigner equation method, and also provide the current–voltage (I–V) curve dependence on various physical parameters such as the gate voltage and the oxide thickness.     

Non-equilibrium scaling properties of a double quantum dot system: Comparison between perturbative renormalization group and flow equation approach

Since the experimental realization of Kondo physics in quantum dots, its far-from-equilibrium properties have generated considerable theoretical interest. This is due to the interesting interplay of non-equilibrium physics and correlation effects in this model, which has now been analyzed using several new theoretical methods that generalize renormalization techniques to non-equilibrium situations. While very good agreement between these methods has been found for the spin-1/2 Kondo model, it is desirable to have a better understanding of their applicability for more complicated impurity models. In this paper the differences and commons between two such approaches, namely the flow equation method out of equilibrium and the frequency-dependent poor man's scaling approach are presented for the non-equilibrium double quantum dot system. This will turn out to be a particularly suitable testing ground while being experimentally interesting in its own right. An outlook is given on the quantum critical behavior of the double quantum dot system and its accessibility with the two methods.     

Spin-flip mesoscopic transport through a carbon nanotube quantum dot system

The pure spin transport in an entire metallic single-wall carbon-nanotube (SWCN) interacting quantum dot (QD) system is investigated by using non-equilibrium Green's function (NEGF) technique. The novel spin current performance introduced by one constant and one rotating magnetic fields shows the unique four-fold degenerate electron shell structure which exists the SWCN QD sensitively. Spin transport properties can be designed by tuning the orbital and Zeeman configuration in the central resonant region, which are greatly influenced by the Coulomb interaction and the magnetic fields.     

Mathematical Theory of Non-Equilibrium Quantum Statistical Mechanics

We review and further develop a mathematical framework for non-equilibrium quantum statistical mechanics recently proposed in refs. 1–7. In the algebraic formalism of quantum statistical mechanics we introduce notions of non-equilibrium steady states, entropy production and heat fluxes, and study their properties. Our basic paradigm is a model of a small (finite) quantum system coupled to several independent thermal reservoirs. We exhibit examples of such systems which have strictly positive entropy production.     

导线非共线的分子器件输运性质的第一性原理研究

分子器件是电子器件向小体积化发展的极限,分子器件中的电子在输运过程中体现出明显的量子效应,分子导线与分子接触的位置和导线间的角度等器件结构因素都会对分子器件的输运性质产生较大的影响.迄今为止,尚未见利用第一性原理量子输运计算方法研究导线非共线的分子器件输运性质的报道.本文以金-苯(硫醇)-金结构的分子器件为例,利用基于非平衡格林函数理论和密度泛函理论的第一性原理量子输运计算方法对其输运性质进行了系统研究,特别注重于研究随着非共线导线间导线夹角角度的变化及导线和苯(硫醇)分子接触位置的不同对器件输运性质的影响.计算表明,金导线与苯(硫醇)的接触位置及导线的夹角等器件结构细节不仅能够定量地影响金-苯(硫醇)-金分子器件的电流大小,还能够定性地改变器件的输运性质,使得部分器件结构出现负微分电阻效应.研究结果对全面理解分子器件的输运性质具有一定的指导意义.     

T型耦合双量子点系统的非对等Kondo共振分裂传输

利用隶玻色子平均场近似理论,并借助于单杂质的Anderson模型的哈密顿量,研究了T型耦合双量子点嵌入正常电极的基态输运性质.结果表明：在体系处于平衡状态时,随着双量子点的耦合强度增加,体系的Kondo 效应被削弱. 当耦合强度足够强时,Kondo量子点态密度的Kondo共振单峰分裂成两个不对等的Kondo共振双峰.在体系处于非平衡状态时,增加两电极的偏压,态密度的Kondo分裂的非对等性明显加强. 关键词： Kondo效应 态密度 格林函数法 耦合双量子点     

量子点双链中电子自旋极化输运性质

利用非平衡格林函数方法, 研究了与单个量子点耦合的量子点双链中电子自旋极化输运性质. 由于系统中Rashba自旋轨道耦合产生的自旋相关的相位, 电子通过上下两种路径时, 自旋不同的电子干涉情况不同, 从而导致了电极中的自旋极化流. 左右两电极间的偏压使单个量子点中的自旋积聚在很大能量区域内能够保持较大的值. 由于系统结构的左右不对称, 正负偏压下自旋积聚情况完全不同. 这些计算结果将有助于实验上设计新型的自旋电子学器件.     

Electronic transport properties of helical macromolecular chains using dihedral orbital model

By applying the non-equilibrium Green’s function method, in combination with the dihedral orbital model, we have theoretically investigated quantum transport properties of organic molecular chains, focusing on the effects of the helical rotation of the chains. The transmission coefficient, the electronic current, as well as the current shot noise were calculated. It was found that the helical rotation modifies the transport properties profoundly. It leads to a diminishing and roughly periodical oscillatory behaviour of both the current and shot noise power.     

Generation of Fabry–Pérot oscillations and Dirac state in two-dimensional topological insulators by gate voltage

We investigate electron transport through Hg Te ribbons embedded by strip-shape gate voltage through using a nonequilibrium Green function technique. The numerical calculations show that as the gate voltage is increased, an edgerelated state in the valence band structure of the system shifts upwards, then hangs inside the band gap and merges into the conduction band finally. It is interesting that as the gate voltage is increased continuously, another edge-related state in the valence band also shifts upwards in the small-k region and contacts the previous one to form a Dirac cone in the band structure. Meanwhile in this process, the conductance spectrum displays as multiple resonance peaks characterized by some strong antiresonance valleys in the band gap, then behaves as Fabry–P′erot oscillations and finally develops into a nearly perfect quantum plateau with a value of 2e~2/h. These results give a physical picture to understand the formation process of the Dirac state driven by the gate voltage and provide a route to achieving particular quantum oscillations of the electronic transport in nanodevices.     

Multichannel Electronic Transport Through a Kondo Dot

Electronic transport through the quantum dot with two energy levels is studied by means of the non-equilibrium Green functions and the slave boson method. Special attention is focused on an interplay between quantum inference of traveling waves and electronic correlations. It is shown that if impurity states are far below the Fermi level the transport is through the highest state only. Interference processes become relevant when the levels are shifted towards the mixed valence regime.     

Simulation of InGaN/GaN multiple quantum well light-emitting diodes with quantum dot model for electrical and optical effects

We report a 2D simulation of electrical and optical characteristics of green color InGaN/GaN multiple quantum well light-emitting diodes by APSYS software with a dot-in-well model. The In-rich quantum dot-like structure in InGaN/GaN multiple quantum wells has been considered in the LED experimental data analysis. Simulation results based on the quantum dot model are in better agreement with the experimental data than those based on the purely quantum well model, indicating that the quantum dot spontaneous emission and the non-equilibrium quantum transport play important roles in the InGaN/GaN multiple quantum well light-emitting diodes.