一类弯曲量子线的量子束缚态

对电子在弯曲量子线中的弹道输运性质进行了理论研究.弯曲量子线由T型量子线和单曲量子线组成.该有限长的量子结构分别与两半无限长的量子通道相连,当施加一偏压时,量子通道分别可作为电子的发射极和收集极.计算结果表明,当入射电子的能量小于量子结构横向上的第一个本征模时,电导存在两个峰.进一步指出,这些峰来自于电子共振隧穿量子结构中的量子束缚态.并详尽地讨论了这些量子束缚态的性质. 关键词： 量子束缚态 共振隧穿 电导 量子线     

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.     

低维纳米材料量子热输运与自旋热电性质 ——非平衡格林函数方法的应用

低维材料不断涌现的新奇性质吸引着科学研究者的目光. 除了电子的量子输运行为之外, 人们也陆续发现和确认了热输运中显著的量子行为, 如 热导低温量子化、声子子带、尺寸效应、瓶颈效应等. 这些小尺度体系的热输运性质可以很好地用非平衡格林函数来描述. 本文首先介绍了量子热输运的特性、声子非平衡格林函数方法及其在低维纳米材料中的研究进展; 其次回顾了近年来在 一系列低维材料中发现的热-自旋输运现象. 这些自旋热学现象展现了全新的热电转换机制, 有助于设计新型的热电转换器件, 同时也给出了用热产生自旋流的新途径; 最后介绍了线性响应理论以及在此理论框架下结合声子、电子非平衡格林函数方法进行的一些有益的探索. 量子热输运的研究对热效应基础研究以及声子学器件、能量转换器件的发展有着不可替代的重要作用.     

磁量子结构中二维自旋电子的隧穿输运

研究了零偏压和偏置电压作用下磁量子结构中自旋电子的隧穿输运性质. 结果表明电子自旋 输运的性质不仅取决于磁量子结构的构型、入射电子的能量和波矢, 而且取决于偏置电压. 在零偏压下, 由等同的磁垒磁阱构成的磁量子结构不具有自旋过滤的特点, 而由不等同的磁 垒磁阱构成的磁量子结构却具有较好的自旋过滤特点. 偏置电压极大地改变了磁量子结构中 电子的极化程度, 使得电子隧穿等同的磁垒磁阱构成的磁量子结构的输运性质也显著地依赖 于电子的自旋指向. 关键词： 磁量子结构 自旋电子 隧穿输运 自旋极化     

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.     

多个侧向链对横向量子线传输性质的调制

利用递推格林函数方法，研究了与多个量子点间无相互作用的量子链相耦合的横向量子线的电子输运特性，发现量子链上格点的个数N_U与侧向无限或有限量子链个数N对横向量子线输运性质的不同调制规律.由计算可知，利用格林函数可以方便的对传输构型及传输链中的格点数进行调解，从而得到各种人为量子器件的输运性质. 关键词： 递推格林函数 无限链 有限链 电子输运     

甚长波量子阱红外探测器的暗电流特性研究

基于载流子在量子结构中的输运理论研究了甚长波量子阱红外探测器(峰值响应波长15μm，量子阱个数大于40)的载流子的输运性质.研究结果表明，在甚长波量子阱红外探测器中，电流密度一般很低，暗电流主要来源于能量高于势垒边的热激发电子.通过薛定谔方程和泊松方程以及电流的连续性方程的自洽求解，发现外加偏压下电子浓度在甚长波器件各量子阱的分布发生较大变化，电场在整个器件结构上呈非均匀分布，靠近发射极层的势垒承担的电压远远高于均匀分布的情形.平带模型假定电压在器件体系上均匀分布，导致小偏压下的理论计算值远远低于实验值. 关键词： 甚长波量子阱红外探测器 量子波输运 暗电流     

基于格林函数的多终端量子链状体系电子输运性质的研究

利用递推格林函数技术计算了多终端量子体系的电子输运特性，首先运用递归方法给出介观 或量子体系的格林函数. 然后利用散射矩阵和输运方程给出体系的电导方程，可以将多终端 的输运简化为双终端的输运方程，以便得到体系电子输运的谱结构. 计算结果表明，由于中 间节点的存在，使器件的传输谱偏离一维链的对称性，在低能量端出现一个新的电导峰值. 此外，本方法可以被应用到各种复杂的带有吸附结构量子体系输运的研究中. 关键词： 格林函数 散射矩阵 量子体系     

磁场对四端量子波导中电子输运性质的影响

采用模匹配方法，研究了非均匀磁场下开放的四端量子波导中的电子输运性质. 结果表明，从一端入射的电子可以透射到两个与之垂直的输出端和一个与之平行的输出端. 在没有外加磁场的情况下，两个垂直输出端的输运概率是相同的，但垂直端与水平端的输运概率不同；在外加磁场下，由于磁边缘态效应，两个垂直输出端的输运概率也有着相当大的差别. 通过施加不同的磁场，我们能获得丰富的电子输运结构，如台阶，宽谷，尖峰等；通过调节磁场的大小和比例以及结构参数可控制该量子结构在各输出端的输运概率. 关键词： 电子输运 介观体系 磁效应     

量子气体中的输运行为

输运测量是了解物质性质的一个重要手段.本文简单介绍最近在量子气体中实现的输运实验及其主要结论,包括在类似于介观物理器件中的Landauer输运和强相互作用费米气体中的自旋输运行为.我们着重讨论自旋动力学的特殊性以及其由于全同粒子相互作用所导致的特殊自旋扩散流的形式.     

Electron Transport Through Double-Dot Aharonov-Bohm Interferometer in the Kondo Regime

By applying the slave boson technique, we have studied the electron transport through double-dot Aharonov-Bohm interferometer in the Kondo regime. For the system with symmetric quantum dots, the linear conductance is shown to be enhanced by Kondo effect, but it is suppressed in the deep dot level regime in the presence of nonzero magnetic flux. The Aharonov-Bohm oscillations of the conductance are also investigated.     

Coherent electron transport in a Si quantum dot dimer

We show that the coherence of charge transfer through a weakly coupled double-dot dimer can be determined by analyzing the statistics of the conductance pattern, and does not require a large phase coherence length in the host material. We present an experimental study of the charge transport through a small Si nanostructure, which contains two quantum dots. The transport through the dimer is shown to be coherent. At the same time, one of the dots is strongly coupled to the leads, and the overall transport is dominated by inelastic cotunneling processes.     

Correlation-induced resonances in transport through coupled quantum dots

We investigate the effect of local electron correlations on transport through parallel quantum dots. The linear conductance as a function of gate voltage is strongly affected by the interplay of the interaction U and quantum interference. We find a pair of novel correlation-induced resonances separated by an energy scale that depends exponentially on U. The effect is robust against a small detuning of the dot energy levels and occurs for arbitrary generic tunnel couplings. It should be observable in experiments on the basis of presently existing double-dot setups.     

Non-Markovian transmission through two quantum dots connected by a continuum

We consider a transport setup that contains a double-dot connected by a continuum. Via an exact solution of the time-dependent Schrödinger equation, we demonstrate a highly non-Markovian quantum-coherence-mediated transport through this dot–continuum–dot (DCD) system, which is in contrast with the common premise since in typical case a quantum particle does not reenter the system of interest once it irreversibly decayed into a continuum (such as the spontaneous emission of a photon). We also find that this DCD system supports an unusual steady state with unequal source and drain currents, owing to electrons irreversibly entering the continuum and floating there.     

Nonadiabatic electron pumping: maximal current with minimal noise

The noise properties of pump currents through an open double-quantum-dot setup with nonadiabatic ac driving are investigated. Driving frequencies close to the internal resonances of the double-dot system mark the optimal working points at which the pump current assumes a maximum while its noise power possesses a remarkably low minimum. A rotating-wave approximation provides analytical expressions for the current and its noise power and allows to optimize the noise characteristics. The analytical results are compared to numerical results from a Floquet transport theory.     

Probing entanglement and nonlocality of electrons in a double-dot via transport and noise

Addressing the feasibility of quantum communication with electrons we consider entangled spin states of electrons in a double-dot which is weakly coupled to leads. We show that the entanglement of two electrons in the double-dot can be detected in mesoscopic transport and noise measurements. In the Coulomb blockade and cotunneling regime the singlet and triplet states lead to phase-coherent current and noise contributions of opposite signs and to Aharonov-Bohm and Berry phase oscillations. These oscillations are a genuine two-particle effect and provide a direct measure of nonlocality in entangled states. We show that the ratio of zero-frequency noise to current is equal to the electron charge.     

Mapping nonequilibrium onto equilibrium: the macroscopic fluctuations of simple transport models

We study a simple transport model driven out of equilibrium by reservoirs at the boundaries, corresponding to the hydrodynamic limit of the symmetric simple exclusion process. We show that a nonlocal transformation of densities and currents maps the large deviations of the model into those of an open, isolated chain satisfying detailed balance, where rare fluctuations are the time reversals of relaxations. We argue that the existence of such a mapping is the immediate reason why it is possible for this model to obtain an explicit solution for the large-deviation function of densities through elementary changes of variables. This approach can be generalized to the other models previously treated with the macroscopic fluctuation theory.     

Spin Thermoelectric Effects in a Three-Terminal Double-Dot Interferometer

We theoretically investigate the thermoelectric properties of a three-terminal double-dot interferometer with Rashba spin-orbit interaction. It is found that with some temperature distributions a thermal spin current can even be produced without the help of magnetic flux and by tuning the spin interference effect in the system, a pure spin or fully spin-polarized current can be driven by temperature differences. For the cases that two of the terminals are held at the same temperature, the charge (spin) thermopower and the charge (spin) figure of merit are defined and calculated in the linear response regime. With some choices of the system parameters the calculated spin and charge thermopowers are of the same order of magnitude and the charge figure of merit can exceed 1.     

The probe technique far from equilibrium: Magnetic field symmetries of nonlinear transport

The probe technique is a simple mean to incorporate elastic and inelastic processes into quantum transport problems. Using numerical simulations, we demonstrate that this tool can be employed beyond the analytically tractable linear response regime, providing a stable solution for the probe parameters: temperature and chemical potential. Adopting four probes: dephasing, voltage, temperature, and voltage-temperature, mimicking different elastic and inelastic effects, we provide a systematic analysis of magnetic field and gate voltage symmetries of charge current and heat current in Aharonov-Bohm interferometers, potentially far from equilibrium. Considering electron current, we prove that in the linear response regime inelastic scattering processes do not break the Onsager symmetry. Beyond linear response, even (odd) conductance terms obey an odd (even) symmetry with the threading magnetic flux, as long as the system acquires a spatial inversion symmetry. When spatial asymmetry is introduced, particle-hole symmetry assures that nonlinear conductance terms maintain certain symmetries with respect to magnetic field and gate voltage. These analytic results are supported by numerical simulations. Analogous results are obtained for the electron heat current. Finally, we demonstrate that a double-dot Aharonov-Bohm interferometer can act as a charge rectifier when two conditions are met simultaneously: (i) many-body effects are included, here in the form of inelastic scattering; and (ii) time reversal symmetry is broken.     

平行耦合双量子点分子A-B干涉仪的电荷及其自旋输运

利用非平衡格林函数方法,理论研究每臂中嵌有一个平行耦合双量子点分子的A-B干涉仪(平行耦合双量子点分子A-B干涉仪)的电荷及其自旋输运性质.无外磁场时,与每臂中嵌有一个量子点的A-B干涉仪相比较,平行耦合双量子点分子A-B干涉仪中电子隧穿变得更加容易发生.当平行耦合双量子点分子A-B干涉仪中引入外磁场时,能够在电导能谱中观察到一个Fano共振和一个反共振,这两种输运状态在磁场取适当数值时能够同时消失.此外,通过调节左右两电极间的偏压、磁通和Rashba自旋轨道相互作用,可以对体系自旋输运进行调控.