一维双方势垒量子隧穿的研究及其数值模拟

量子隧穿效应在实际技术中具有重要应用,本文首先展示了如何求解一维任意边界非对称以及对称双方势垒的透射系数,然后研究了对称双方势垒透射系数对垒宽、垒间距以及微观粒子入射能量与垒高比值(E/U 0)的变化依赖关系.最终得出以下结论,随着双方势垒垒宽的增加,透射系数从最大值1衰减至最小值0.随着垒间距的增加,透射系数呈现周期振荡,本文首次推导得出透射系数最大时对应的垒间距解析表达式,并给出振荡的周期,进一步证明得到它等于微观粒子的德布罗意波长.当垒宽越小时,随着E/U 0的增大,透射系数更容易达到1,并且保持不变,当垒间距越大时,随着E/U 0的增大,透射系数振荡周期变大,而振幅变小,粒子更容易实现共振隧穿.     

一维梯形势垒透射系数的计算

构造了在超晶格物理中具有潜在应用价值的一维梯形势垒模型并解析地得到了粒子隧穿势垒的透射系数.给出了该透射系数在低能近似和微斜近似下的近似表达式,并指出它可以视为方势阱透射系数在低能下的修正.此外,区别于方势垒模型,梯形势垒透射系数的峰值并不一定对应于共振透射,但是峰值处对应的粒子入射能量近似地满足势垒高度和相应一维无限深梯形势阱中粒子能级之和的规律.     

处理具有任意形状势垒的磁性隧道结中电子输运的一个简单方法

在Slonczewski自由电子模型的基础上,提出了一个可用于处理具有任意形状势垒的磁性隧道结中磁电子输运的简单方法,并以三种常见构形的势垒,即梯形势垒,计入了镜像势的梯形势垒和抛物线势垒为例,讨论了势垒形状对隧穿磁电阻及其随偏压变化的影响. 关键词： 磁性隧道结 隧穿磁电阻 任意形状势垒 非零偏压     

一维三方势垒量子隧穿特性的研究

本文通过严格求解定态薛定谔方程,研究了一维对称三方势垒的量子隧穿特性,解析地给出透射系数的精确表达式,并且数值模拟了势垒高度、势垒间距以及粒子入射能量对透射系数的影响.结果表明：当取不同的势垒宽度,或者不同的粒子入射能量时,透射系数随着势垒间距的增加而呈现出明显的周期式振荡.将一维对称双方势垒和三方势垒进行比较,透射系数随着势垒间距的增大,均呈现周期性振荡,并且振荡周期相同,但三方势垒振荡更剧烈,振幅越大,并且三方对称势垒是双峰曲线,而双方对称势垒是单峰曲线.该特性为设计新型纳米器件以及共振隧穿量子器件等提供理论指导.     

N重方势垒结构共振隧道效应研究

本文从理论上研究了N重方势垒结构的共振隧道效应,推导出透射系数及共振隧道条件的解析表达式,结果发现,由于多势垒结构(n≥3)量子阱间的耦合,共振能级不同于量子阱的本征值。此外,由透射系数表示式证实了多势垒结构电子透射谱在共振能级附近为Lorentzian型。所得结果对于分析透射系数随能量的变化关系,估计共振能级以及制造共振隧道器件都具有十分重要的意义。 关键词：     

电子横向运动对共振隧穿的影响

讨论了电子横纵方向运动耦合时的隧穿现象,对CdSe/Zn1-xCdxSe方形双势垒结构和抛物形双势垒结构的数值计算表明,在零偏压和非零偏压情况下,电子横向运动对共振隧穿的影响是不容忽略的。     

对称双势垒量子阱中自旋极化输运的时间特性

电子的隧穿时间是描述量子器件动态工作范围的重要指标. 本文考虑k³ Dresselhaus 自旋轨道耦合效应对系统哈密顿量的修正, 结合转移矩阵方法和龙格-库塔法来解含时薛定谔方程, 进而讨论了电子在非磁半导体对称双势垒结构中的透射系数及隧穿寿命等问题. 研究结果发现:由于k³ Dresselhaus 自旋轨道耦合效应使自旋简并消除, 并在时间域内得到了表达, 导致自旋向上和自旋向下电子的透射峰发生了自旋劈裂; 不同自旋取向的电子构建时间和隧穿寿命不同, 这是导致自旋极化的原因之一; 电子的自旋极化在时间上趋于稳定. 关键词： 自旋极化输运 透射系数 隧穿寿命 自旋极化率     

半导体量子器件物理讲座第四讲 共振隧穿器件及其电路应用

当一个电子的能量低于势垒高度时,它仍可以隧穿通过势垒,在一定条件下,双势垒结构中电子的隧穿几率甚至可以接近1,利用这种共振隧穿现象可以做成共振隧穿二极管,它的电流－电压特性曲线中会出现负微分电阻,利用这种负阻效应可以做成高频振荡器和倍频器等电子器件,双势垒结构与通常的双极晶体管结合可以做成共振隧穿双极晶体管,它们可以用来做成多态记忆器和模数转换器等器件。     

非对称方势垒量子隧穿研究及其数值模拟

为解决传统量子力学方法在研究非对称双势垒问题上计算过于繁琐的问题,利用转移矩阵的方法分别研究了电子对于非对称单势垒和非对称双势垒的量子隧穿特性,对两种情况分别得出了简洁的透射率公式.数值计算结果表明,在合适的参数下,单、双结结构都表现出良好的电导开关效应,且在入射电子能量大于势垒高度时,透射系数呈现显著的周期变化的量子...     

电子双势垒量子隧穿的散射矩阵方法及其数值模拟

采用散射矩阵的方法研究了电子在由两个方势垒组成的双势垒结构中的隧穿特性.将电子在双势垒中的隧穿过程分为相干输运和非相干输运两部分来研究,相干输运导致了隧穿透射系数随中间层厚度变化产生量子振荡,而非相干输运导致了振荡振幅的衰减.双势垒总的透射系数与势垒高度、入射和出射波矢的匹配性有关,数值计算的结果证实了相关结论.     

Electrofluorescence polarity in a molecular diode

The kinetic equations describing the transmission of an electron in the molecular compound “electrode 1–molecule–electrode 2” (1M2 system) are derived using the method of a nonequilibrium density matrix. The steady-state transmission regime is considered, for which detailed analysis of the kinetics of electrofluorescence formation in systems with symmetric and asymmetric couplings between the molecule and the electrodes is carried out. It is shown that the optically active state of the molecule is formed as a result of electron hops between the molecule and each of the electrodes, as well as due to inelastic interelectrode tunneling of the electron. The electrofluorescence power for a molecular diode (asymmetric 1M2 system) depends on the polarity of the voltage bias applied to the electrodes. The polarity is explained using a model in which the optically active part of the molecule (chromophore group) is represented by the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Two mechanisms of the emergence of polarity are revealed. One mechanism is associated with nonidentical Stark shifts of the HOMO and LUMO levels relative to the Fermi levels of the electrodes. The second mechanism is associated with the fact that the rates of an electron hopping between HOMO (LUMO) and one of the electrodes are much higher than the rates of such a hopping with the other electrode. The conditions in which each mechanism can be implemented experimentally are indicated.     

Resonant tunneling properties of inverted Morse double quantum barrier

We study resonant tunneling characteristics of inverted Morse double quantum barrier structures. The effect of electric bias and structure parameters is calculated by using non-equilibrium Green's function method. Results for the transmission coefficients are compared with the structure parameters. Our results show that the widths of the wells and heights of barriers have a significant effect on the transmission properties. We found that the resonant peak of the transmission coefficient decreases with increasing electric field bias. Moreover, resonant energy level increases with increasing barrier height and increasing width parameters.     

Electron transport through a molecular conductor with center-of-mass motion

The linear conductance of a molecular conductor oscillating between two metallic leads is investigated numerically both for Hubbard interacting and noninteracting electrons. The molecule-leads tunneling barriers depend on the molecule displacement from its equilibrium position. The results present an interesting interference which leads to a conductance dip at the electron-hole symmetry point that could be experimentally observable. It is shown that this dip is caused by the destructive interference between the purely electronic and phonon-assisted tunneling channels, which are found to carry opposite phases. When an internal vibrational mode is also active, the electron-hole symmetry is broken but a Fano-like interference is still observed.     

分子结电学特性的理论研究

在第一性原理的基础上，对共扼分子2-氨基-5-硝基-1，4-二乙炔基-4’，-苯硫醇基苯(2-amino-5-nitro-1,4-diethyny-4’-benzenethiol-benzene)与金表面形成的分子结的电学特性进行了理论研究.利用密度泛函理论计算了该分子及扩展分子的电子结构；讨论了分子与金表面的相互作用，定量地确定了耦合常数，求出了电子的迁移强度；利用弹性散射格林函数法研究了该分子结的伏—安特性.计算结果表明，当外加偏压小于0.9V时分子结存在电流禁区，随着偏压升高，分子结的电导出现平台特 关键词： 化学吸附 分子结 分子电子学     

Magnetotransport in a graphene monolayer with two tunable magnetic barriers

The magnetotransport property for a monolayer graphene with two turnable magnetic barriers has been investigated by the transfer-matrix method. We show that the parameters of barrier height, width, and interval between two barriers affect the electron wave decaying length, which determine the conductance with parallel or antiparallel magnetization configuration, and consequently the tunneling magnetoresistance (TMR) for the system. Interestingly, a graphene attached by two barriers with different heights can produce a resonant TMR peak at low energy region one order of magnitude larger than that for the system with two same height barriers because that the asymmetry of magnetic barriers block the electron transmission in the case of antiparallel magnetization configuration. The results obtained here may be useful in understanding of electron tunneling in graphene and in designing of graphene-based nanodevices.     

Bias voltage effect on electron tunneling across a junction with a ferroelectric-ferromagnetic two-phase composite barrier

The effect of bias voltage on electron tunneling across a junction with a ferroelectric-ferromagnetic composite barrier is investigated theoretically. Because of the inversion symmetry breaking of the spontaneous ferroelectric polarization, bias voltage dependence of the electron tunneling shows significant differences between the positive bias and the negative one. The differences of spin filtering or tunnel magnetoresistance increase with the increasing absolute value of bias voltage. Such direction preferred electron tunneling is found intimately related with the unusual asymmetry of the electrical potential profile in two-phase composite barrier and provides a unique change to realize rectifying functions in spintronics.     

1,8-二巯基芘分子电学特性的理论研究

在第一性原理的基础上 ,对 1,8 二巯基芘分子的电学特性进行了理论研究 .采用了 3个Au原子构成的团簇来模拟Au表面 .首先利用密度泛函理论计算了 1,8 二巯基芘分子的电子结构及其和Au表面的相互作用 ,再利用前线轨道理论和微扰理论定量地确定了该分子和Au表面的相互作用能常数 .最后利用弹性散射格林函数法研究了该分子结的伏 安特性 .计算结果表明 ,分子中的硫原子和Au原子形成很强的共价键 .当外加偏压小于 1V时分子结存在电流禁区 ,随着偏压升高 ,分子结的电导出现平台结构 .分子结的电导特性和其电子结构密切相关 ,扩展分子轨道为电荷的迁移提供了通道 ,而局域轨道对电流贡献很小     

Coherence destruction of tunneling in a quantum-dot molecule with bias

This paper studies the constraint conditions for coherence destruction in tunneling by using perturbation theory and numerical simulation for an AC-field with bias and Coulomb interaction between electrons in a quantum dot molecule. Such conditions can be described by using the roots of a Bessel function Jn(x), where n is determined by both the bias and the Coulomb interactions, and x is the ratio of the amplitude to the frequency of the AC-field. Under such conditions, a coherent suppression of tunneling occurs between localized electronic states, which results from the dynamical localization phenomenon. All the conditions are verified with numerical simulations.     

Resonant tunneling through electronic trapping states in thin MgO magnetic junctions

We report an inelastic electron tunneling spectroscopy study on MgO magnetic junctions with thin barriers (0.85-1.35 nm). Inelastic electron tunneling spectroscopy reveals resonant electronic trapping within the barrier for voltages V>0.15 V. These trapping features are associated with defects in the barrier crystalline structure, as confirmed by high-resolution transmission electron microscopy. Such defects are responsible for resonant tunneling due to energy levels that are formed in the barrier. A model was applied to determine the average location and energy level of the traps, indicating that they are mostly located in the middle of the MgO barrier, in accordance with the high-resolution transmission electron microscopy data and trap-assisted tunneling conductance theory. Evidence of the influence of trapping on the voltage dependence of tunnel magnetoresistance is shown.