两端线型双量子点分子Aharonov-Bohm干涉仪电输运

设计一个两端线型双量子点分子Aharonov-Bohm (A-B)干涉仪. 采用非平衡格林函数技术, 理论研究无含时外场作用下的体系电导和引入含时外场作用下的体系平均电流. 在不考虑含时外场时, 调节点间耦合强度或磁通可以诱导电导共振峰劈裂. 控制穿过A-B干涉仪磁通的有无, 实现了共振峰电导数值在0与1之间的数字转换, 为制造量子开关提供了一个新的物理方案. 同时借助磁通和Rashba自旋轨道相互作用, 获得了自旋过滤. 当体系引入含时外场时, 平均电流曲线展示了旁带效应. 改变含时外场的振幅, 实现了体系平均电流的大小与位置的有效控制, 而调节含时外场的频率, 则可以实现平均电流峰与谷之间的可逆转换. 通过调节磁通与Rashba自旋轨道相互作用, 与自旋相关的平均电流亦得到有效控制. 研究结果为开发利用耦合多量子点链嵌入A-B 干涉仪体系电输运性质提供了新的认知. 上述结果可望对未来的量子器件设计与量子计算发挥重要的指导作用.     

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     

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.     

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.     

Nonequilibrium spin-polarized transport through two parallel-coupled quantum dots

Spin-polarized transport through an Aharonov–Bohm ring containing two quantum dots (QDs) in each of its arms is studied by using the nonequilibrium Green’s function technique. We take both the Rashba spin-orbit interaction that exists in one of the QDs, and an inhomogeneous magnetic flux penetrating through the ring, into consideration. It is found that a 100% spin-polarized current can be driven out of the QDs ring, and both the spin directions and the magnitude of the outgoing current can be controlled. The origin of the pure spin-up or spin-down current is interpreted in terms of the spin accumulation in the QDs. This device is realizable by presently available technologies and can be used as a spin filter.     

Flux-dependent transport through an Aharonov-Bohm interferometer with embedded multiple coupled quantum dots

The electron transport through an Aharonov-Bohm (AB) interferometer with embedded four coupled quantum dots (QDs) is studied with the Green's function technique. The QDs are coupled to each other by the hopping integral tc. Two among them connect with the left lead and the other two with the right lead by the tunneling matrix element T which incorporates the effects of the applied magnetic field φ. The linear conductance spectra swap between the molecular levels and the atomic states by adjusting tc and T. Fano effect appears when the electrons tunnel through different channels contributed by different QDs energy levels, and the Fano resonance peaks split for large tc. The Fano factor can be manipulated by tc, T, φ, and the QD energy levels.     

AlGaN/GaN量子阱中子带的Rashba自旋劈裂和子带间自旋轨道耦合作用研究

首先把本征值方程投影到导带的子空间中, 进而得到AlGaN/GaN量子阱中第一、二子带的Rashba自旋劈裂系数(α ₁, α ₂)和子带间自旋-轨道耦合系数η₁₂. 然后自恰求解薛定谔方程和泊松方程计算了不同栅压的量子阱中的α ₁, α ₂和η₁₂, 并分别讨论了量子阱阱层、左右异质结界面和垒层对它们的贡献. 结果表明可以通过栅压来调节自旋-轨道耦合系数, 子带间自旋轨道耦合系数η₁₂比Rashba自旋劈裂系数α ₁, α ₂小, 但基本在同一数量级.     

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.     

Polarized spin transport in mesoscopic quantum rings with electron--phonon and Rashba spin--orbit coupling

The influence of electron--phonon (EP) scattering on spin polarization of current output from a mesoscopic ring with Rashba spin--orbit (SO) interaction is numerically investigated. There are three leads connecting to the ring at different positions; unpolarized current is injected to one of them, and the other two are output channels with different bias voltages. The spin polarization of current in the outgoing leads shows oscillations as a function of EP coupling strength owing to the quantum interference of EP states in the ring region. As temperature increases, the oscillations are evidently suppressed, implying decoherence of the EP states. The simulation shows that the magnitude of polarized current is sensitive to the location of the lead. The polarized current depends on the connecting position of the lead in a complicated way due to the spin-sensitive quantum interference effects caused by different phases accumulated by transmitting electrons with opposite spin states along different paths.     

对称抛物势阱磁性隧道结中的自旋输运及磁电阻效应

研究了两端具有铁磁接触的对称抛物势阱磁性隧道结(F/SPW/F)中自旋相关的隧穿概率和隧穿磁电阻,讨论了量子尺寸效应和Rashba 自旋轨道耦合作用对自旋极化输运特性的影响.研究结果表明:隧穿概率和隧穿磁电阻随抛物势阱宽度的增加发生周期性的振荡.抛物势阱深度的增加减小了隧穿概率和隧穿磁电阻的振荡频率.Rashba 自旋轨道耦合强度的增加加大了隧穿概率和隧穿磁电阻的振荡频率.隧穿概率和隧穿磁电阻的振幅和峰谷比强烈依赖于两铁磁电极中磁化方向的夹角. 关键词： 磁性隧道结 Rashba 自旋轨道耦合 隧穿概率 隧穿磁电阻     

Spin-dependent transport in four-terminal rings in the presence of the Rashba spin-orbit coupling

Spin dependent transport in one-dimensional four-terminal rings (FTRs) is investigated in the presence of the Rashba spin-orbit coupling (RSOC). In the absence of the RSOC, the conductances as a function of the electron wave vector show resonant behavior for symmetrical configurations. For asymmetrical configurations, the conductances show peculiar zero-conductance resonances, and two kinds of conductance zeros have been found. In the presence of the RSOC, the original conductance zeros disappear as new conductance zeros are generated. Moreover, two kinds of symmetry relations have been found in the conductances, the spin dependent conductances and the spin polarizations. For the FTRs with axial or central symmetry, the phase-locking effect has been found in terminal 2, where there is no spin polarization. Under a weak magnetic field, the Zeeman term is treated by a perturbation, and it is found that the Zeeman effect is obvious for weak RSOC.     

Si4团簇电子输运性质的第一性原理计算

采用密度泛函理论和非平衡格林函数相结合的方法对Si₄ 团簇与Au(100)电极空位相连的纳米结点的电子输运性质进行了理论模拟计算, 得到了纳米结点在不同距离下的几何结构、电子结构、电导、透射谱、电荷转移量; 讨论了当距离d_z=12.004 Å时纳米结点的电导、电流随电压的变化关系. 关键词： 密度泛函理论 非平衡格林函数 4团簇')" href="#">Si₄团簇 电子输运     

太赫兹场辅助的单量子阱自旋共振输运

在考虑自旋轨道耦合的情况下,研究了通过一非磁性半导体异质结的太赫兹光子驱动的电子输运,研究结果显示频率相关的电导谱出现不对称的Fano型共振的劈裂;随着振荡场幅度的增加,多光子辅助过程出现;通过改变外加场参数,可以实现自旋过滤目的,特别是在干涉相消Fano型共振的频率位置时,出现了100%纯的自旋极化流.这些属性有助于大范围可调的自旋过滤器的实现并且可以得到纯的自旋透射流.     

半导体量子阱中电子自旋弛豫和动量弛豫

根据电子自旋轨道耦合对自旋极化弛豫影响的DP机理进一步导出了半导体中电子自旋弛豫与动量弛豫及载流子浓度的关系，并采用飞秒抽运探测技术在室温下测量AlGaAs/GaAs 多量子阱中载流子浓度在 1×10¹⁷—1×10¹⁸cm^-3范围内，电子自旋弛豫时间由58ps增加至82 ps的变化情况，与理论计算值符合，说明了随着载流子浓度的增加，载流子间的频繁散射加速了电子动量驰豫，减弱了电子自旋轨道耦合作用，从而延长了电子自旋寿命. 关键词： 电子自旋轨道耦合 电子自旋弛豫和动量弛豫 飞秒光谱技术     

硼或氮掺杂的锯齿型石墨烯纳米带的非共线磁序与电子输运性质

基于非共线磁序密度泛函/非平衡格林函数方法,研究了硼或氮掺杂的锯齿型石墨烯纳米带的非共线磁序与电子透射系数.未掺杂的石墨烯纳米带的计算结果表明磁化分布主要遵循类似于Neel磁畴壁的螺旋式磁化分布.相比于未掺杂的情况,硼/氮掺杂的石墨烯纳米带的磁化分布出现了双区域的特征,即杂质原子附近的磁化较小,杂质原子左(右)侧区域的磁化分布更接近于左(右)电极的磁化方向,这为通过掺杂手段在石墨烯纳米带边缘上构建不同磁畴壁提供了可能性.与未掺杂的透射系数不同的是,硼/氮掺杂的石墨烯纳米带的透射系数在费米面附近随着磁化偏转角增大而减小,表明非共线磁序引起的自旋翻转散射占据主导地位.而在E=±0.65 eV处,出现了一个较宽的dip结构,投影电子态密度的分析表明其来源于杂质原子形成的束缚态所引起的背散射.我们的研究结果对于理解石墨烯纳米带中的非共线磁序与杂质散射以及器件设计具有一定的意义.     

Effect of well thickness on the Rashba spin splitting and intersubband spin-orbit coupling in AlGaN/GaN/AlGaN quantum wells with two subbands

By projecting the characteristic equation into the subspace of the conduction band, the Rashba spin splitting coefficient for the first two subbands (α₁, α₂) and the intersubband spin-orbit coupling coefficient (η₁₂) in AlGaN/GaN quantum well structure are obtained. Then sizable α₁, α₂ and η₁₂ in QWs are calculated by solving the Schrödinger and Poisson equations self-consistently. We find that the internal electric field is crucial for considerable spin-orbit coupling effect in III-nitride QWs and the spin-orbit coupling coefficient can be greatly modulated by the well thickness. Compared with the Rashba coefficient, the intersubband spin-orbit coupling coefficient is basically of the same order of magnitude. The results show the great possibility of spin manipulation in low-dimensional semiconductors, and III-nitride QWs are candidates for the design of spintronic devices.     

Spin-dependent Breit-Wigner and Fano resonances in photon-assisted electron transport through a semiconductor heterostructure

We theoretically investigate the electron transmission through a seven-layer semiconductor heterostructure with the Dresselhaus spin-orbit coupling under two applied oscillating fields. Numerical results show that both of the spin-dependent symmetric Breit-Wigner and the asymmetric Fano resonances appear and that the properties of these two types of resonance peaks are dependent on the amplitude and the relative phases of the two applied oscillating fields. The modulation of the spin-polarization efficiency of transmitted electrons by the relative phase is also discussed.     

The electronic properties and electron transport of sawtooth penta-graphene nanoribbon under uniaxial strain: ab-initio study

ABSTRACT

The electronic properties and electron transport of a sawtooth penta-graphene nanoribbon (SSPGNR) under uniaxial strains are theoretically studied by density-functional theory (DFT) in combination with the non-equilibrium Green's function formalism. We investigated the electronic structures and the current–voltage (I–V) characteristics of the SSPGNRs under a sequence of uniaxial strains in range from 10% compression to 10% stretch. In this strained range, carbon atoms still keep a pentagon network, but with the changing bond lengths. The C–C bond lengths change almost linearly with the tolerable strain. The value of the band gap of SSPGNRs can be depicted as a parabola under uniaxial strain. Our calculations show that the current is monotonous increase with compressive strain at the same applied bias voltage. In case of tensile strain, the variable rule of the current is different that it increases at first and decrease later. The fundamental physical properties (band structure, I–V characteristic) of SSPGNRs seem to be more sensitive to compressive strain than the stretch strain. The current intensity of the compressive-SSPGNR is by 2 orders of magnitude compared to that of the tensile-SSPGNR at the same strain in range from 6% to 10%. The results obtained from our calculations are beneficial to practical applications of these strained structures in SSPGNRs-based electromechanical devices.     

B/N掺杂对于石墨烯纳米片电子输运的影响

选用锯齿(zigzag)型石墨烯纳米片为研究对象, Au作为电极, 分子平面与Au的(111)面垂直, 并通过末端S原子化学吸附于金属表面, 构成两种分子器件: 一种是在纳米片的边缘掺杂N(B)原子, 发现电流-电压具有非线性行为, 但是整流系数较小, 特别是掺杂较多时, 整流具有不稳定性; 另一种是用烷链把两个石墨烯片连接, 在烷链附近和石墨烯片的边缘进行N(B)掺杂, 发现在烷链附近掺杂具有较大的整流, 但是掺杂的原子个数和位置会影响整流性能. 研究表明: 整流主要为正负电压下分子能级的移动方向和空间轨道分布不同导致. 部分体系中的负微分电阻现象主要由于偏压导致能级移动和透射峰形态的改变, 并且在某些偏压下主要透射通道被抑制而引起. 关键词： 石墨烯纳米片 电子输运 整流行为 非平衡格林函数方法     

铁磁/半导体(绝缘体)/铁磁异质结中渡越时间与两铁磁层磁矩夹角变化的关系

在群速度概念的基础上, 研究了自旋极化电子隧穿通过铁磁体/半导体(绝缘体)/铁磁体异质结时, 渡越时间随两端铁磁层中磁矩夹角变化的关系. 研究结果表明: 当中间层为半导体层时, 由于半导体层中的Rashba自旋轨道耦合强度的影响, 自旋向上电子和自旋向下电子的渡越时间差会在两铁磁层相对磁矩夹角为π/2和3π/2附近出现一个极小值. 当中间层为绝缘体层时, 势垒高度的变化会导致不同取向的自旋极化电子渡越时间差的变化, 并当势垒高度超过一临界值时发生翻转. 关键词： 铁磁体/半导体(绝缘体)/铁磁体异质结 Rashba自旋轨道耦合强度 渡越时间 磁矩