基于应变锗空穴双量子点的可调耦合和自旋阻塞研究

应变锗空穴量子点是实现超大规模量子计算最有前景的平台之一.由于锗空穴不受超精细相互作影响，有着较长的自旋弛豫时间和量子退相干时间，且锗中本征的强旋轨道耦合和空穴载流子的低有效质量，使得全电场操控空穴自旋量子比特得以实现，极大地降低了器件加工难度，增加了量子点的可扩展性.本文介绍了一种使用应变锗异质结制备重叠栅空穴双量子点器件的方法，完成了应变锗异质结性质测量，空穴双量子点器件制作，单量子点输运性质和双量子点输运性质研究，双量子点耦合可研究调节性研究，以及外磁场存在下的漏电流性质研究和泡利自旋阻塞解除机制的研究.这些工作为未来实现高质量自旋量子比特制备和高保真度量子逻辑门操控提供了实验平台和基本参数.     

嵌入不同耦合强度的串联双量子点铁磁臂系统中的自旋反转输运

采用slave-boson平均场近似方法,通过对嵌入不同耦合强度的串联双量子点铁磁臂系统中的自旋反转输运这一问题的研究,发现无论双量子点系统处于弱耦合还是强耦合状态,Kondo共振峰都会出现不同结构的分裂,这主要是由于自旋反转作用改变了量子点的能级以及量子点间的耦合使得Kondo共振峰发生分裂。这些新奇的分裂现象使得这一双量子点系统的物理特性更有意义,它们将有助于解释自旋电子学中的电子强关联问题。     

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     

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.     

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     

旁耦合于介观环与铁磁电极量子点系统中的自旋极化输运

借助单杂质Anderson模型哈密顿量,及利用格林函数和运动方程等理论,研究旁耦合于介观环和铁磁电极的量子点系统中的极化输运特性.结果表明,通过调节点-环耦合强度、铁磁电极中的极化强度、磁矩相对取向及温度等,均能实现控制体系中自旋极化电流的目的,达到自旋阀效应.为此系统作为一种新的自旋电子材料提供理论依据.     

磁性纳米电机单电子晶体管的自旋输运特性

文章作者在研究磁性隧道结的自旋输运中引入量子点的机械振动自由度,将单电子隧穿和振动自由度耦合所导致的shuttle输运理论应用到自旋电子学中,研究结果表明,shuttle输运对自旋极化输运有很大的影响,其独特的输运性质可以用来设计自旋电子器件,文章在理论上提出具有巨磁效应的自旋阀、高性能的半导体自旋注入器以及电流的整流器.     

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     

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.     

含stubs量子波导系统的电子自旋极化输运性质

理论上研究了含stubs的Rashba自旋轨道耦合（spin-orbit coupling, SOC）量子波导系统的自旋极化输运性质. 利用晶格格林函数方法,发现由于stubs和SOC产生的势阱使系统中出现束缚态,这些束缚态与传播态之间相互干涉导致电导中出现Fano共振结构,同时在对应的自旋极化率中也出现Fano共振或反共振结构. 此外,由于系统结构的突变使电子被反向散射和量子干涉效应,电导中出现一系列的共振峰. 但是,当系统加上外磁场后,所有这些效应都被抑制, 系统重新出现量子化电导, 同时自旋电导也出 关键词： 量子波导 自旋极化输运 自旋轨道耦合     

Spin–orbit interaction induced current dip in a single quantum dot coupled to a spin

Experiments on semiconductor quantum dot systems have demonstrated the coupling between electron spins in quantum dots and spins localized in the neighboring area of the dots. Here we show that in a magnetic field the electrical current flowing through a single quantum dot tunnel-coupled to a spin displays a dip at the singlet–triplet anticrossing point which appears due to the spin–orbit interaction. We specify the requirements for which the current dip is formed and examine the properties of the dip for various system parameters, such as energy detuning, spin–orbit interaction strength, and coupling to leads. We suggest a parameter range in which the dip could be probed.     

A spin switch device based on a triple quantum dots superlattice using the Dicke effect

Spin-dependent transport in a triple quantum dots superlattice system with a bridge coupling to two leads is studied. There exists an odd-even parity oscillation of spin polarization at the central dot level εc = 0 due to the spin-dependent Fano and Dicke effects induced by the quantum interference and the Rashba spin-orbit interaction. In the case of even numbers of triple quantum dots, the device can be used as a spin switch by tuning the energy difference h between the energies of the central and the lateral dots. These results may be helpful to design and fabricate practical spintronic devices.     

Fano--Kondo effect in the T-shaped double quantum dots coupled to ferromagnetic leads

Using an equation-of-motion technique, we theoretically study the Fano--Kondo effect in the T-shaped double quantum dots coupled to two ferromagnetic leads by the Anderson Hamiltonian. We calculate the density of states in this system with both parallel and antiparallel lead-polarization alignments, and our results reveal that the interdot coupling, the spin-polarized strength and the energy level of the side coupled quantum dot greatly influence the density of states of the central quantum dot. This system is a possible candidate for spin valve transistors and may have potential applications in the spintronics.     

Nonequilibrium spintronic transport through an artificial Kondo impurity: conductance,magnetoresistance, and shot noise

We investigate the nonequilibrium transport properties of a quantum dot when spin flip processes compete with the formation of a Kondo resonance in the presence of ferromagnetic leads. Based upon the Anderson Hamiltonian in the strongly interacting limit, we predict a splitting of the differential conductance when the spin flip scattering amplitude is of the order of the Kondo temperature. We discuss how the relative orientation of the lead magnetizations strongly influences the electronic current and the shot noise in a nontrivial way. Furthermore, we find that the zero-bias tunneling magnetoresistance becomes negative with increasing spin flip scattering amplitude.     

Stabilization of the electron-nuclear spin orientation in quantum dots by the nuclear quadrupole interaction

The nuclear quadrupole interaction eliminates the restrictions imposed by hyperfine interaction on the spin coherence of an electron and nuclei in a quantum dot. The strain-induced nuclear quadrupole interaction suppresses the nuclear spin flip and makes possible the zero-field dynamic nuclear polarization in self-organized InP/InGaP quantum dots. The direction of the effective nuclear magnetic field is fixed in space, thus quenching the magnetic depolarization of the electron spin in the quantum dot. The quadrupole interaction suppresses the zero-field electron spin decoherence also for the case of nonpolarized nuclei. These results provide a new vision of the role of the nuclear quadrupole interaction in nanostructures: it elongates the spin memory of the electron-nuclear system.     

Direct spin–phonon coupling of spin-flip relaxation in quantum dots

Within the frame of the Pavlov–Firsov spin–phonon coupling model, we study the spin-flip assisted by the acoustical phonon scattering between the first-excited state and the ground state in quantum dots. We analyze the behaviors of the spin relaxation rates as a function of an external magnetic field and lateral radius of quantum dot. The different trends of the relaxation rates depending on the magnetic field and lateral radius are obtained, which may serve as a channel to distinguish the relaxation processes and thus control the spin state effectively.     

Using a quantum dot system to realize perfect state transfer

There are some disadvantages to Nikolopoulos et al.'s protocol [Nikolopoulos G M, Petrosyan D and Lambropoulos P 2004 Europhys. Lett. 65 297] where a quantum dot system is used to realize quantum communication. To overcome these disadvantages, we propose a protocol that uses a quantum dot array to construct a four-qubit spin chain to realize perfect quantum state transfer (PQST). First, we calculate the interaction relation for PQST in the spin chain. Second, we review the interaction between the quantum dots in the Heitler-London approach. Third, we present a detailed program for designing the proper parameters of a quantum dot array to realize PQST.