Design of Electron-Spintronics devices

纳米制造技术的最新进展使我们能够将电子限制在量子点或量子线等纳米结构中。我们的目标是设计纳米电子自旋电子学器件，其中电子的自旋自由度起着重要作用。我们的主要结果如下所示。我们研究了由半导体量子点和量子线组成的人工分子基态的自旋组态。我们从理论上研究了嵌入一维或二维Rashba自旋轨道耦合的局域自旋之间的RKKY相互作用。我们还研究了由局域自旋通过扭曲的RKKY相互作用耦合而成的线形和环状人工分子的自旋组态。我们发现人工分子的总自旋的平方模沿扭角0振荡，并且在0=奇数倍II处有极大值。环状人工分子的总自旋的平方范数在一定的值0处发生了很大的变化，其中…最低的两个能级相互交叉的越多。我们分析了半导体量子点中从光子极化量子比特到电子自旋极化量子比特的量子态转移(QST)的保真度，特别关注了电子与同时产生的空穴之间的交换作用。为了实现高保真QST，我们必须尽快分离电子和空穴，因为电子-空穴交换作用改变了电子自旋的取向。因此，我们提出了一种双点结构来快速分离电子和空穴，并证明了在满足共振隧穿条件的情况下，量子温度扫描的保真度可以高达0.996。我们分析了实验观察到的Co纳米粒子在库仑阻挡下的TMR比，发现纳米粒子中的自旋弛豫时间比体相中的要大得多。4.从理论上研究了铁磁注入器和探测器电极连接的非磁性金属中的自旋输运。我们推导出了从金属到隧穿区域的自旋累积信号的一般表达式。这使我们能够讨论最近关于自旋注入和探测实验的争议。将结果推广到超导器件，我们发现，由于带隙超导体是用于自旋输运而不是电荷输运的低载流子系统，所以打开超导能隙可以强烈地增强自旋累积信号。这种增强也有望在半导体器件中实现。较少

Recent progress in nano-fabrication technology enables us to confine electrons in nano-structures such quantum dots or quantum wires. Our objective is to design the nano-electronspintoronics devices where the spin degrees of freedom of electrons play an important role. Our main results are listed below.1. We studied the spin configuration of the ground state of an artificial molecule consisting of semiconductor quantum dots and quantum wires. We theoretically study the RKKY interaction between localized spins embedded in a 1D-or 2DEG with Rashba spin-orbit coupling. We also study the spin-configuration of linear and ring-shaped artificial molecules consisting localized spins coupled via the twisted RKKY interaction. We find that the squaremorm of the total spin of an artificial molecule oscillates with the twist angle 0 and has maxima at 0 = an odd integer times II. The square-norm of the total spin of the ring-shaped artificial molecules change drastically at certain values of 0 where … More the lowest two energy levels cross each other.2. We analyzed the fidelity of the quantum state transfer (QST) from a photon-polarization qubit to an electron-spin-polarization qubit in a semiconductor quantum dot, with special attention to the exchange interaction between the electron and the simultaneously created hole. In order to realize a high-fidelity QST we had to separate the electron and hole as soon as possible, since the electron-hole exchange interaction modifies the orientation of the electron spin. Thus, we propose a double-dot structure to separate the electron and hole quickly and show that the fidelity of the QST can reach as high as 0.996 if the resonant tunneling condition is satisfied.3. We analyzed the experimentally observed TMR ratio of Co nanoparticles in the Coulomb blockade regime and found that the spin relaxation time in nano-partides is highly enhanced in comparison with that in the bulk. 4. We studied theoretically the spin transport in a nonmagnetic metal connected to ferromagnetic injector and detector electrodes. We derived a general expression for the spin accumulation signal which covers from the metallic to the tunneling regime. This enables us to discuss reoent controversy on spin injection and detection experiments. Extending the result to a superconducting device, we found that the spin accumulation signal was strongly enhanced by opening of the superconducting gap since a gapped superconductor was a low carrier system for spin transport but not for charge. The enhancement is also expected in semiconductor devices. Less