Design of Electron-Spintronics devices

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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