Theoretical Study Of Spin Transport Properties Of The Spin - Orbit Coupling System

Spin-orbit coupling (SOC) effect has generated great interest from both academic and practical perspectives. In particular, SOC allows for purely electric manipulation of the electron spin, i.e. magnetic material or external magnetic field is not required, which may result in the ail-electrically controlled spin device, such as the Datta-Das spin field-effect-transistor. In semiconductor two-dimensional electron gas system, there are several typical SOC mechanisms: Rashba SOC, Dresselhaus SOC, and the SOC induced by impurities. Both Rashba and Dresselhaus SOC belong to the intrinsic SOC, and the impurity-induced SOC belongs to the extrinsic SOC. Electronic systems with strong spin-orbit couplings exhibit exotic effects, such as the spin Hall effect, the spin current, etc. Also, the well developed techniques on the fabrication of nanostructures with high mobility have provided the necessary basis for the experimental investigation on the spin-orbit coupling effect.In this dissertation, we investigate the spin transport properties of electrons in systems with spin-orbit couplings. The main results of our investigation are listed as follows, which include four parts. The former three are related to the model design of spin devices with special spin transport properties, and the last is about the study of lifetime of the spin helix.I. Model design of flying spin-qubit logic gate.We propose a solid state proposal for quantum computation with mobile spin qubits in one-dimensional systems, based on recent advances in spintronics. Two simple device units are utilized: one-dimensional semiconductor wires with Dresselhaus SOC and Rashba SOC, separately. Qubit information coded by the electron spin can be manipulated effectively by the SOC when passing through the semiconductor wire. The different manipulative behaviors in Dresselhaus and Rashba wires enable us to make the diverse quantum logic gates. By connecting the Dresselhaus and Rashba units in series, we obtain a universal set of single qubit gates: Hadamard, phase, and 7r/8 gates, inferring that an arbitrary single qubit gate can be achieved in the semiconductor nanowires. We also discuss the conditions for the total transmission of the incident electrons, which ensures all the gates obtained are lossless. II. Bistable switching effect for ballistic transport.In the spin field-effect-transistor (spin-FET), the outgoing current can be tuned by the spin-orbit coupling. We investigate the spin transport properties in the spin-FET with spatially periodic Rashba structure, and obtain an ideal switching effect. When an appropriate magnitude of Rashba strength is provided, an energy gap can be formed due to the periodic Rashba potential. This causes the incident electrons with energies in the gap to be totally reflected. If the Rashba strength is tuned to be smaller than a critical value, all the incident electrons can be transmitted. Therefore, a stable 'rectangle-type' switching effect can be obtained by controlling the Rashba SOC. The switching effect realized through the periodic Rashba structure is found to be more stable than that realized through a single Rashba segment. The ideal switching effect might be applicable in future nanoelectronic devices.III. Model design of the spin filter.The spin filter can generate spin-polarized current out of an unpolarized source. We present two theoretical schemes for spin filters in one-dimensional semiconductor quantum wires with spatially modulated Rashba SOC and weak magnetic field. In the first scheme, the SOC is periodic and the weak magnetic field is applied uniformly along the wire. The periodic Rashba potential results in two coinciding energy gaps for spin-up and spin-down electrons, while the weak magnetic field can separate the two gaps by breaking the time reversal symmetry, and therefore full spin polarizations with opposite signs are obtained within the two separated energy intervals. In the second scheme, the weak magnetic field is periodic while the SOC is uniform. The periodic magnetic field induces two separated energy gaps, and we obtain the spin-up and spin-down current in the two energy intervals. An ideal negative/positive switching effect for spin polarization is realized by tuning the strength of SOC.IV. Investigation on the lifetime of the spin helix.Although strong spin-orbit interaction is useful for manipulating the electron spin, it also induces undesired effect of spin relaxation. Bernevig et al discovered a persistent spin helix (PSH) in two special SOC systems, and the lifetime of the spin helix is robust against the spin-independent impurities (Phys. Rev. Lett. 97, 236601 (2006)). However, influence of the impurity-induced SOC, which actually is a very important problem, was not considered in their proposal. In the present work, we investigate the influence of the impurity-induced spin-orbit scattering on the properties of the spin helix, using the method of equations of motion of Green's function. The results show that lifetime of the spin helix is not divergent, and it is closely dependent on the strength of the extrinsic SOC and the impurity density. The intrinsic SOC also has great influence on the lifetime of the spin helix, if the extrinsic SOC is present.