Spin Dynamics And Spin Manipulation In Confined Semiconductor Nanostructures

Spintronics, whose central theme is the active manipulation of spin degree of freedom in solid-state system, is a multidisciplinary field which aims to develop a new kind of spintronic devices to replace the traditional electronic devices, and has achieved much progress in past decade. Two of the important issues on realizing the spintronic device are how to prolong the spin decoherence time and the spin diffusion length, and how to manipulate the spin efficiently. So it is necessary to understand the mechanism of the spin relaxation/dephasing and the spin diffusion/transport. In this dissertation, we focus on the spin relaxation/dephasing in quantum dots, quantum wires and quantum wells, and the spin diffusion/transport in quantum wells. We also investigate the effect of the intense THz field on the two dimensional electron gas with spin orbit coupling, and the kinetic properties of the extrinsic spin Hall effect.We first give a simple review of the spintronics on spin generation, spin detection and spin relaxation/transport inside semiconductors. Then we give a detailed derivation of the physical origin of the spin orbit coupling in Zinc-blende semiconductors which plays an very important role in spin relaxation。We derive the Dresselhuas term, the Rashba term and the Elliott-Yafet spin orbit coupling, from the 8×8 Kane model. Then we give the exactly expression of the electron Hamiltonian for bulk and two dimension systems, including the electron-impurity scattering, the electron-phonon scattering and the electron-electron Coulomb scattering. All the spin relaxation mechanisms and the theoretical approaches are reviewed: For the two-level system, the relaxation time can be calculated directly by the Fermi golden rule or the time evolution of the equation of motion. While for spin ensemble, the spin relaxation mechanisms include the Elliott-Yafet mechanism, the D'yakonov-Perel' mechanism, the Bir-Aronov-Pikus mechanism, the spin dephasing induced by the hyperfine interaction, and the spin relaxation induced by the inhomogeneous broadening. Then we review the progress of the kinetic spin Bloch equation on the spin relaxation/diffusion.For the single-particle approach, we first consider the spin relaxation time in quantum dots. The spin-relaxation time due to the electron-acoustic phonon scattering in GaAs quantum dots is calculated by the Fermi golden rule after the exact diagonalization of the electron Hamiltonian with the spin-orbit coupling. The spin relaxation time limited by the pizeoelectric phonon is dominant. We show that the perturbation method widely used in the literature is inadequate in accounting for the electron structure and therefore the spin-relaxation time. Different effects such as the magnetic field, dot size, and the temperature on the spin-relaxation time are investigated in detail. Then we investigated the spin-dependent density of states and the density of spin polarization of an InAs-based two-dimensional electron gas with the Rashba spin-orbit coupling under an intenseterahertz laser field by utilizing the Floquet states to solve the time-dependent Schr(?)ingerequation. It is found that both densities are strongly affected by the terahertz laser field. A terahertz magnetic moment perpendicular to the external terahertz laser field in the electron gas is induced. This effect can be used to convert terahertz electric signals into terahertz magnetic ones efficiently.For the many-body investigation, we study the spin relaxation/dephasing or the spin diffusion/transport in quantum wires and quantum wells by numerically solving the kinetic spin Bloch equation. We first consider the spin relaxation in quantum wire due to the Rashba spin orbit coupling. We find that the spin dephasing is strongly affected by the angle of Rashba effective magnetic field and the applied magnetic field. The nonlinearity in spin dephasing time versus the direction of the electric field shows the potential to manipulate the spin lifetime in spintronic device. Moreover, we figure out a quantity that can well represent the inhomogeneous broadening of the system which may help us to understand the many-body spin dephasing due to the Rashba effect. Then we study the spin relaxation under identical Dresselhaus and Rashba coupling strengths in n-type (001) GaAs quantum wells in both the traditional collinear statistics, where the energy spectra do not contain the spin-orbit coupling terms, and the helix statistics, where the spin-orbit couplings are included in the energy spectra. We show that there is only marginal difference between the spin relaxation times obtained from these two different statistics. We further show that with the cubic term of the Dresselhaus spin-orbit coupling included, the spin relaxation time along (110) direction becomes finite, although it is still much longer than that along the other two perpendicular directions. The properties of the spin relaxation along this special direction under varied conditions are studied in detail.With all scattering, especially the electron-electron scattering included, we reinvestigate the spin diffusion in n-type (001) GaAs quantum wells at high temperatures by solving the kinetic spin Bloch equation together with the Poisson equation self-consistently by developing a new numerical scheme. We find that the spin polarization/coherence oscillates along the transport direction even when there is no external magnetic field, and the oscillation period is independent of the electric field. We show that when the scattering is strong enough, electron spins with different momentums oscillate in the same phase which leads to an equal transversal spin injection length and an ensemble transversal injection length. The intrinsic scattering is already strong enough for such feature. The scattering, temperature, quantum well width, and external magnetic/electric field dependences of the spin diffusion are studied in detail. Then we explore the effect of the anisotropy of the spin-orbit coupling on the competition between the Rashba and the Dresselhaus spin-orbit couplings in the spin diffusion. We find the spin-diffusion length shows strong anisotropy not only for the spin-polarization direction but also for the spin-diffusion direction. Without the cubic term of the Dresselhaus spin-orbit coupling and with the identical Dresselhaus and Rashba strengths, infinite diffusion lengths can be obtained either for the spin-diffusion/injection direction along ((?)10), regardless of the direction of spin polarization, or for the spin polarization along (110), regardless of the direction of the spin diffusion/injection. With the cubic Dresselhaus term included, although the spin-diffusion length becomes finite and decreases with the temperature and the electron density, the anisotropy for the spin-diffusion direction and spin-polarization direction is maintained. Due to the contribution of the cubic term of the Dresselhaus spin orbit coupling, the maximum spin diffusion length for the injection direction along ((?)10) occurs when the Rashba strength is slightly smaller than (instead of equal to) the Dresselhaus strength,Finally, we study the kinetics of the extrinsic spin Hall conductivity induced by the skew scattering from the fully microscopic kinetic spin Bloch equation approach in (001) GaAs symmetric quantum well. In the steady state, the extrinsic spin Hall current/conductivity vanishes for the linear-k dependent spin-orbit coupling and is very small for the cubic-κdependent spin-orbit coupling. The spin precession induced by the Dresselhaus/Rashba spin-orbit coupling plays a very important role in the vanishment of the extrinsic spin Hall conductivity in the steady state. An in-plane spin polarization is induced by the skew scattering, thanks to the spin-orbit coupling. This spin polarization is very different from the current-induced spin polarization.