Investigations On Several Problems In Spintronics From Mesoscopic And Many-Body Approaches

In recent years, much attention has been devoted to spintronics, whose purpose is to manipulate the spin degree of freedom instead of (or together with) the charge degree of freedom in solid-state system. In this multidisciplinary field, people try to develop spintronic devices to substitute the traditional electronic devices. Two of the most important issues in this field are prolonging the spin decoherence time and efficient generating spin polarization. So it is important to understand the mechanism of the spin relaxation/dephasing and spin generation. In this dissertation, we focus on the spin relaxation/dephasing in quantum wells, the spin Hall effect in two-dimensional hole system, and we propose three schemes for spin filters.We first review the development of spintronics, including spintronic devices, spin generation, spin detection, and spin relaxation/dephasing. We also simply introduce the spin-orbit coupling in III-V semiconductors that includes the Dresselhaus, Rashba, and Elliott-Yafet terms. The spin relaxation/dephasing mechanisms for spin ensemble are reviewed, including the Elliott-Yafet mechanism, the D'yakonov-Perel' mechanism, the Bir-Aronov-Pikus mechanism, and the hyperfine interaction mechanism. Especially, we emphasize importance of the spin relaxation/dephasing induced by inhomogeneous broadening. The progress of the kinetic spin Bloch equation theory from a fully microscopic many-body approach is also reviewed. Then we review the Coulomb Singwi-Tosi-Land-Sj(o|¨)lander local field correction beyond the random phase approximation. Finally, we introduce the basic knowledge of mesoscopic physics such as the Landauer-Büttiker formula.Prom many-body approach, we first perform a fully microscopic investigation on the spin relaxation/dephasing in n-type (001) GaAs quantum wells with Al_0.4Ga_0.6As barrier due to the D'yakonov-Perel' mechanism from very low temperature to room temperature, by constructing and numerically solving the kinetic spin Bloch equations. We consider all possible scattering such as the electron-longitudinal-optical-phonon scattering, the electron-nonmagnetic-impurity scattering, the electron-electron Coulomb scattering, and the electron-acoustic-phonon scattering in our calculation. The spin relaxation/dephasing times calculated from our theory with one fitting spin splitting parameter are in good agreement with the experimental data by Ohno et al. [Physica E 6, 817 (2000)] over the whole temperature regime, from 20 K to 300 K. We further show the temperature dependence of the spin relaxation/dephasing time under various conditions such as electron density, impurity density and well width. We predict a peak solely due to the Coulomb scattering in the spin relaxation/dephasing time at low temperature in samples with low electron density but high mobility. The hot-electron spin kinetics at low temperature is also addressed with many features quite different from the high temperature case predicted.Then we study the electron spin relaxation/dephasing in intrinsic and p-type (001) GaAs quantum wells. We include the spin-flip electron-heavy hole exchange scattering which leads to the Bir-Aronov-Pikus spin relaxation/dephasing. We show that, due to the absebce of the nonlinear terms in the electron-heavy hole exchange scattering in the Fermi-golden-rule approach, the spin relaxation due to the Bir-Aronov-Pikus mechanism is greatly exaggerated at moderately high electron density and low temperature in the literature. We compare the spin relaxation/dephasing time due to the Bir-Aronov-Pikus mechanism with that due to the D'yakonov-Perel' mechanism which is also calculated from the kinetic spin Bloch equations with all the scatterings. We find that, in intrinsic quantum wells, the contribution from the Bir-Aronov-Pikus mechanism is much smaller than that from the D'yakonov-Perel' mechanism at low temperature, and it is comparable at high temperature. In p-type quantum wells, the spin relaxation/dephasing due to the Bir-Aronov-Pikus mechanism is also much smaller than the one due to the D'yakonov-Perel' mechanism at low temperature and becomes comparable to each other at higher temperature when the hole density and the width of the quantum well are large enough. We claim that unlike in the bulk samples which still need to be reexamined, the Bir-Aronov-Pikus mechanism hardly dominates the spin relaxation/dephasing in two-dimensional samples.Weng and Wu predicted that the Hartree-Fock term of the Coulomb interaction serves as an effective magnetic field which can be greatly increased with the spin polarization and therefore blocks the spin precession as a result of the lack of detuning [Phys. Rev. B 68, 075312 (2003)]. We study the spin dynamics of a high-mobility GaAs/Al_0.3Ga_0.7As quantum well by time-resolved Faraday rotation and time-resolved Kerr rotation in dependence on the initial degree of spin polarization of the electrons. By increasing the initial spin polarization, P, from the low P regime to a large P of several percent, we find that the spin dephasing time increases from about 20 ps to 200 ps; moreover, it increases with temperature at small spin polarization but decreases with temperature at large spin polarization. All these features are in good agreement with theoretical predictions by Weng and Wu. Measurements as a function of spin polarization at fixed electron density are performed to further confirm their theory. In calculation, we include both the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, with all the scatterings explicitly included. We reproduce all principal features of the experiments, i.e., a dramatic decrease of spin dephasing with increasing P and the temperature dependences for different spin polarizations.Finally, we investigate the effect of the Singwi-Tosi-Land-Sj(o|¨)lander local field correc- tion on spin relaxation/dephasing in n-type GaAs quantum wells at low temperature. We calculate the local field factor G(q) in quantum wells by numerically solving three equations which link the local field factor, the structure factor, and the dielectric function, self-consistently. Such a correction reduces both the electron-electron Coulomb scattering and the Coulomb Hartree-Fock term. We compare the spin relaxation/dephasing time with and without this correction under different conditions such as temperature, electron density, well width and spin polarization. We find that, when the reduction of scattering is larger than the reduction of Hartree-Fock term, this correction leads to a decrease of the spin relaxation/dephasing time in the strong scattering limit, and an increase in the weak scattering limit; when the reduction of Hartree-Fock is larger than the reduction of scattering, it leads to a decrease of the spin relaxation/dephasing time.From mesoscopic approach, we first study the spin Hall effect in two dimensional hole systems by using the four-terminal Landauer-Büttiker formula with the help of Green functions. We show that the heavy (light) hole spin Hall effect exists even when there is no correlation between the spin-up and -down heavy (light) holes and when theΓ-point degeneracy of the heavy hole and light hole bands is lifted due to the confinement or recovered by the strain. When only a heavy hole charge current without any spin polarization is injected from one lead, under right choice of lead voltages, one can get a pure heavy (light) hole spin current, combined with a possible impure light (heavy) hole spin current from two transverse leads. The spin Hall coefficients of both heavy and light holes depend on the Fermi energy, device size and the disorder strength. It is also shown that the spin Hall effect of two dimensional hole systems is much more robust than that of electron systems with the Rashba spin-orbit coupling and the spin Hall coefficients do not decrease to zero with the system size but tend to some nonzero values when the disorder strength is smaller than some critical value.Then we propose three schemes for spin filters.Quantum interference in Aharonov-Bohm ring structure provides additional control of spin at mesoscopic scale. The first scheme for spin filter is proposed by studying coherent transport through the Aharonov-Bohm structure with lateral magnetic modulation on both arms of the ring structure. Large spin polarized current can be obtained within many energy channels.The second scheme for the spin filter is proposed by studying the coherent transport of electrons through a double-bend structure in a quantum wire with a weak lateral magnetic potential which is much weaker than the Fermi energy of the leads. Extremely large spin polarized current in the order of micro-Ampere can be obtained because of the strong resonant behavior from the double bends. Further study suggests the robustness of this spin filter. Aharonov-Bohm effect in two-dimensional mesoscopic frame in hole systems has also been studied. We show that differing from the Aharonov-Bohm effect in electron systems, due to the presence of both the heavy and light holes, the conductances not only show the normal spin-unresolved Aharonov-Bohm oscillations, but also become spin-separated. Some schemes for spin filter based on the abundant interference characteristics are proposed and the robustness against the disorder of the proposed schemes are discussed.