Spin Dynamics In Ⅲ-Ⅴ Semiconductors And Their Nanostructures

The aim for a new generation of electronics with much higher operation speed and lower power dissipation via exploiting the carrier spin degree of freedom motivates the studies in the field "semiconductor spintronics". Spin dynamics is one of the central issues in semiconductor spintronics, not only due to its relevance in spin-based devices, but also due to its rich phenomena and physics. In most cases spin dynamics consists of the coherent part and the dissipative part. The coherent part is usually simple and trivial, whereas the dissipative part is complicated and interesting. The complexity and variety of genuine spin phenomena usually roots in the dissipative dynamics. In this dissertation we focus on spin dynamics in III-V semiconductors and their nanostructures which are the widely studied materials in semiconductor spintronics community. Especially, we focus on spin relaxation and spin dephasing in III-V semiconductors and their nanostructures.We first briefly introduce the field of semiconductor spintronics in Chapter I by ex-ploring its history, aims, achievements and challenges. We then introduce some prototype spintronic devices, which historically stimulated the community a lot. We also introduce the methods of generating and detecting spin polarization. We then devote a lot of para-graphs to spin interactions in III-V semiconductors, which is the physical origin of all spin phenomena. After that we introduce the relevant spin relaxation mechanisms in III-V semiconductors.After that, we comprehensively review the literature of experimental studies and single-particle theories on spin relaxation in III-V semiconductors in Chapter II. We review spin relaxation in both metallic and insulating regimes. We also review spin relaxation in paramagnetic (III,Mn)Ⅴsemiconductors. It should be pointed out that most of the previous theoretical studies are based on the single-particle approach, which fails in many cases. Many interesting features can only be obtained by the many-body theory. We also mention some experimental results which can only be understood in the many-body framework, besides explicitly pointing out the problems in the single-particle theories.In Chapter III we briefly introduce the kinetic spin Bloch equation approach, which is the fundamental of our work on spin dynamics in semiconductors. Kinetic spin Bloch equations describe spin dynamics in a fully microscopic fashion:they include both spin precession due to all spin interactions and all relevant scatterings. Furthermore, screening is treated carefully as it will affect both scattering and the Coulomb Hartree-Fock term. We introduce the kinetic spin Bloch equations including the optical and spin correlations in intrinsic quantum wells as well as the kinetic spin Bloch equations with only the spin correlations in n-type quantum wells as examples.After that, we elaborate on our comprehensive research on electron spin relaxation in bulk III-V semiconductors based on kinetic spin Bloch equation approach in Chapter IV. Our studies cover n-type, intrinsic and p-type III-V semiconductors. We find that the dependences of spin relaxation time in degenerate regime is qualitatively different from that in non-degenerate regime. To understand the various dependences of the spin relaxation time, the first job is to determine in which regime the electron/hole system is. Besides, we find that, due to factors such as screening and Pauli blocking, depen-dences of momentum scattering rates are complicated, which lead to intricate and various behaviors in spin relaxation. Especially the behavior of the D'yakonov-Perel'spin relax-ation time is much more complicated than what was understood in the literature. Our main findings are:In n-type, intrinsic and most of the p-type semiconductors, the Elliott-Yafet mechanism is less important than the D'yakonov-Perel'mechanism, even for the narrow band-gap semiconductors such as InSb and InAs; Due to the crossover from non-degenerate regime to degenerate regime, the density dependence of spin relaxation time exists a peak in the metallic regime around TF～T in both n-type and intrinsic materi-als; In n-type III-V semiconductors, the temperature dependence of spin relaxation time varies for different densities; Specifically, in low-density case with strain, the temperature dependence can be nonmonotonic, whereas in the case without strain the spin lifetime de-creases with increasing temperature; In high density case, spin lifetime can increase with increasing temperature; In common intrinsic III-V semiconductors such as GaAs, GaSb and InSb, the Bir-Aronov-Pikus mechanism is found to be negligible compared with the D'yakonov-Perel'one; In the case of small initial spin polarization, there is a peak in the temperature dependence of spin relaxation time located around T～TF/3, which is due to the nonmonotonic temperature dependence of the electron-electron Coulomb scatter-ing in intrinsic semiconductors; In p-type semiconductors, under high excitation density, due to the screening effect, the D'yakonov-Perel'spin relaxation time first increases then decreases with increasing temperature with a peak around T～TF; The D'yakonov-Perel' spin relaxation time exhibits intriguing behaviors in hole density dependence—it first in-creases then decreases, and then increases with increasing hole density; Finally, in n-type semiconductors, high electric field leads to shorter spin lifetime and the effect of electric field increases with increasing mobility. We elaborate thoroughly on the underlying physics of the above intriguing behaviors. We point out that some of the behaviors are universal which also exist in low-dimensional structures,Ⅱ-VI semiconductors and Wurtzite semi-conductors, or other materials with similar structures (given that the spin-orbit coupling is finite). It should be mentioned that some of our predictions have been confirmed by recent experiments [1-7].We then describe our investigation (in collaboration with experimentalists in the Schuller group in Regensburg University) on anisotropic spin relaxation in (001) GaAs quantum wells in Chapter V. The experimentalists measured the magneto-anisotropy of spin relaxation in high mobility GaAs quantum wells. They found that the anisotropy in spin relaxation can be tuned largely by the magnetic field. Specifically, when the magnetic field is along the [110] direction, spin relaxation time exists a valley at B= 0.2 T in the magnetic field dependence, whereas there is a peak when the magnetic field is along [110] direction at B= 0.5 T. The observed phenomena can not be explained by previous theory. We employed the kinetic spin Bloch equation approach to calculate spin relaxation time under the experimental conditions. We find good agreement with the experimental data. We explored the underlying physics. Moreover, we predicted the lifetime for the spin com-ponent along the [110] direction to be several nanoseconds—two orders larger than that of the spin component along [110]. These findings are valuable to spin lifetime manipulation in semiconductor spintronics.We present our systematic studies on electron spin relaxation in paramagnetic GaM-nAs quantum wells based on kinetic spin Bloch equation approach in Chapter VI. We study the spin relaxation in both the n-type GaMnAs quantum wells where most of Mn ions take the interstitial positions as well as the p-type GaMnAs quantum wells where most of Mn ions substitute Ga atoms. For n-type GaMnAs quantum wells, we find that spin relaxation is completely dominated by the D'yakonov-Perel'mechanism. Remark-ably, the Mn concentration dependence of the spin relaxation time is nonmonotonic and exhibits a peak. This is due to the fact that the momentum scattering and the inhomoge-neous broadening have different density dependences in the non-degenerate and degenerate regimes. Interestingly, in p-type GaMnAs quantum wells, there also exists a peak in the Mn concentration dependence. Differently, this peak is due to the competition between the D'yakonov-Perel'mechanism and the other mechanisms such as the s-d exchange, Elliott-Yafet and Bir-Aronov-Pikus mechanisms. We reproduced the peak position measured by Awschalom group [8]. Moreover, we determine the dominant spin relaxation mechanisms in various regimes, which offer very important information for further studies. The tem-perature, photo-excitation density and magnetic field dependences of the spin relaxation time are investigated systematically with the underlying physics revealed. Our results are consistent with the recent experimental findings [8-11].In Chapter VII, we review kinetics in driven time-dependent system, including the theoretical methods. This chapter serves as a background introduction for our studies in spin dynamics under intense THz driving field. We first briefly introduce several driven time-dependent systems in condensed matter physics. We then introduce THz technology and related physics. We briefly review the effects of intense THz field on the transport and optics of semiconductors. We then review the kinetics in driven time-dependent system in the dissipation-free limit. We introduce the Floquet-Fourier approach to time-dependent Schrodinger equation and discuss briefly the properties of the Floquet wavefunction. After that, we present the the Floquet-Markov theory for the dissipative kinetics in driven time-dependent system.After that we review our studies on spin dynamics in quantum dots under intense THz field in Chapter VIII. We first obtain the exact solution to the Schrodinger equations, and then study the effect of intense THz field on the density of states. We show that in the presence of spin-orbit coupling the THz electric field can be used to manipulate spin and induce a spin polarization perpendicular to the electric field. After that we include the electron-phonon scattering and investigate the effect of intense THz field on spin relaxation. We find that intense THz magnetic field can strongly affect the spin relaxation via the sideband modulated spin-flip electron-phonon scattering.In Chapter IX, we present our investigations on spin kinetics in many-electron system under intense THz driving field. We consider the two-dimensional system confined in an InAs quantum well. We first construct the kinetic spin Bloch equation via Floquet-Markov theory and nonequilibrium Green function theory. In the kinetic spin Bloch equations, we treat the THz field nonperturbatively, where all the sidebands are included. We treat the scattering beyond the rotating wave approximation. We include all relevant scatterings, including the electron-impurity, electron-phonon and electron-electron scatterings. Our approach is quite general and can be applied to other many-carrier system with arbitrary spin-orbit coupling. By numerically solving the kinetic spin Bloch equations, we study the effect of intense THz laser field on spin kinetics. Specifically, we mainly discuss the THz field-induced spin polarization and the effect of THz field on spin relaxation. We find that the THz field induces a steady-state spin polarization which first predicted by Cheng and Wu [12] in dissipation free case, still exists in the presence of scattering. The steady-state spin polarization can be as large as 7%, which indicates that intense THz field is an efficient tool for generating spin polarization. Our research reveals that there are two physical origins of the induced spin polarization. The first part is induced by the effective magnetic field directly due to the spin-orbit coupling. The second part is induced by the effective magnetic field due to the combined effect of THz field-induced current and spin-orbit coupling. As we treating the scattering beyond the rotating wave approximation, we find many interesting features, which are absent within the rotating wave approximation. The first feature is that there is always a retardation of the spin polarization in response to the THz field-induced effective magnetic field. Another is that the THz field can induce a current which leads to an effective magnetic field in the presence of spin-orbit coupling. If the scattering is treated in the rotating wave approximation, there is no retardation. More importantly, within such approximation the scattering will keep the kx→-kx symmetry, and there is no THz-field-induced current. We investigate the dependence of the amplitude of the steady-state spin polarization on the strength and frequency of THz field. We find that the main factors affecting the amplitude of spin polarization are that:(1) THz field-induced hot-electron effect, (2) the THz field-induced effective magnetic field. Both effects increase with increasing THz strength or decreasing frequency. As the spin polarization is induced by the effective magnetic field, higher effective magnetic field leads to higher spin polarization. However, the hot-electron effect reduces the spin polarization. We find that the amplitude of the induced spin polarization first increases then decreases with increasing THz field strength or decreasing THz frequency. The decrease is due to that the prominent hot-electron effect at high THz field or low THz frequency. We also find that the intense THz field strongly affects the spin relaxation. The effect also comes from (1) the hot-electron effect and (2) the THz field-induced effective magnetic field. We find that in the case with very small impurity density, the two factors compete to each other and lead to the nonmonotonic dependence of spin relaxation time with increasing THz field strength or decreasing frequency. At high impurity density the hot-electron effect dominates and the spin relaxation time decreases monotonically with increasing THz field strength or decreasing frequency.We review our systematic research on spin relaxation and dephasing in GaAs quantum dots in Chapter X. In our investigation we include various spin decoherence mechanisms, such as the hyperfine interaction, spin-orbit coupling together with the electron-phonon scattering, the g-factor fluctuations, the direct spin-phonon coupling due to the phonon-induced strain, and the coaction of the electron-phonon interaction together with the hyperfine interaction. The relative contributions to spin relaxation and dephasing from these mechanisms are compared under various conditions, where their dependences are studied with the underlying physics revealed. We find that both spin relaxation and spin dephasing are not determined by one spin decoherence mechanism solely. Each mechanism dominates one regime. In some situations, several mechanisms are comparable. Our calculation agrees well with experiments [13]. We also give the parameter regimes where the Fermi Golden rule is valid. The predicted temperature dependence of spin dephasing time has been confirmed by recent experiment [14]. Our study is valuable for the understanding of spin relaxation and dephasing in GaAs quantum dots, which serves as the ground for further studies on the manipulation of spin decoherence and for quantum information processing.