The Effects Of The Spin-orbit Coupling On The Electronic Spin State Of Quasi-one-dimensional Nanowire Quantum Dot

Confined electron spins in quantum dots can be used for implementing quantum information processing owing to their relatively long decoherence times.Generally,the very small electron spin magnetic moment dictates that a strong alternating-current(AC)magnetic field is required to reach reasonable rate of spin rotation.Interestingly,in semiconductor materials with spatial inversion asymmetry,because of the spin-orbit coupling(SOC),the spin degree of freedom can be effectively manipulated using an external AC electric field.In recent decade,the SOC in semiconductor nanowire structures plays an importan-1 role in exploring new physical mechanisms.Experimentally,the SOC in nanowire quantum dot makes it possible to implement the fast electric-field manipulation of the spin and offers an effective platform for searching the Majorana fermion in a semiconductor-superconductor hybrid topological system.Meanwhile,combined with the electron-phonon interaction,the SOC in semiconductor materials also results in spin relaxation,which is harmful to the spin life-time.Thus,in order to make a comprehen-sive analysis of the effect of the SOC on the spin dynamics in nanowire,one should precisely take the spin-orbit interactions into consideration.However,in most cases,the SOC term is always regarded as a perturbation term to facilitate the calculation process,such that the strong SOC effects are always overlooked.In this PhD thesis,we investigate the effects of the strong SOC on the spin dynamics in nanowire quantum dots,and the thesis is composed of the following seven chapters:In chapter 1,we provide a brief introduction to the quantum-dot system and outline the work principles of the spin-based quantum information processing in quantum dots.In chapter 2,we present the basic knowledge of the SOC,the electric-dipole spin resonance(EDSR),and the phonon-induced spin relaxation in semiconductor quantum dots.Moreover,the analytical formulas for calculating the EDSR frequency and the spin relaxation rate in the weak SOC regime are given.In chapter 3,we study the electric-dipole transitions and the phonon-induced relax-ations for a single electron in a nanowire DQD.Compared with a single QD,enabled by the SOC there are two mechanisms leading to the EDSR in the DQD:the intradot pseudo-spin states mixing and the interdot spin-flipped tunneling.The EDSR frequency and strength are determined by these mechanisms together,and the dominate mecha-nism in the spin resonance can be varied by changing the magnetic field strength or the interdot distance for the double dot.Finally,we find that in the nanostructures with large g-factor and strong SOC,the phonon-induced spin relaxations can be effectively suppressed even at relatively small magnetic fields because of the phonon bottleneck effect.In chapter 4,we use an exactly solvable model to investigate the impact of the magnetic-field direction on the SOC effects in a nanowire QD.The energy spectrum and the corresponding eigenstates of an electron confined in the quantum dot are ob-tained exactly.We find that no matter how large the SOC is,the EDSR frequency as a function of the magnetic field direction always has a ? periodicity.However,the phonon-induced spin relaxation rate as a function of the magnetic-field direction has a 7r periodicity only in the weak SOC regime,and the periodicity is prolonged to 2? in the strong SOC regime.In chapter 5,we study the effects of the confining potential on the EDSR frequency in a one-dimensional square-well quantum dot with strong SOC.Based on the exact eigenstates of the confined electron,we can find that there exists an optimal height of the confinement potential,in which the EDSR frequency between the lowest Zeeman sublevels reaches its maximum.In chapter 6,we study the two-electron exchange interaction in a nanowire DQD un-der the influence of the strong SOC and the magnetic field.We find that the exchange interaction between the two electrons is shown to possess the form of the Moriya's anisotropic superexchange interaction.In the presence of a uniform external magnetic field,when the anisotropic exchange interaction is transformed to an isotropic Heisen-berg interaction,the uniform magnetic field becomes an effective inhomogeneous field,and the inhomogeneity of the effective magnetic field reflects the SOC strength.Also,we show that the strength of the two-electron exchange interaction can be effectively controlled by tuning the SOC strength and the direction of the anisotropic exchange interaction can be manipulated by varying the magnetic-field direction.Finally,chapter 7 gives a summary of the thesis and outlines the prospects of future work.