Electron–phonon-induced spin relaxation in InAs quantum dots

We have calculated spin-relaxation rates in parabolic quantum dots due to the phonon modulation of the spin–orbit interaction in the presence of an external magnetic field. Both deformation potential and piezoelectric electron–phonon coupling mechanisms are included within the Pavlov–Firsov spin–phonon Hamiltonian. Our results have demonstrated that, in narrow gap materials, the electron–phonon deformation potential and piezoelectric coupling give comparable contributions to spin-relaxation processes. For large dots, the deformation potential interaction becomes dominant. This behavior is not observed in wide or intermediate gap semiconductors, where the piezoelectric coupling, in general, governs the spin-relaxation processes. We have also demonstrated that spin-relaxation rates are particularly sensitive to the Landé g-factor.     

Hole spin relaxation in Ge quantum dots

Hole spin relaxation in an isolated Ge quantum dot due to interaction with phonons is investigated. Spin relaxation in this case occurs through the mechanism of the modulation of the spin-orbit interaction by lattice vibrations. According to the calculations performed, the spin relaxation time due to direct single-phonon processes for the hole ground state equals 1.4 ms in the magnetic field H = 1 T at the temperature T = 4 K. The dependence of the relaxation time on the magnetic field is described by the power function H^?5. At higher temperatures, a substantial contribution to spin relaxation is made by two-phonon (Raman) processes. Because of this, the spin relaxation time decreases to nanoseconds as the temperature is raised to T = 20 K. Analysis of transition probabilities shows that the third and twelfth excited hole states, which are intermediate in two-step relaxation processes, play the main part in Raman processes.     

Anisotropy of spin splitting and spin relaxation in lateral quantum dots

Inelastic spin relaxation and spin splitting epsilon(s) in lateral quantum dots are studied in the regime of strong in-plane magnetic field. Because of both the g-factor energy dependence and spin-orbit coupling, epsilon(s) demonstrates a substantial nonlinear magnetic field dependence similar to that observed by Hanson et al. [Phys. Rev. Lett. 91, 196802 (2003)]. It also varies with the in-plane orientation of the magnetic field due to crystalline anisotropy of the spin-orbit coupling. The spin relaxation rate is also anisotropic, the anisotropy increasing with the field. When the magnetic length is less than the "thickness" of the GaAs dot, the relaxation can be an order of magnitude faster for B ||[100] than for B || [110].     

Signature of symmetry of laterally coupled quantum dots in far-infrared spectrum

We study the effect of structure asymmetry on the energy spectrum and the far-infrared spectrum (FIR) of a lateral coupled quantum dot. The calculated spectrum shows that the parity break of coupled quantum dot results in more coherent superpositions in the low-lying states and exhibits unique anti-crossing in the two-electron FIR spectrum modulated by a magnetic field. We also find that the Coulomb correlation effect can make the FIR spectrum of coupled quantum dot without strict parity deviate greatly from Kohn theorem, which is just contrary to the symmetric case. Our results therefore suggest that FIR spectrum may be used to determine the symmetry of coupled quantum dot and to evaluate the degree of Coulomb interaction.     

Field dependence of the electron spin relaxation in quantum dots

The interaction of the electron spin with local elastic twists due to transverse phonons is studied. The universal dependence of the spin-relaxation rate on the strength and direction of the magnetic field is obtained in terms of the electron gyromagnetic tensor and macroscopic elastic constants of the solid. The theory contains no unknown parameters and it can be easily tested in experiment. At high magnetic field it provides a parameter-free lower bound on the electron spin relaxation in quantum dots.     

Coulomb "Blockade" of nuclear spin relaxation in quantum dots

We study the mechanism of nuclear spin relaxation in quantum dots due to the electron exchange with the 2D gas. We show that the nuclear spin relaxation rate 1/T(1) is dramatically affected by the Coulomb blockade (CB) and can be controlled by gate voltage. In the case of strong spin-orbit (SO) coupling the relaxation rate is maximal in the CB valleys, whereas for the weak SO coupling the maximum of 1/T(1) is near the CB peaks.     

Role of orbital dynamics in spin relaxation and weak antilocalization in quantum dots

We develop a semiclassical theory for spin-dependent quantum transport to describe weak (anti)localization in quantum dots with spin-orbit coupling. This allows us to distinguish different types of spin relaxation in systems with chaotic, regular, and diffusive orbital classical dynamics. We find, in particular, that for typical Rashba spin-orbit coupling strengths, integrable ballistic systems can exhibit weak localization, while corresponding chaotic systems show weak antilocalization. We further calculate the magnetoconductance and analyze how the weak antilocalization is suppressed with decreasing quantum dot size and increasing additional in-plane magnetic field.     

Direct observation of the electron spin relaxation induced by nuclei in quantum dots

We have studied the electron spin relaxation in semiconductor InAs/GaAs quantum dots by time-resolved optical spectroscopy. The average spin polarization of the electrons in an ensemble of p-doped quantum dots decays down to 1/3 of its initial value with a characteristic time T(Delta) approximately 500 ps, which is attributed to the hyperfine interaction with randomly oriented nuclear spins. We show that this efficient electron spin relaxation mechanism can be suppressed by an external magnetic field as small as 100 mT.     

Excitons in coupled quantum dots

Properties of excitons in vertically coupled GaAs/AlGaAs quantum dots were investigated using the variational method within the envelope function and effective mass approximations. It was found that when the thickness of the spacer layer becomes less than about one exciton Bohr radius, both the exciton binding energy and the fundamental optical transition energy are reduced compared to those in isolated quantum dots. This is a result of increased space extension of exciton due to the penetration of carrier wave functions into the spacer layer and corresponding reduction in confinement energy which dominates over the Coulomb interaction between the electron and the hole.     

Spin pump effects on the spin current through two coupled quantum dots

We theoretically study the spin pump effects of the rotating magnetic field on the spin current through two coupled quantum dots. Owing to the interdot coupling, two molecular states with different bands can be formed, resulting asymmetric spin current peaks. The possibility of manipulating the spin current is explored by tuning the strength, the frequency, and the direction of the rotating magnetic field. The number and location of the spin current peaks can be controlled by making use of various tunings. Furthermore, the normal 2π period of the spin current with respect to the magnetic flux can be destroyed by the interdot coupling.     

Geometrical spin dephasing in quantum dots

We study spin-orbit mediated relaxation and dephasing of electron spins in quantum dots. We show that higher order contributions provide a relaxation mechanism that dominates for low magnetic fields and is of geometrical origin. In the low-field limit relaxation is dominated by coupling to electron-hole excitations and possibly 1/f noise rather than phonons.     

Electrical control of hole spin relaxation in charge tunable InAs/GaAs quantum dots

We report on optical orientation of singly charged excitons (trions) in charge-tunable self-assembled InAs/GaAs quantum dots. When the charge varies from 0 to -2, the trion photoluminescence of a single quantum dot shows up and under quasiresonant excitation gets progressively polarized from zero to approximately 100%. This behavior is interpreted as the electric control of the trion thermalization process, which subsequently acts on the hole-spin relaxation driven in nanosecond time scale by the anisotropic electron-hole exchange. This is supported by the excitation spectroscopy and time-resolved measurements of a quantum dot ensemble.     

Photoexcited electron and hole dynamics in semiconductor quantum dots: phonon-induced relaxation, dephasing, multiple exciton generation and recombination

Photoexcited dynamics of electrons and holes in semiconductor quantum dots (QD), including phonon-induced relaxation, multiple exciton generation, fission and recombination (MEG, MEF and MER), were simulated by combining ab?initio time-dependent density functional theory and non-adiabatic molecular dynamics. These nonequilibrium phenomena govern the optical properties and photoexcited dynamics of QDs, determining the branching between electronic processes and thermal energy losses. Our approach accounts for QD size and shape as well as defects, core-shell distribution, surface ligands and charge trapping, which significantly influence the properties of photoexcited QDs. The method creates an explicit time-domain representation of photoinduced processes and describes various kinetic regimes owing to the non-perturbative treatment of quantum dynamics. QDs of different sizes and materials, with and without ligands, are considered. The simulations provide direct evidence that the high-frequency ligand modes on the QD surface play a pivotal role in the electron-phonon relaxation, MEG, MEF and MER. The insights reported here suggest novel routes for controlling the photoinduced processes in semiconductor QDs and lead to new design principles for increasing the efficiencies of photovoltaic devices.     

Influences of spin–orbit interaction on quantum speed limit and entanglement of spin qubits in coupled quantum dots

Quantum speed limit and entanglement of a two-spin Heisenberg XYZ system in an inhomogeneous external magnetic field are investigated. The physical system studied is the excess electron spin in two adjacent quantum dots. The influences of magnetic field inhomogeneity as well as spin–orbit coupling are studied. Moreover, the spin interaction with surrounding magnetic environment is investigated as a non-Markovian process. The spin–orbit interaction provides two important features: the formation of entanglement when two qubits are initially in a separated state and the degradation and rebirth of the entanglement.     

Exciton relaxation in PbS quantum dots

Exciton relaxation in PbS quantum dots (QDs) in glass and tetrachloroethylene have been investigated and the radiative and non‐radiative relaxation rates of the lowest 1S–1S state have been measured. An estimate of the carrier intra‐band transition rates and their transfer efficiency is calculated. The dependence of the exciton dynamics on the QD surface properties is presented and discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Local field effects on phonon-induced transparency in quantum dots embedded in a semiconductor medium

We study theoretically the influence of local fields on phonon-induced transparency (PIT) in quantum-dot systems embedded in a semiconductor matrix. As compared with our previous work without local field effects, we present analytical and numerical results from solution of the generalized optical Bloch equations including the local field effects. It is shown that the local field effects broaden the transparency window due to PIT and reduce the group velocity of light. For some specific parameters of the light and quantum dots, fast light can be obtained in such systems. The results also demonstrate that Kerr nonlinearity is enhanced greatly due to the local field effects. PACS 42.50.Gy; 78.67.Hc; 73.21.La; 03.67.-a     

InAs量子点中自旋-轨道相互作用下电子自旋弛豫的参量特征

本文在处理InAs单电子量子点哈密顿模型时,将自旋-轨道(SO)相互作用作为微扰项,计算在Fock-Darwin本征函数下SO相互作用的矩阵元,利用其对能级和波函数的二阶修正,并且考虑新的能级对g因子和有效质量m^*的影响,计算得到在声子协助下电子的自旋弛豫率Γ的表达式.给出了InAs量子点中声子协助的电子自旋弛豫率Γ对于限制势频率ω₀、温度T、纵向高度z_{0关键词： 自旋弛豫率 自旋-轨道(SO)相互作用 InAs量子点 Fock-Darwin本征函数}