Electron spin waves of the surface of a nanotube

The spin-wave spectrum of a non-ferromagnetic electron gas on the surface of a nanotube in a magnetic field has been considered. The electron-electron interaction has been taken into account in the Hartree-Fock approximation. The dynamic spin susceptibility tensor of the electron gas has been calculated in the random-phase approximation. The spectra of intrasubband and intersubband magnons have been calculated in the quantum and semi-classical cases. It has been shown that, for a large number of occupied subbands, the spin-wave frequencies exhibit de Haas-van Alphen and Aharonov-Bohm oscillations with a change in the electron density and the magnetic flux through the cross section of the nanotube.     

Decoherence in solid-state qubits

The interaction of solid-state qubits with environmental degrees of freedom strongly affects the qubit dynamics, and leads to decoherence. In quantum information processing with solid-state qubits, decoherence significantly limits the performances of such devices. Therefore, it is necessary to fully understand the mechanisms that lead to decoherence. In this review, we discuss how decoherence affects two of the most successful realizations of solid-state qubits, namely, spin qubits and superconducting qubits. In the former, the qubit is encoded in the spin 1/2 of the electron, and it is implemented by confining the electron spin in a semiconductor quantum dot. Superconducting devices show quantum behaviour at low temperatures, and the qubit is encoded in the two lowest energy levels of a superconducting circuit. The electron spin in a quantum dot has two main decoherence channels, a (Markovian) phonon-assisted relaxation channel, due to the presence of a spin–orbit interaction, and a (non-Markovian) spin bath constituted by the spins of the nuclei in the quantum dot that interact with the electron spin via the hyperfine interaction. In a superconducting qubit, decoherence takes place as a result of fluctuations in the control parameters, such as bias currents, applied flux and bias voltages, and via losses in the dissipative circuit elements.     

Theory of g-factor enhancement in narrow-gap quantum well heterostructures

We report on the study of the exchange enhancement of the g-factor in the two-dimensional (2D) electron gas in n-type narrow-gap semiconductor heterostructures. Our approach is based on the eight-band k?p Hamiltonian and takes into account the band nonparabolicity, the lattice deformation, the spin-orbit coupling and the Landau level broadening in the δ-correlated random potential model. Using the 'screened' Hartree-Fock approximation we demonstrate that the exchange g-factor enhancement not only shows maxima at odd values of Landau level filling factors but, due to the conduction band nonparabolicity, persists at even filling factor values as well. The magnitude of the exchange enhancement, the amplitude and the shape of the g-factor oscillations are determined by both the screening of the electron-electron interaction and the Landau level width. The 'enhanced' g-factor values calculated for the 2D electron gas in InAs/AlSb quantum well heterostructures are compared with our earlier experimental data and with those obtained by Mendez et?al (1993 Phys.?Rev.?B 47 13937) in magnetic fields up to 30?T.     

Precession spin relaxation mechanism caused by frequent electron-electron collisions

It is shown that the D’yakonov-Perel’ spin relaxation mechanism in a two-dimensional electron gas is controlled not only by the electron-momentum relaxation that accounts for the electron mobility but also by the electron-electron collisions. The kinetic equation describing the mixing of electron spin in the k space was solved, and the spin relaxation time τ_s caused by frequent electron-electron collisions was determined. The time τ _s was calculated for a nondegenerate electron gas both with and without allowance for the exchange interaction.     

Indirect interaction of solid-state qubits via two-dimensional electron gas

We propose a mechanism of long-range coherent coupling between nuclear spin qubits in semiconductor-heterojunction quantum information processing devices. The coupling is via localized donor electrons which interact with the two-dimensional electron gas. An effective interaction Hamiltonian is derived and the coupling strength is evaluated. We also discuss mechanisms of decoherence and consider gate control of the interaction between qubits. The resulting quantum computing scheme retains all the control and measurement aspects of earlier approaches, but allows qubit spacing at distances of the order of 100 nm, attainable with the present-day semiconductor device technologies.     

可控磁性量子点杂质引起的单电子自旋过滤器

本文提出一种基于电子-电子自旋交换相互作用获得自旋极化电流的模型. 该方案中, 需要两个距离相近的量子点. 其中一个是开放系统, 另一个是封闭系统. 开放系统能完成单电子输运, 封闭系统产生比较强的局域磁场, 两个系统之间有电子-电子自旋交换相互作用. 该相互作用会影响电子输运, 从而可以对电子输运产生自旋过滤效应. 我们用量子主方程描述开放系统的演化, 在有效哈密顿量的基础上, 可以得到解析结果. 结果显示, 在低温条件下, 交换相互作用足够强的时候, 系统给出的自旋过滤效率接近1 .     

量子点接触中有关电导0.7结构的研究

研究了由鞍形势描述的量子点接触中的电子输运性质,利用Hartree-Fock近似与低维纳米器件中Thomas-Fermi近似方法处理了电子-电子之间的相互作用,采用格点格林函数方法计算了零温下体系的电导、电子的自旋积累以及散粒噪声,重现了0.7结构这个反常的实验现象.计算结果加深了我们关于半导体纳米器件中的强关联互作用对自旋输运影响的理解.     

Spin polarization in parallel double dots with spin-orbit interaction

Spin polarization in parallel double quantum dots embedded in arms of Aharonov-Bohm interferometer is investigated. The spin-orbit interaction exists in quantum dots. We find that the spin polarization is quite large even with a weak spin-orbit interaction. The direction and the strength of the spin polarization are well controllable and manipulatable by simply varying the strength of spin-orbit interaction or the energy levels in quantum dots. Moreover, electron-electron interaction strengthens the spin accumulation when the energy levels of the two quantum dots are identical. As the energy levels are unequal, electron-electron interaction cannot increase the spin accumulation. It is worth mentioning that the device is free of a magnetic field or a ferromagnetic material and it can be easily realized with present technology.     

Microscopic calculation of the fermi liquid interaction constants for electrons in the surface inversion layer of a semiconductor

The leading Fourier coefficients of both the spin dependent and spin independent Landau interaction function of a quasi-two-dimensional electron gas are evaluated. Three different schemes for calculating the interaction function are studied for both a pure two-dimensional Coulomb interaction and an effective electron-electron interaction appropriate to the actual metal-insulator-semiconductor structure.     

Effect of exchange interaction on the spin fluctuations of localized electrons

A microscopic theory of spin fluctuations in an ensemble of electrons localized on donors in a bulk semiconductor has been developed. Both the hyperfine interaction of the electron spin with spins of host lattice nuclei and the exchange interaction between the electrons have been taken into account. A model of clusters to calculate spin noise spectra of the ensemble of localized charge carriers has been proposed. It has been shown that the electron-electron exchange interaction leads to an effective averaging of random nuclear fields and a shift of the peak in the spin-fluctuation spectrum towards lower frequencies.     

Spin dynamics of Rashba-Dresselhaus two-dimensional electron systems with electron-electron interactions

Spin dynamics of Rashba-Dresselhaus two-dimensional electron systems is studied by taking account of electron-electron interactions under the D’yakonov-Perel’ mechanism. The diffusion equations for charge and spin densities are obtained through decoupling of the interactions using the auxiliary Bose field. We show that the electron-electron interaction has no effect on the infinite spin lifetime when the Rashba and Dresselhaus coupling constants satisfy the condition α = ±β. If the general condition α≠±β is satisfied, the spin lifetime is finite and enhanced by the electron-electron interaction with the increment of the temperature in the ballistic regime. The increasing amplitude of the spin lifetime depends on the ratio of the temperature to the Fermi temperature.     

Femtosecond coherent spectroscopy of four-wave mixing and photon echoes in a GaAs/AlGaAs heterostructure at room temperature

The results from experiments employing coherent femtosecond spectroscopy in a layer of two-dimensional electron gas at the boundary of the GaAs/AlGaAs heterojunction at room temperature are presented. The decay curves of primary femtosecond photon echo are obtained. The decoherence time in two-dimensional electron gas depends strongly on the power of the exciting pulse and varies from 36 to 54 fs. The dephasing time is studied for the first time as a function of the power of exciting pulses at room temperature. It is established that this dependence obeys the law T ₂ ∼ N ^−0.22, which differs from the typical law T ₂ ∼ N ⁻¹ for unscreened electron-electron interaction in semiconductor crystals. Analysis shows that electron-phonon interaction plays an important part along with electron-electron interaction. The induced spin gratings in the GaAs/AlGaAs heterostructure are studied with an eye to their possible application in spintronics.     

Probing electron-electron interaction in quantum Hall systems with scanning tunneling spectroscopy

Using low-temperature scanning tunneling spectroscopy applied to the Cs-induced two-dimensional electron system (2DES) on p-type InSb(110), we probe electron-electron interaction effects in the quantum Hall regime. The 2DES is decoupled from bulk states and exhibits spreading resistance within the insulating quantum Hall phases. In quantitative agreement with calculations we find an exchange enhancement of the spin splitting. Moreover, we observe that both the spatially averaged as well as the local density of states feature a characteristic Coulomb gap at the Fermi level. These results show that electron-electron interaction can be probed down to a resolution below all relevant length scales.     

Dynamical Theory of X-Ray Diffraction for Restricted Beams: I. Coherent Scattering by a Porous Crystal

The processes of electron spin dynamics in a hybrid nonresonance structure, which includes a layer of a diluted magnetic II–Mn–VI semiconductor and an asymmetric quantum well (QW) of a nonmagnetic III–V semiconductor, are experimentally studied. The nonresonance of the structure is determined by the fact that the level of the ground state of the magnetic layer falls into the range of the excited states of the nonmagnetic QW. The electron polarization in the ground thermalized state of QW is found not to depend on the magnetic part of the structure. However, the magnetic part affects the electron polarization in the excited state via spin injection from the magnetic semiconductor and the mixing of the electronic states of the magnetic and nonmagnetic subsystems of the structure. The possibility of controlling the polarization of an electron spin by carrier excitation toward the region of mixed states along with the absence of depolarizing influence of the magnetic semiconductor on carriers in the thermalized state of QW can be applied to design new spintronic devices along with those that use spin injection, optical orientation, and depolarization.     

Quantum Hall ferrimagnetism in lateral quantum dot molecules

We demonstrate the existence of ferrimagnetic and ferromagnetic phases in a spin phase diagram of coupled lateral quantum dot molecules in the quantum Hall regime. The spin phase diagram is determined from the Hartree-Fock configuration interaction method as a function of electron number N and magnetic field B. The quantum Hall ferrimagnetic phase corresponds to spatially imbalanced spin droplets resulting from strong interdot coupling of identical dots. The quantum Hall ferromagnetic phases correspond to ferromagnetic coupling of spin polarization at filling factors between nu=2 and nu=1.     

Restoring coherence lost to a slow interacting mesoscopic spin bath

For a two-state quantum object interacting with a slow mesoscopic interacting spin bath, we show that a many-body solution of the bath dynamics conditioned on the quantum-object state leads to an efficient control scheme to recover the lost quantum-object coherence through disentanglement. We demonstrate the theory with the realistic problem of one electron spin in a bath of many interacting nuclear spins in a semiconductor quantum dot. The spin language can be easily generalized to a quantum object in contact with a bath of interacting multilevel quantum units with the caveat that the bath is mesoscopic and its dynamics is slow compared with the quantum object.     

Spin Hall effect and spin current generation in two-dimensional systems with random Rashba spin–orbit coupling

We consider a new effect induced by spin–orbit coupling in a two-dimensional electron gas confined in a semiconductor quantum well, i.e. the possibility of spin current generation by fluctuating random Rashba spin–orbit interaction, with the corresponding mean value of the interaction being equal to zero. Our main results suggest that – in contrast to the spatially uniform Rashba spin–orbit interaction – the spin Hall effect does not vanish for typical disorder strengths. We also point out some other possibilities of using such a random Rashba coupling for the generation of spin density and spin current in two-dimensional nonmagnetic structures.     

Electron spins in an array of tunnel-coupled quantum dots

Mechanisms of electron spin relaxation in semiconductor arrays with tunnel-coupled quantum dots are reviewed. The contribution for anisotropic exchange interaction is shown for asymmetrical quantum dots having no inversion axis relative to their plane. The configuration of vertically coupled double Ge/Si quantum dots is found where anisotropic exchange coupling does not contribute to spin decoherence. It could be a basic configuration of spin-based quantum computation schemes.     

Spin waves on the surface of the nonferromagnetic nanotube in magnetic field

Propagating in the nonferromagnetic electron gas on the cylindrical nanotube's surface spin waves in longitudinal magnetic field are considered. The spectrum of electrons in the Hartree-Fock approximation was applied. The dynamic spin susceptibility of a degenerate electron gas was derived using the random phase approximation. The spectra of intra-subband and inter-subband magnons were calculated in quasiclassical and quantum limits. The quantity of spin waves spectrum branches depends on the amount of filled subbands. In case the filled subband numbers are large, the wave's frequencies undergo oscillations of de Haas-van Alphen and Aharonov-Bohm types with the electron density and the magnetic induction changes.     

Electron energy state spin-splitting in 3D cylindrical semiconductor quantum dots

In this article we study the impact of the spin-orbit interaction on the electron quantum confinement for narrow gap semiconductor quantum dots. The model formulation includes: (1) the effective one-band Hamiltonian approximation; (2) the position- and energy-dependent quasi-particle effective mass approximation; (3) the finite hard wall confinement potential; and (4) the spin-dependent Ben Daniel-Duke boundary conditions. The Hartree-Fock approximation is also utilized for evaluating the characteristics of a two-electron quantum dot system. In our calculation, we describe the spin-orbit interaction which comes from both the spin-dependent boundary conditions and the Rashba term (for two-electron quantum dot system). It can significantly modify the electron energy spectrum for InAs semiconductor quantum dots built in the GaAs matrix. The energy state spin-splitting is strongly dependent on the dot size and reaches an experimentally measurable magnitude for relatively small dots. In addition, we have found the Coulomb interaction and the spin-splitting are suppressed in quantum dots with small height. Received 15 May 2001 / Received in final form 14 May 2002 Published online 13 August 2002