磁场调制下的双电子量子点qubit

研究了磁场中二维有限深抛物形量子点中双电子在总自旋分别为 S=0或 S=1时的电子态 ,在有效质量近似下 ,利用精确的对角化方法计算了系统的能级结构 .发现系统的基态总自旋 S可以通过改变磁场的大小进行调制 ,由此可以设计利用 S=0和 S=1两个自旋态组成一个量子比特     

Applications of Huang–Rhys theory in semiconductor optical spectroscopy

A brief review of Huang–Rhys theory and Albrechtos theory is provided,and their connection and applications are discussed.The former is a first order perturbative theory on optical transitions intended for applications such as absorption and emission involving localized defect or impurity centers,emphasizing lattice relaxation or mixing of vibrational states due to electron–phonon coupling.The coupling strength is described by the Huang–Rhys factor.The latter theory is a second order perturbative theory on optical transitions intended for Raman scattering,and can in-principle include electron–phonon coupling in both electronic states and vibrational states.These two theories can potentially be connected through the common effect of lattice relaxation – non-orthonormal vibrational states associated with different electronic states.Because of this perceived connection,the latter theory is often used to explain resonant Raman scattering of LO phonons in bulk semiconductors and further used to describe the size dependence of electron–phonon coupling or Huang–Rhys factor in semiconductor nanostructures.Specifically,the A term in Albrechtos theory is often invoked to describe the multi-LO-phonon resonant Raman peaks in both bulk and nanostructured semiconductors in the literature,due to the misconception that a free-exciton could have a strong lattice relaxation.Without lattice relaxation,the A term will give rise to Rayleigh or elastic scattering.Lattice relaxation is only significant for highly localized defect or impurity states,and should be practically zero for either single particle states or free exciton states in a bulk semiconductor or for confined states in a semiconductor nanostructure that is not extremely small.     

Intersubband optical absorption in strained double barrier quantumwell infrared photodetectors

A systematic theoretical investigation of intersubband optical absorption in AlGaAs-AlAs-InGaAs strained double barrier quantum well is presented for the first time. Electron states are calculated within the effective mass approximation which includes the effects of subband nonparabolicity and strain, and found to be in good agreement with experiments. Intersubband optical absorption is investigated using the density matrix formalism with the intrasubband relaxation taken into account. Analytical formulas are given for electron energies, absorption coefficient, and responsivity. Subband nonparabolicity and elastic strain are found to significantly influence both electron states and intersubband optical absorption. The peak absorption wavelength is found to decrease linearly if the In composition is increased, and an approximate formula is given. Electron states and optical absorption are affected by the inner barrier thickness if it is less than 40 Å. The results are useful for design and improvement of the performance of quantum well infrared photodetectors operating in the important wavelength region between 1.5 and 4 μm     

Effect of spin-orbit coupling on the spectrum of two-dimensional electrons in a magnetic field

The electron states in semiconductors with zinc-blende structure in an external magnetic field are studied taking into account the intrinsic spin splitting. The Hamiltonian of the spin-orbit coupling is written out using the method of invariants. Effects of modification of the energy spectrum of two-dimensional electrons in single and double quantum wells in a magnetic field oriented parallel to the interface planes are considered.     

Optical orientation of electrons in compensated semiconductors

The theory of the optical orientation of charge carriers in compensated III-V semiconductors and quantum wells for the case where electrons are excited to the conduction band from Mn-charged acceptor states is presented. It is shown that, in GaAs/AlGaAs quantum wells, the degree of the spin orientation of conduction-band electrons in this excitation scheme can be as high as 85%. This spin-orientation enhancement results from an increase in the heavy-hole contribution to the acceptor state in the vicinity of the defect center rather than from level splitting caused by quantum confinement. It is shown that the degree of circular polarization of the photoluminescence emitted upon the recombination of electrons thermalized at the bottom of the band with holes occupying the acceptor ground state in a quantum well can exceed 70%.     

Nonequilibrium aspects of armchair graphene nanoribbon conduction

Electron quantum transport is theoretically studied for finite-size armchair graphene nanoribbons biased within source and drain metallic electrodes, using an extended-Hückel-based Green's function coupled to a three-dimensional Poisson solver. The analysis evidences dynamic nonequilibrium electron charging phenomena that can affect the conduction mechanism by provoking electronic structure alterations. The origin of such process can be traced in a tracking relationship between the device's local density of states and the electrochemical potentials of the contacts. Such effect has no equivalent in the semiclassical limit.     

Encoding Information on the Excited State of a Molecular Spin Chain

The quantum states of nano-objects can drive electrical transport properties across lateral and local-probe junctions. This raises the prospect, in a solid-state device, of electrically encoding information at the quantum level using spin-flip excitations between electron spins. However, this electronic state has no defined magnetic orientation and is short-lived. Using a novel vertical nanojunction process, these limitations are overcome and this steady-state capability is experimentally demonstrated in solid-state spintronic devices. The excited quantum state of a spin chain formed by Co phthalocyanine molecules coupled to a ferromagnetic electrode constitutes a distinct magnetic unit endowed with a coercive field. This generates a specific steady-state magnetoresistance trace that is tied to the spin-flip conductance channel, and is opposite in sign to the ground state magnetoresistance term, as expected from spin excitation transition rules. The experimental 5.9 meV thermal energy barrier between the ground and excited spin states is confirmed by density functional theory, in line with macrospin phenomenological modeling of magnetotransport results. This low-voltage control over a spin chain's quantum state and spintronic contribution lay a path for transmitting spin wave-encoded information across molecular layers in devices. It should also stimulate quantum prospects for the antiferromagnetic spintronics and oxides electronics communities.     

Spin filtration of unpolarized electrons by impurity centers in semiconductors

It is shown that unpolarized paramagnetic centers can implement the spin filtration of unpolarized conduction electrons in semiconductors. This ability of paramagnetic centers is caused by the difference in the spin evolution of the states of electron-paramagnetic-center pairs and by the spin selectivity of electron capture exclusively from singlet pairs. The electron spin polarization should be opposite to the paramagneticcenter polarization. To implement spin filtration, an external magnetic field is necessary. The polarization can attain the largest values (∼10%) if the probability of spin-selective electron capture from singlet pairs exceeds the pair-decay rate by a factor of 5–7.     

量子台阶上准束缚电子能态光学性质研究

给出量子台阶上准呸缚电子能态间的跃迁特性，通过理论计算模型中等效电子波函数相干长度对量子台阶光学特性的影响，提出了一种对电子波函数相干长度行为的实验研究途径。     

Spin-polarized currents in the tunnel contact of a normal conductor and a two-dimensional topological insulator

The spin filtering of electrons tunneling from the edge states of a two-dimensional topological insulator into a normal conductor under a magnetic field (external or induced due to proximity to a magnetic insulator) is studied. Calculations are performed for a tunnel contact of finite length between the topological insulator and an electronic multimode quantum strip. It is shown that the flow of tunneling electrons is split in the strip, so that spin-polarized currents arise in its left and right branches. These currents can be effectively controlled by the contact voltage and the chemical potential of the system. The presence of a magnetic field, which splits the spin subbands of the electron spectrum in the strip, gives rise to switching of the spin current between the strip branches.     

Magneto-polaron induced intersubband optical transition in a wide band gap II–VI semiconductor quantum dot

Effects of LO-phonon contribution on the electronic and the optical properties are investigated in a Cd_0.8Zn_0.2Se/ZnSe quantum dot in the presence of magnetic field strength. The magneto-polaron induced hydrogenic binding energy as a function of dot radius in the wide band gap quantum dot is calculated. The oscillator strength and the spontaneous lifetime are studied taking into account the spatial confinement, magnetic field strength and the phonon contribution. Numerical calculations are carried out using variational formulism within the single band effective mass approximation. The optical properties are computed with the compact density matrix method. The magneto-polaron induced optical gain as a function of photon energy is observed. The results show that the optical telecommunication wavelength in the fiber optic communications can be achieved using CdSe/ZnSe semiconductors and it can be tuned with the proper applications of external perturbations.     

Chiral Photodetector Based on GaAsN

The detection of light helicity is key for various applications, from drug production to optical communications. However, the light helicity direct measurement is inherently impossible with conventional photodetectors based on III–V or IV–VI non-chiral semiconductors. The prior polarization analysis by often moving optical elements is necessary before light is sent to the detector. A method is here presented to effectively give the conventional dilute nitride GaAs-based semiconductor epilayer a chiral photoconductivity. The detection scheme relies on the giant spin-dependent recombination of conduction electrons and the accompanying spin polarization of the engineered defects to control the conduction band. As the conduction electron spin polarization is, in turn, intimately linked to the excitation light polarization, the light polarization state and intensity can be determined by a simple conductivity measurement. This approach, removing the need for any optical elements in front of a non-chiral detector, could offer easier integration and miniaturization. This new chiral photodetector could potentially operate in a spectral range from the visible to the infra-red using (InGaAl)AsN alloys or ion-implanted nitrogen-free III–V compounds.     

正方体GaAs量子点电子特性的数值自洽求解

采用抛物势作为量子点对电子有效约束势，使用有限差分法对Schrodinger-Poisson方程进行离散化，根据自旋密度泛函，进行数值自洽求解，得到三维正方体GaAs量子点电子总基态能、电子密度等电子特性，并与相同条件(电子数、自旋、尺寸)的二维正方形GaAs量子点的电子密度进行了对比．     

Structure Inversion Asymmetry and Rashba Effect in Quantum Confined Topological Crystalline Insulator Heterostructures

Structure inversion asymmetry is an inherent feature of quantum confined heterostructures with non-equivalent interfaces. It leads to a spin splitting of the electron states and strongly affects the electronic band structure. The effect is particularly large in topological insulators because the topological surface states are extremely sensitive to the interfaces. Here, the first experimental observation and theoretical explication of this effect are reported for topological crystalline insulator quantum wells made of Pb_1−xSn_xSe confined by Pb_1−yEu_ySe barriers on one side and by vacuum on the other. This provides a well defined structure asymmetry controlled by the surface condition. The electronic structure is mapped out by angle-resolved photoemission spectroscopy and tight binding calculations, evidencing that the spin splitting decisively depends on hybridization and, thus, quantum well width. Most importantly, the topological boundary states are not only split in energy but also separated in space—unlike conventional Rashba bands that are splitted only in momentum. The splitting can be strongly enhanced to very large values by control of the surface termination due to the charge imbalance at the polar quantum well surface. The findings thus, open up a wide parameter space for tuning of such systems for device applications.     

New mechanism of excitonic enhanced optical gain for wide-gap quantum wells

New mechanism of optical gain in quantum wells are proposed using excitonic effects. Exciton in wide-gap semiconductors plays an important role in optical phenomena since it has a large binding energy and could be stable at room temperature. However, its bound state is constructed by the electron-hole Coulomb interaction and should be related to the electron and hole distributions when the ground state has many electron and holes. We have evaluated the current-current correlation function, i.e. conductivity, treating the mechanism of optical gain and exciton on equal footing. It is shown that the recombination of the exciton does not yield optical gain directly but that excitonic effects enhance an oscillator strength of the coupled transition. Taking into account a localized level in the energy gap, the optical gain in terms of the population inversion between the localized level and one of the band edge subband states is produced with the very small carrier concentration. Simultaneously, the excitonic absorption occurs due to the band edge electron-hole interaction. It is found that the former optical gain is enhanced extremely by the latter excitonic effect through the coupling between the two transitions. This enhanced optical gain might show a possibility of very low threshold current density for wide-gap laser diodes.     

Peculiarities of the Properties of III–V Semiconductors in a Multigrain Structure

The systematized results of studies of the properties of InAs, InSb, and GaAs semiconductors in a multigrain structure based on measurement and analysis of the current–voltage and spectral characteristics are presented. It is established that electron emission and injection are determined by the localization effects of states in the bulk and surface region of submicron grains. The phenomena of current limitation and lowfield emission characteristic of quantum dots are revealed and studied. The results can be used in studies and in the development of multigrain structures for gas and optical sensors, detectors, and emitters of infrared and terahertz ranges.     

QHE key to spin electronics and quantum computing

Researchers Shuichi Murakami, University of Tokyo and Shoucheng Zhang, Stanford University may have discovered a spin current associated with holes rather than electrons in semiconductors. The predicted current would be able to inject spin momentum into quantum dots and would interact with conventional electron currents, bridging electronics and spin-based quantum circuits.Visit www.three-fives.com for the latest advanced semiconductor industry news     

On the physics of semiconductor quantum dots for applications in lasers and quantum optics

The progression of carrier confinement from quantum wells to quantum dots has received considerable interests because of the potential to improve the semiconductor laser performance at the underlying physics level and to explore quantum optical phenomena in semiconductors. Associated with the transition from quantum wells to quantum dots is a switch from a solid-state-like quasi-continuous density of states to an atom-like system with discrete states. As discussed in this paper, the transition changes the role of the carrier interaction processes that directly influence optical properties. Our goals in this review are two-fold. One is to identify and describe the physics that allows new applications and determines intrinsic limitations for applications in light emitters. We will analyze the use of quantum dots in conventional laser devices and in microcavity emitters, where cavity quantum electrodynamics can alter spontaneous emission and generate nonclassical light for applications in quantum information technologies. A second goal is to promote a new connection between physics and technology. This paper demonstrates how a first-principles theory may be applied to guide important technological decisions by predicting the performances of various active materials under a broad set of experimental conditions.     

Small Components of the Wave Function of Electron

In this paper, via an analogy between the wave functions of free electron and free electromagnetic fields, we study the relation between the large components and the small components of the wave function of electron, and show that in some cases the small components cannot be ignored. As an application, we will demonstrate that not only the spin quantum states of a moving electron but also those of a motionless electron can be affected by some special electrostatic fields. This may provide a ne…     

光学泵作用下两个耦合量子点的输运性质

运用Keldysh格林函数,理论研究了在光学泵作用下的两个耦合量子点的电子输运性质.发现了电流-电压曲线上的平台结构以及透射系数的共振峰,可以由量子点的局域态密度来解释.讨论了光学泵的频率以及强度对系统输运性质的影响,发现当光学泵的频率等于空穴的分立能级时,发生电子的动力学局域化.这个结果可以用来实现光学控制开关.