Magic numbers in vertically coupled quantum dots

A coupled quantum dot system has been studied by numerical diagonalization of the Hamiltonian. Discontinuous ground-state transitions induced by an external magnetic field have been predicted. Series of magic numbers of angular momentum which minimize the ground-state electron-electron interaction energy have been discovered. Theoretical explanations derived from the first principles have been formulated. Received: 13 July 1997 / Accepted: 7 October 1997     

Control of vertically coupled InGaAs/GaAs quantum dots with electric fields

Controllable interactions that couple quantum dots are a key requirement in the search for scalable solid state implementations for quantum information technology. From optical studies of excitons and corresponding calculations, we demonstrate that an electric field on vertically coupled pairs of In(0.6)Ga(0.4)As/GaAs quantum dots controls the mixing of the exciton states on the two dots and also provides controllable coupling between carriers in the dots.     

Antibonding ground state of holes in double vertically coupled Ge/Si quantum dots

The existence of the antibonding ground state of holes in artificial molecules, which are formed by the vertically coupled Ge/Si quantum dots, has been proved experimentally. This phenomenon is absent in natural molecules and double quantum dots containing electrons. It is a consequence of spin-orbit interaction and deformation effects in the valence band of vertically aligned quantum dots.     

Electrically driven reverse overhauser pumping of nuclear spins in quantum dots

We propose a new mechanism for polarizing nuclear spins in quantum dots, based on periodic modulation of the hyperfine coupling by electric driving at the electron spin resonance frequency. Dynamical nuclear polarization results from resonant excitation rather than hyperfine relaxation mediated by a thermal bath, and thus is not subject to Overhauser-like detailed balance constraints. This allows polarization in the direction opposite to that expected from the Overhauser effect. Competition of the electrically driven and bath-assisted mechanisms can give rise to spatial modulation and sign reversal of polarization on a scale smaller than the electron confinement radius in the dot.     

Optical properties of modulation-doped InGaAs vertically coupled quantum dots

Optical spectroscopy including photoluminescence, electroluminescence, photocurrent, and differential absorption, have been investigated for the triple-layer InGaAs vertically coupled quantum dots (VCQDs) by adding modulation doping (MD) in the 5 nm GaAs spacer layers. In addition to the QDs fundamental and excited transitions, a coupled-state transition is observed for the VCQDs. For the VCQDs of p-type MD, the optical transitions at ground state and coupled state are enhanced by the improvement of hole capture for the valence subbands. For the VCQDs of n-type MD, the main absorption change occurs at the coupled state, consistent with the dominant emission peak observed in EL spectra.     

Molecule-type phases and Hund's rule in vertically coupled quantum dots

We study the ground state of two vertically coupled quantum dots as a function of the interdot distance within the spin density functional theory. The tunneling between the dots is included. For small and large interdot distances the atomic phases are recovered. For intermediate distances new molecule-type phases are predicted which can be observed experimentally in the addition energies. The results are interpreted in terms of an effective single particle picture and we find that Hund's rule breaks down for 11 and 12 electrons. The results are summarized in a phase diagram in which spin and isospin blockade regions are also found.     

Zero-energy modes in serially coupled double quantum dots

We investigate symmetrically coupled double quantum dots via the hierarchical equations of motion method and propose a novel zero-energy mode(ZEM) at a temperature above the spin singlet–triplet transition temperature. Owing to the resonance of electron quasi-particle and hole quasi-particle, ZEM has a peak at ω = 0 in the spectral density function.We further examine the effect of the magnetic field on the ZEM, where an entanglement of spin and charge has been determined; therefore, the magnetic field can split the ZEM in the spectra.     

Thermoelectric effects through weakly coupled double quantum dots

The electrical conductance, the thermal conductance, the thermopower and the thermoelectrical figure of merit are analyzed through a double quantum dot system weakly coupled to metal electrodes, by means of density matrix approach. The effects of interdot tunneling, intra- and interdot Coulomb repulsions on the figure of merit are examined. Results show that increase of interdot tunneling gives rise to a reduction in figure of merit. On the other hand, increase of Coulomb repulsion results in enhancement of figure of merit because of reduce of bipolar effect.     

Electron dynamics in the coupled double quantum dots system

We have studied the electron dynamics in different geometrical arrangements of the two coupled double quantum dot structures. Applying the equation of motion method for appropriate correlation functions the occupation probabilities of different quantum dots of the considered system has been theoretically investigated. The numerical calculations were performed for different forms of the time-dependent tunneling amplitudes and quantum dot energy levels. We found, among others, that under some conditions for the tunneling amplitudes changed in the form of Gaussian pulses it is possible to localize the electron in a controlled manner on the given dot of the considered system.     

Quantum phase transition in capacitively coupled double quantum dots

We investigate two equivalent, capacitively coupled semiconducting quantum dots, each coupled to its own lead, in a regime where there are two electrons on the double dot. With increasing interdot coupling, a rich range of behavior is uncovered: first a crossover from spin- to charge-Kondo physics, via an intermediate SU(4) state with entangled spin and charge degrees of freedom, followed by a quantum phase transition of Kosterlitz-Thouless type to a non-Fermi-liquid "charge-ordered" phase with finite residual entropy and anomalous transport properties. Physical arguments and numerical renormalization group methods are employed to obtain a detailed understanding of the problem.     

Anomalous Stark shifts in single vertically coupled pairs of InGaAs quantum dots

Vertically coupled Stranski Krastanow quantum dots (QDs) are predicted to exhibit strong tunnelling interactions that lead to the formation of hybridised states. We report the results of investigations into single pairs of coupled QDs in the presence of an electric field that is able to bring individual carrier levels into resonance and to investigate the Stark shift properties of the excitons present. Pronounced changes in the Stark shift behaviour of exciton features are identified and attributed to the significant redistribution of the carrier wavefunctions as resonance between two QDs is achieved. At low electric fields coherent tunnelling between the two QD ground states is identified from the change in sign of the permanent dipole moment and dramatic increase of the electron polarisability, and at higher electric fields a distortion of the Stark shift is attributed to a coherent tunnelling effect between the ground state of the upper QD and the excited state of the lower QD.     

The microstructure of vertically coupled quantum dots ensembles by EXAFS spectroscopy

The microstructure of samples containing Ge/Si, GaN/AlN, and InAs/AlAs (Ge, GaN, and InAs quantum dots (QDs) in the Si, AlN, and AlAs matrices, respectively) multilayer heterostructures has been investigated by EXAFS spectroscopy. The effect of the effective thicknesses of Si (or AlN) barrier layers, number of Ge (or GaN) layers in a sandwich, and annealing temperature on the size and shape of Ge (or GaN) clusters, interfacial diffusion in these systems, and formation of three-dimensional ordered ensembles of QDs has been established.     

Radiation characteristics of injection lasers based on vertically coupled quantum dots

We have studied injection lasers based on InGaAs/GaAs vertically coupled quantum dots (QD) grown by molecular beam epitaxy. The threshold current density decreases by one order of magnitude down to 90 A cm⁻²(300 K) with an increase of the number of QD stacks (N) up to 10. ForN≥ 3 lasing occurs via the QD ground state up to room temperature. Differential efficiency increases withNup to 50%. No change in range of high temperature stability of threshold current density (J_th) was observed, while the characteristic temperature (T₀) measured at 300 K increases from 60 to 120 K. Using InGaAs-AlGaAs QD with higher localization energy allowed us to decreaseJ_thdown to 60 A cm⁻²and to increase the differential efficiency up to 70%.     

Electronic structure of asymmetric vertically coupled InAs/GaAs quantum dots

In this paper, the electronic structure of an asymmetric self-assembled vertically coupled quantum dots heterostructure has been investigated. The structure consists of two ellipsoidal quantum dot (QDs) caps made with InAs embedded in a wetting layer InAs and surrounded by GaAs. Using the strain dependent k·p theory, the energy of the two lowest states of a single electron/hole which is confined within the coupled QD structure has been calculated. As a result, it can be estimated the energy gap for different geometry parameters and for tuning the external magnetic field. The numerical results show that the energy gap is very sensitive to the size asymmetry of the structure and to the small separation distance of the dots but less sensitive to the existence of an external magnetic field and large interdot distance.     

耦合锗量子点中空穴态对称特性研究

分别采用单带重空穴近似和六带Kronig-Penney模型, 对垂直耦合锗量子点在不同耦合距离下的空穴态特性进行了计算, 并探讨了自旋-轨道的相互作用对空穴态对称性的影响. 计算结果表明: 多带耦合的框架下, 随着量子点垂直间距的增大, 空穴基态从成键态转变为反键态, 而且价带基态能级和第一激发态能级发生反交叉现象, 这与单带模型下得到的相应结果存在较大差异. 通过分析六带模型计算得到的成、反键态波函数, 轻、重空穴态和自旋-轨道分裂态对特征空穴态波函数的贡献比例随着量子点垂直间距的增大发生了转变, 并最终导致量子点空穴基态波函数由成键态转变为反键态. 关键词： 耦合量子点 空穴态 成健态-反健态 自旋-轨道     

Exciton states in strongly coupled asymmetric semimagnetic double quantum dots

Exciton states in a pair of strongly coupled artificial asymmetric quantum dots (QDs) have been studied in magnetic fields up to B = 8T by means of photoluminescence spectroscopy. The QD molecules have been fabricated using a selective interdiffusion technique applied to asymmetric CdTe/(Cd,Mg,Mn)Te double quantum wells. The lateral confinement potential within the plane induced by the diffusion gives rise to effective zero-dimensional exciton localization. Incorporation of the Mn ions in only one dot results in a pair of QDs with a markedly different spin splitting. In contrast to a positive value of the exciton Lande g factor in nonmagnetic (Cd,Mg)Te-based single QDs, the ground exciton transition in the nonmagnetic QD demonstrates nearly zero g factor, thus, indicating a strong electron coupling between the dots. A new low-energy band with a strong red shift appears at high B signifying formation of the indirect exciton in accordance with our calculations. The text was submitted by the authors in English.     

Quantum entanglement transition in vertically coupled two single-electron quantum dots with charged impurity

Effects of a charged impurity on the ground state of two vertically coupled identical single-electron quantum dots with and without applied magnetic field are investigated. In the absence of the magnetic field, the investigations of the charged impurity effect on the quantum entanglement (QE) in some low-lying states are carried out. It is found that, both the positive charged impurity (PCI) and the negative charged impurity (NCI)reduce the QE in the low-lying states under consideration except that the QE in the ground state is enhanced by the NCI. Additionally, in the domain of B from 0 Tesla to 15 Tesla, the ground state energy E, the ground state angular momentum L and the ground state QE entropy S are worked out. As far as the ground state are concerned, the PCI (NCI) blocks (induces) the angular momentum phase transition and the QE phase transition besides the known fact (i. e., the PCI/NCI decreases/increases the energy) in the magnetic field.     

Hole states in artificial molecules formed by vertically coupled Ge/Si quantum dots

In the tight binding approximation, the spatial configuration of the ground state and the binding energy of a hole in a “diatomic” artificial molecule formed by vertically coupled Ge/Si(001) quantum dots are studied. The inhomogeneous spatial distribution of elastic strain arising in the medium due to the lattice mismatch between Ge and Si is taken into account. The strain is calculated using the valence-force-field model with a Keating interatomic potential. The formation of the hole states is shown to be determined by the competition of two processes: the appearance of a common hole due to the overlapping of “atomic” wavefunctions and the appearance of asymmetry in the potential energy of a hole in the two quantum dots because of the superposition of the elastic strain fields from the vertically aligned Ge nanoclusters. When the thickness of the Si layer separating the Ge dots (t _Si) is greater than 2.3 nm, the binding energy of a hole in the ground state of the two-dot system proves to be lower than the ionization energy of a single quantum dot because of the partial elastic stress relaxation due to the coupling of the quantum dots and due to the decrease in the depth of the potential well for holes. For the values of the parameter t _Si, an intermediate region is revealed, where the covalent molecular bond fails and the hole is localized in one of the two quantum dots, namely, in the dot characterized by the highest strain values.     

Entanglement and the Kondo effect in serially coupled double quantum dots

We investigate entanglement between electrons in serially coupled double quantum dots attached to noninteracting leads. In addition to local repulsion we consider the influence of capacitive inter-dot interaction. We show how the competition between extended Kondo and local singlet phases determines the ground state and thereby the entanglement. The results are additionally discussed in connection with the linear conductance through the system.     

Self-polarization and dynamical cooling of nuclear spins in double quantum dots

The spin-blockade regime of double quantum dots features coupled dynamics of electron and nuclear spins resulting from the hyperfine interaction. We explain observed nuclear self-polarization via a mechanism based on feedback of the Overhauser shift on electron energy levels, and propose to use the instability toward self-polarization as a vehicle for controlling the nuclear spin distribution. In the dynamics induced by a properly chosen time-dependent magnetic field, nuclear spin fluctuations can be suppressed significantly below the thermal level.