Ground State Transitions in Vertically Coupled Four-Layer Single Electron Quantum Dots

We study a four-electron system in a vertically coupled four-layer quantum dot under a magnetic field by the exact diagonalization of the Hamiltonian matrix. We find that discontinuous ground-state energy transitions are induced by an external magnetic field. We find that dot-dot distance and electron-electron interaction strongly affect the low-lying states of the coupled quantum dots. The inter-dot correlation leads to some sequences of possible disappearances of ground state transitions, which are present for uncoupled dots.     

Four-Electron Systems in a Coupled Double-Layer Quantum Dots

Making use of the method of few-body physics, the energy spectrum of a four-electron system consisting in a vertically coupled double-layer quantum dot as a function of the strength ofa magnetic field is investigated. Discontinuous ground-state transitions induced by an external magnetic field are shown. We find that, in the strong coupling case, the ground-state transitions depend not only on the external magnetic field B but also on the distance d between double-layer quantum dots. However, in the case of weak coupling, the ground-state transitions occur in the new sequence of the values of the magic angular momentum. Hence, the interlayer separation d and electron-electron interaction strongly affect the ground state of the coupled quantum dots.     

Quantum cascade transitions in nanostructures

In this article we review the physical characteristics of quantum cascade transitions (QCTs) in various nanoscopic systems. The quantum cascade laser which utilizes such transitions in quantum wells is a brilliant outcome of quantum engineering that has already demonstrated its usefulness in various real-world applications. After a brief introduction to the background of this transition process, we discuss the physics behind these transitions in an externally applied magnetic field. This has unravelled many intricate phenomena related to intersubband resonance and electron relaxation modes in these systems. We then discuss QCTs in a situation where the quantum wells in the active regions of a quantum cascade structure are replaced by quantum dots. The physics of quantum dots is a rapidly developing field with its roots in fundamental quantum mechanics, but at the same time, quantum dots have tremendous potential applications. We first present a brief review of those aspects of quantum dots that are likely to be reflected in a quantum-dot cascade structure. We then go on to demonstrate how the calculated emission peaks of a quantum-dot cascade structure with or without an external magnetic field are correlated with the properties of quantum dots, such as the choice of confinement potentials, shape, size and the low-lying energy spectra of the dots. Contents PAGE 1 Introduction 456 2 Intersubband transitions in quantum wells 458 3 Quantum cascade transitions 462 3.1. Basic principles 462 3.1.1. Minibands and minigaps 464 3.1.2. Vertical transitions 464 3.1.3. GaAs/AlGaAs quantum cascade lasers 464 3.1.4. QCLs based on superlattice structures 465 3.1.5. Type-II quantum cascade lasers 466 3.1.6. Recent developments 466 3.2. Applications: sense-ability and other qualities 466 4 Quantum cascade transitions in novel situations 467 4.1. External magnetic field 467 4.1.1. Parallel magnetic field 468 4.1.2. Many-body effects: depolarization shift 470 4.1.3. The role of disorder 471 4.1.4. Tilted magnetic field 475 4.2. Magneto-transport experiments and phonon relaxation 479 4.3. Magneto-optics experiment and phonon relaxation 484 5 A brief review of quantum dots 485 5.1. From three- to zero-dimensional systems 485 5.2. Making the dots 487 5.2.1. Lithographic patterning 487 5.2.2. Self-assembled quantum dots 488 5.3. Shell filling in quantum dots 489 5.4. Electron correlations: spin states 490 5.5. Anisotropic dots 491 5.6. Influence of an external magnetic field 491 5.6.1. The Fock diagram 491 5.6.2. The no-correlation theorem 492 5.6.3. Correlation effects and magic numbers 492 5.6.4. Spin transitions 493 5.7. Quantum dots in novel systems 494 5.8. Potential applications of quantum dots 494 5.8.1. Single-electron transistors (SETs) 494 5.8.2. Single-photon detectors 494 5.8.3. Single-photon emitters 495 5.8.4. Quantum-dot lasers 495 6 Quantum cascade transitions in quantum-dot structures 496 6.1. Quantum dots versus quantum wells 496 6.2. QCT with rectangular dots 497 6.2.1. Vertical transitions 500 6.2.2. Diagonal transitions 501 6.3. QCT in a parabolic dot 504 6.4. Magnetic field effects on intersubband transitions 506 6.5. Mid-IR luminescence from a QD cascade device 512 7 Summary and open questions 513 Acknowledgements 515 References 515     

Few-electron molecular states and their transitions in a single InAs quantum dot molecule

We study electronic configurations in a single pair of vertically coupled self-assembled InAs quantum dots, holding just a few electrons. By comparing the experimental data of nonlinear single-electron transport spectra in a magnetic field with many-body calculations, we identify the spin and orbital configurations to confirm the formation of molecular states by filling both the quantum mechanically coupled symmetric and antisymmetric states. Filling of the antisymmetric states is less favored with increasing magnetic field, and this leads to various magnetic field induced transitions in the molecular states.     

Two interacting spins in external fields and application to quantum computation

We study the four-level-system given by two quantum dots immersed in a time-dependent magnetic field, which are coupled to each other by an effective Heisenberg-type interaction. We describe the construction of the corresponding evolution operator in a special case of different time-dependent parallel external magnetic fields. We find a relation between the external field and the effective interaction function. The obtained results are used to analyze the theoretical implementation of a universal quantum gate.     

Off—Center D—Centers in a Quantum Dot in the Presence of a Perpendicular Magnetic Fields

We investigate the effect of the position of the donor in quantum dots on the energy spectrum in the presence of a perpendicular magnetic field by using the method of few-body physics,As a function of the magnetic field,we find,when D^- centers are placed sufficiently off-center,discontinuous ground-state transitions which are similar to those found in many-electron parabolic quantum dots.Series of magic numbers of angular momentum which minimize the ground-state electron-electron interaction energy have been discovered.The dependence of the binding energy of the gound-state of the D^- center on the dot radius for a few values of the magnetic field strength is obtained and compared with other results.     

Biexcitons in artificial molecules with in-plane magnetic field

We theoretically investigate the effect of a magnetic field perpendicular to the tunneling direction on the ground-state properties of biexcitons in coupled quantum dots. The single-particle states are computed by numerically solving the 3D Scrödinger equation. The biexciton states are obtained by means of a configuration-interaction approach, which fully accounts for the intra- and inter-dot Coulomb correlations. We show that the biexciton ground state undergoes non-trivial transitions as a function of the applied magnetic field, which can be traced back to unexpected carrier localizations.     

Fixed-distance coupling and encapsulation of heterogeneous quantum dots using phonon-assisted photo-curing

We propose a novel method of coupling heterogeneous quantum dots at fixed distances and capsulating the coupled quantum dots by utilizing nanometric local curing of a photo-curable polymer caused by multistep electronic transitions based on a phonon-assisted optical near-field process between quantum dots. Because the coupling and the capsulating processes are triggered only when heterogeneous quantum dots floating in a solution closely approach each other to induce optical near-field interactions between them, the distances between the coupled quantum dots are physically guaranteed to be equal to the scale of the optical near fields. To experimentally verify our idea, we fabricated coupled quantum dots, consisting of CdSe and ZnO quantum dots and a UV-curable polymer. We also measured the photoluminescence properties due to the quantum-dot coupling and showed that the individual photoluminescences from the CdSe and ZnO quantum dots exhibited a trade-off relationship.     

Molecular states observed in a single pair of strongly coupled self-assembled InAs quantum dots

Molecular states in a single pair of strongly coupled self-assembled InAs quantum dots are investigated using a sub-micron sized single-electron transistor containing just a few pairs of coupled InAs dots embedded in a GaAs matrix. We observe a series of well-formed Coulomb diamonds with charging energy of less than 5 meV, which are much smaller than those reported previously. This is because electrons are occupied in molecular states, which are spread over both dots and occupy a large volume. In the measurement of ground and excited state single-electron transport spectra with a magnetic field, we find that the electrons are sequentially trapped in symmetric and anti-symmetric states. This result is well explained by numerical calculation using an exact diagonalization method.     

The Electron－Hole Pair in a Single Quantum Dot and That in a Vertically Coupled Quantum Dot

The energy spectra of low-lying states of an exciton in a single and a vertically coupled quantum dots are studied under the influence of a perpendicularly applied magnetic field. Calculations are made by using the method of numerical diagonalization of the Hamiltonian within the effective-mass approximation. We also calculated the binding energy of the ground and the excited states of an exciton in a single quantum dot and that in a vertically coupled quantum dot as a function of the dot radius for different vaJues of the distance and the magnetic field strength.     

串型耦合双量子点处于自旋阻塞区时磁输运性质的研究

使用双杂质安德森模型的哈密顿量,从理论上研究了串型耦合双量子点系统处于自旋阻塞区时的磁输运性质,并用主方程近似方法求解了哈密顿量.结果表明,自旋轨道耦合作用导致的双量子点间的自旋反转隧穿能够解除系统的自旋阻塞.同时也研究了超精细相互作用导致的在量子点内自旋反转和双量子点之间的自旋关联对系统的磁输运性质的影响,取得了一些有价值的结果,并对相关的物理问题进行了讨论.     

Entanglement and Zeeman interaction in diluted magnetic semiconductor quantum dot

We present theoretically the Zeeman coupling and exchange-induced swap action in spin-based quantum dot quantum computer models in the presence of magnetic field. We study the valence and conduction band states in a double quantum dots made in diluted magnetic semiconductor. The latter have been proven to be very useful in building an all-semiconductor platform for spintronics. Due to a strong p–d exchange interaction in diluted magnetic semiconductor (Cd_0.57Mn_0.43Te), the relative contribution of this component is strongly affected by an external magnetic field, a feature that is absent in nonmagnetic double quantum dots. We determine the energy spectrum as a function of magnetic field within the Hund–Mulliken molecular-orbit approach and by including the Coulomb interaction. Since we show that the ground state of the two carriers confined in a vertically coupled quantum dots provide a possible realization for a gate of a quantum computer, the crossing between the lowest states, caused by the giant spin splitting, can be observed as a pronounced jump in the magnetization of small magnetic field amplitude. Finally, we determine the swap time as a function of magnetic field and the inter dot distance. We estimate quantitatively swap errors caused by the field, establishing that error correction would, in principle, be possible in the presence of nonuniform magnetic field in realistic structures.     

Magnetic field and symmetry effects in small quantum dots

Shell phenomena in small quantum dots with a few electrons under a perpendicular magnetic field are discussed within a simple model. It is shown that various kinds of shell structures, which occur at specific values for the magnetic field lead to a disappearance of the orbital magnetization for particular magic numbers for noninteracting electrons in small quantum dots. Including the Coulomb interaction between two electrons, we found that the magnetic field gives rise to dynamical symmetries of a three-dimensional axially symmetric two-electron quantum dot with a parabolic confinement. These symmetries manifest themselves as near-degeneracy in the quantum spectrum at specific values of the magnetic field and are robust at any strength of the electron-electron interaction. A remarkable agreement between experimental data and calculations exhibits the important role of the thickness for the two-electron quantum dot for analysis of ground state transitions in a perpendicular magnetic field. The text was submitted by the author in English.     

Nonadiabatic two-parameter charge and spin pumping in a quantum dot

We study dc charge and spin transport through a weakly coupled quantum dot, driven by a nonadiabatic periodic change of system parameters. We generalize the model of Tien and Gordon to simultaneously oscillating voltages and tunnel couplings. When applying our general result to the two-parameter charge pumping in quantum dots, we find interference effects between the oscillations of the voltage and tunnel couplings. We show that these interference effects may explain recent measurements in metallic islands. Furthermore, we discuss the possibility to electrically pump a spin current in presence of a static magnetic field.     

Evaluation of electron--electron interactions in coupled quantum dots by using far-infrared spectra

We have studied the far-infrared spectra of two-electron vertically coupled quantum dots in an axial magnetic field by exact diagonalization. The calculated results show an obvious difference in role between the interactions for spin S = 1 and for spin S = O. The results support the possibility to evaluate the interactions by far-infrared spectroscopy in vertically coupled quantum dots.     

Four Electrons in a Coupled Three-Layer Quantum Dot

We investigate four electrons confined in a coupled three-layer quantum dot, by the exact diagonalization method. A vertical magnetic field to the confinement plane is considered. The ground-state electronic structures and angular momentum transitions are investigated. We find that for four-electron Q Ds, the Series of the magic numbers in three-layer QDs are different from those in one-, and two-layer Q Ds. These are connected to the exchange and rotational symmetries of the systems.     

Coherent spectroscopy of a strongly driven triple quantum dot molecule

Based on a calculation model,we study the interference phenomena of serially coupled V-type and Λ-type triple quantum dots (CTQDs) driven simultaneously by a strong driving field and a weak probe field.Strongly depending on the configuration of the three-level CTQD,the probe absorption spectra,which are shown in the tunneling current,exhibit various quantum coherence properties.In the case where the two pairs of transitions of the CTQD have a small eigenfrequency difference △ω,the double-coupling effect of the driving field results in two Autler-Townes doublets and one weak Mollow triplet in one spectrum.With the value of △ω increasing,only one Autler-Townes splitting remains due to the single-coupling of the field.We also find that the effect of spontaneous emission of phonons may lead to an obvious background current,which can be used to distinguish which transition is driven by the driving field in experiment.The interesting quantum property of a CTQD revealed in our results suggests its potential applications in quantum modulators and quantum logic devices.     

Electron–Nuclear Spin Dynamics in Semiconductor QDs

This work presents an overview of investigations of the nuclear spin dynamics in nanostructures with negatively charged InGaAs/GaAs quantum dots characterized by strong quadrupole splitting of nuclear spin sublevels. The main method of the investigations is the experimental measurements and the theoretical analysis of the photoluminescence polarization as a function of the transverse magnetic field (effect Hanle). The dependence of the Hanle curve profile on the temporal protocol of optical excitation is examined. Experimental data are analyzed using an original approach based on separate consideration of behavior of the longitudinal and transverse components of the nuclear polarization. The rise and decay times of each component of the nuclear polarization and their dependence on transverse magnetic field strength are determined. To study the role of the Knight field in the dynamic of nuclear polarization, a weak additional magnetic field parallel to the optical axis is used. We have found that, only taking into account the nuclear spin fluctuations, we can accurately describe the measured Hanle curves and evaluate the parameters of the electron–nuclear spin system in the studied quantum dots. A new effect of the resonant optical pumping of nuclear spin polarization in an ensemble of the singly charged (In,Ga)As/GaAs quantum dots subjected to a transverse magnetic field is discussed. Nuclear spin resonances for all isotopes in the quantum dots are detected in that way. In particular, transitions between the states split off from the ±1/2 doublets by the nuclear quadrupole interaction are identified.     

Optical spectroscopy on semiconductor quantum dots in high magnetic fields

We review the recent literature on the use of optical spectroscopy of semiconductor quantum dots in high magnetic fields. We address both self-assembled epitaxial dots and colloidal nanocrystal quantum dots, each of which has its own characteristic optical response. Combining simple theoretical models for quantum confinement with the effect of high magnetic fields we describe the basic optically allowed transitions expected for epitaxial and colloidal quantum dots. Within these models we discuss the effects of quantum confinement and orbital and spin Zeeman effects on the optical spectra, illustrated by experimental examples. Finally, effects of electron–electron and exchange interactions are addressed.