Time-dependent transport through a quantum dot with the over-dot (bridge) additional tunneling channel

The time-dependent electron transport through a quantum dot with the additional over-dot (bridge) tunneling channel within the evolution operator technique has been studied. The microwave field applied to the leads and quantum dot has been considered and influence of the time-dependent shift of corresponding energy levels on the quantum dot charge and current flowing in the system, its time-averaged values and derivatives of the average current with respect to the gate and source–drain bias voltages have been investigated. The influence of the over-dot tunneling channel on the photon-assisted tunneling has been also studied.     

Spin-Polarized Electron Tunneling TD:\pdf\.PDFhrough a Quantum Dot

Nonequilibrium Green's function is uscd to study spin-polarized electron tunneling through a quantum dot connected to two ferromagnetic electrodes with different orientations via two insulating barriers (FM/I/QD/I/FA.f). Intra-level Coulomb interaction in the dot is considered. General formula of tunneling current which can be used for arbitrary angle between the two electrodes' magnetizations is derived for both the weak and strong intra-dot interactions.We find that the transport current can be divided into two parts: the current with the spin-flip and the current without the spin-flip, which critically depend on the linewidth function near the Fermi level of the ferromagnetic electrodes. If a magnetic field is applied in the quantum dot, different behaviors will be found for weak and strong interactions.     

一种新型的高频半导体量子点单电子泵

除了直流负电压外,还在浅法刻蚀出的GaAs/AlGaAs量子线上的两个金属指形门上分别叠加两个相位相差π的正弦信号,从而对形成量子点的两个势垒作不等幅调制.在无源漏偏压的情况下,通过周期形成的量子点实现了单电子的搬运.由于新的半导体量子点单电子泵不是依赖库仑阻塞效应通过隧穿进行单电子输运,因此,该器件就不会受到固定隧穿时间引起的低工作频率限制.在1.7K温度下,频率达到3GHz仍然可以观测到量子化电流平台,对应的电流值达到0.5nA量级.这种新器件提供了实现高速度、高精度搬运单电子的另一种可能途径. 关键词： 单电子输运 单电子旋转门 单电子泵 量子化电流平台     

Charge and current beats in T-shaped qubit–detector systems

The time evolution of a charge qubit coupled electrostatically with different detectors in the forms of single, double and triple quantum dot linear systems in the T-shaped configuration between two reservoirs is theoretically considered. The correspondence between the qubit quantum dot oscillations and the detector current is studied for different values of the inter-dot tunneling amplitudes and the qubit–detector interaction strength. We have found that even for a qubit coupled with a single QD detector, the coherent beat patterns appear in the oscillations of the qubit charge. This effect is more evident for a qubit coupled with double or triple-QD detectors. The beats can be also observed in both the detector current and the detector quantum dot occupations. Moreover, in the presence of beats the qubit oscillations hold longer in time in comparison with the beats-free systems with monotonously decaying oscillations. The dependence of the qubit dynamics on different initial occupations of the detector sites (memory effect) is also analyzed.     

Tunneling current of the Coulomb-coupled quantum dots embedded in n-n junction

Taking account of the electron--electron (hole) and electron--hole interactions, the tunneling processes of the main quantum dot (QD) Coulomb-coupled with a second quantum dot embedded in n--n junction have been investigated. The eighteen resonance mechanisms involved in the tunneling processes of the system have been identified. It is found that the tunneling current depends sensitively on the electron occupation number in the second quantum dot. When the electron occupation number in the second dot is tiny, both the tunneling current peaks and the occupation number plateaus in the main QD are determined by the intra-resonance mechanism. The increase of the electron occupation number in the second dot makes the inter-resonance mechanism participate in the transport processes. The competition between the inter and intra resonance mechanisms persists until the electron occupation number in the second dot reaches around unity, leading to the consequence that the inter-resonance mechanisms completely dominate the tunneling processes.     

Coupled quantum dots: manifestation of an artificial molecule

Transport spectroscopy reveals the microscopic features of few-electron quantum dots which justify the nameartificial atoms. New physics evolve when two quantum dots are coupled by a tunneling barrier. We study, both theoretically and experimentally, the tunneling spectroscopy on a double quantum dot. A detailed lineshape analysis of the conductance resonances proves that off-resonant coherent interdot tunneling governs transport through this system, while tunneling into the double quantum dot occurs resonantly. This coherent interdot tunneling witnesses the evolution of a delocalized electronic state which can be compared to a valence electron of thisartificial molecule.     

点间耦合强度对三耦合量子点系统微分电导的影响

本文利用非平衡格林函数运动方程方法,研究了与两个电极耦合在一起的三耦合量子点系统的微分电导及量子干涉的AB振荡问题.通过理论计算发现,由于量子点上的局域态密度的不同从而导致系统电导或隧穿性质的不同,而且量子点间耦合强度、量子点能级等都会对输运性质产生影响. 关键词： 量子点 非平衡格林函数 运动方程 局域态密度     

Correlation-induced resonances and population switching in a quantum-dot coulomb valley

Strong correlation effects on electron transport through a spinless quantum dot are considered. When two single-particle levels in the quantum dot are degenerate, a conserved pseudospin degree of freedom appears for generic tunneling matrix elements between the quantum dot and leads. Local fluctuations of the pseudospin in the quantum dot give rise to a pair of asymmetric conductance peaks near the center of a Coulomb valley. An exact relation to the population switching is provided.     

Controlling the Goos-H?nchen Shift via Incoherent Pumping Field and Electron Tunneling in the Triple Coupled InGaAs/GaAs Quantum Dots

We study the controlHng of the Goos-H?nchen(GH) shifts in reflected and transmitted light beams in the triple coupled InGaAs/GaAs quantum dot(QD) nanostructures with electron tunneling and incoherent pumping Geld.It is shown that the lateral shift can become either large negative or large positive,which can be controlled by the electron tunneling and the rate of incoherent pump Geld in different incident angles.It is also demonstrated that the properties of the GH shifts are strongly dependent on the probe absorption beam of the intracavity medium due to the switching from superluminal Hght propagation to subluminal behavior or vice versa.Our suggested system can be considered as a new theoretical method for developing a new nano-optoelectronic sensor.     

A numerical approach for analyzing quantum dot infrared photodetectors' parameters

Quantum dot infrared photodetectors (QDIPs) have many advantages over other types of semiconductor-based photodetectors. However some of its characteristics have been investigated theoretically, there are many unstudied points. In this paper a new approach is presented to evaluate quantum dot infrared photodetectors dark current and photocurrent. In this study, it is assumed that both thermionic emission and field-assisted tunneling mechanisms determine the dark current of quantum dot detectors. Based on these assumptions, new formula for average number of electron in a quantum dot for both, dark and illumination condition is calculated, which is more accurate than the previous reported formulas; because in deriving previous reported formulas, it was assumed only thermionic emission determines dark current but field-assisted tunneling mechanisms has not been considered. Then numerical method is used to calculate the average number of electron in a quantum dot and to determine dark current and photocurrent. The theoretical results are compared with experimental data. They have good agreement with available experimental data.     

Antiresonance of electron tunneling through a quantum dot array

Electronic transport through a one-dimensional quantum dot array is theoretically studied. In such a system both electron reservoirs of continuum states couple with the individual component quantum dots of the array arbitrarily. When there are some dangling quantum dots in the array outside the dot(s) contacting the leads, the electron tunneling through the quantum dot array is wholly forbidden if the electron energy is just equal to the molecular energy levels of the dangling quantum dots, which is called as antiresonance of electron tunneling. Accordingly, when the chemical potential of the reservoir electrons is aligned with the electron levels of all quantum dots, the linear conductance at zero temperature vanishes if there are odd number dangling quantum dots; Otherwise, it is equal to 2e²/h due to resonant tunneling if the total number of quantum dots in the array is odd. This odd–even parity is independent of the interdot and the lead–dot coupling strength.     

Dynamics of a wave packet in a system with a quantum dot in the Coulomb blockade regime

The nonstationary problem of electron tunneling through a quantum dot in the Coulomb block-ade regime is studied. The temporal Schrödinger equation is solved and the dynamics of the wave packet in a system consisting of a quantum dot connected to two one-dimensional contacts is investigated. The transmission coefficient is calculated. Dependences of the transmission on the tunneling electron energy are constructed.     

Coherent resonant transport through a mesoscopic system with quantum ac microwave field

The time-dependent transport through an ultrasmall quantum dot coupling to two electron reservoirs is investigated. The quantum dot is perturbed by a quantum microwave field (QMF) through gate. The tunneling current formulae are obtained by taking expectation values over coherent state (CS), and SU(1,1) CS. We derive the transport formulae at low temperature by employing the nonequilibrium Green function technique. The currents exhibit coherent behaviors which are strongly associated with the applied QMF. The time-dependent currents appear compound effects of resonant tunneling and time-oscillating evolution. The time-averaged current and differential conductance are calculated, which manifest photon-assisted behaviors. Numerical calculations reveal the similar properties as those in classical microwave field (CMF) perturbed system for the situations concerning CS and squeezed vacuum SU(1,1) CS. But for other squeezed SU(1,1) CS, the tunneling behavior is quite different from the system perturbed by a single CMF through gate. Due to the quantum signal perturbation, the measurable quantities fluctuate fiercely. Received 28 May 1998     

Electronic Green's functions in a T-shaped multi-quantum dot system

We developed a set of equations to calculate the electronic Green's functions in a T-shaped multi-quantum dot system using the equation of motion method. We model the system using a generalized Anderson Hamiltonian which accounts for finite intradot on-site Coulomb interaction in all component dots as well as for the interdot electron tunneling between adjacent quantum dots. Our results are obtained within and beyond the Hartree–Fock approximation and provide a path to evaluate all the electronic correlations in the multi-quantum dot system in the Coulomb blockade regime. Both approximations provide information on the physical effects related to the finite intradot on-site Coulomb interaction. As a particular example for our generalized results, we considered the simplest T-shaped system consisting of two dots and proved that our approximation introduces important corrections in the detector and side dots Green's functions, and implicitly in the evaluation of the system's transport properties. The multi-quantum dot T-shaped setup may be of interest for the practical realization of qubit states in quantum dot systems.     

Fabrication of quantum dot transistors incorporating a single self-assembled quantum dot

We report on the fabrication and the characterization of quantum dot transistors incorporating a single self-assembled quantum dot. The current–voltage characteristics exhibit clear staircase structures at room temperature. They are attributed to electron tunneling through the quantized energy levels of a single quantum dot.     

Energy-dependent tunneling in a quantum dot

We present measurements of the rates for an electron to tunnel on and off a quantum dot, obtained using a quantum point contact charge sensor. The tunnel rates show exponential dependence on drain-source bias and plunger gate voltages. The tunneling process is shown to be elastic, and a model describing tunneling in terms of the dot energy relative to the height of the tunnel barrier quantitatively describes the measurements.     

Interference Behaviors of Resonant Transport Through a Mesoscopic System with ac Responses

Time-dependent interference behaviors on currents transporting through a mesoscopic system are investigated by using the Keldysh nonequilibrium Green function technique. The system is composed of a quantum dot coupled with two electron reservoirs. The electrons in the quantum dot are perturbed by two microwave fields (MWFs) through gate. The MWFs cause the energy level splitting in the quantum dot to form multi-channel for the tunneling current, and these branches of current interfere to produce stable oscillation. The resulting oscillation of current is strongly associated with frequency relations between MWFs. The timedependent current is the consequence of resonant effects for electrons resonating with quantum dot state and with MWFs. We present numerical calculations for the cases where the Coulomb interaction U = 0. Negative temporal current and differential conductance are observed even if the dc bias is not small. We compare the results with corresponding quantities in the system perturbed by single MWF.     

Multi-functional edge driven nano-scale cellular automata based on semiconductor tunneling nano-structure with a self-assembled quantum dot layer

We present a feasibility study of the semiconductor tunneling nano-structure consisting of multiple layers of two semiconductors along with a quantum dot layer for potential application in a cellular automata logic module. The elementary logic cell of the proposed CA module consists of a couple of tunnel diodes connected in series through a quantum dot. The charge of the quantum dot is considered as a logic variable. The local interconnections of nano-cells are achieved via the in-plane tunneling in the quantum dot layer. On the basis of approximate tunneling characteristics, multiple associative states and state dynamics are simulated. There are two ultimate advantages of the proposed CA scheme: (i) potential realization of a number of logic functions in one module, and (ii) reduced number of cell contacts required for read-in and read-out procedures (only edge cells have individual contacts). Examples of image processing using different logic functions are presented.     

Effect of measurement on the decay rate of a quantum system

We investigated the electron tunneling out of a quantum dot in the presence of continuous monitoring by a detector. It is shown that the Schrodinger equation for the whole system can be reduced to new Bloch-type rate equations describing the time development of the detector and the measured system at once. Using these equations we find that the continuous measurement of the unstable system does not affect its exponential decay, exp(-gammat), contrary to expectations based on the quantum Zeno effect. However, the width of the energy distribution of the tunneling electron is no longer gamma, but increases due to the decoherence, generated by the detector.