微波作用下有直接隧穿量子点系统中的泵流特性

利用演化算符的方法,研究了量子点体系中的电流以及自旋流,该体系中量子点和左右磁性电极耦合并且受到微波作用,且两电极之间有直接隧穿,得到了体系电流的解析表达式.发现对于无直接隧穿和零偏压情况,无论对称结构还是非对称结构,电流和自旋流总为零.对于直接隧穿和零偏压情况,对于两边为非对称结构,微波场辐射在量子点上可以导致自旋流而非零的总电流,给出了平行和反平行磁构型下的结果并进行了讨论;对于两边为对称结构结构,平行磁构型下,量子点上加微波场时自旋流和总电流均为零;在反平行磁构型下,量子点上加微波场可以导致自旋流而 关键词： 微波场 直接隧穿 量子点 泵流     

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.     

The effects of two-dimensional bifurcations and quantum beats in a system of combined atomic force and scanning tunneling microscopes with quantum dots

The field and temperature dependence of the probability of two-dimensional dissipative tunneling is studied in the framework of one-instanton approximation for a model double-well oscillator potential in an external electric field at finite temperature with account for the influence of two local phonon modes for quantum dots in a system of a combined atomic force and a scanning tunneling microscope. It is demonstrated that in the mode of synchronous parallel transfer of tunneling particles from the cantilever tip to the quantum dot the two local phonon modes result in the occurrence of two stable peaks in the curve of the 2D dissipative tunneling probability as a function of the field. Qualitative comparison of the theoretical curve in the limit of weak dissociation and the experimental current–voltage characteristic for quantum dots that grow from colloidal gold under a cantilever tip at the initial stage of quantum-dot formation when the quantum dot size does not exceed 10 nm is performed. It is established that one of the two stable peaks that correspond to interaction of tunneling particles with two local phonon modes in the temperature dependence of the 2D dissipative tunneling probability can be split in two, which corresponds to the tunneling channel interference mechanism. It is found that the theoretically predicted and experimentally observed mode of quantum beats occurs near the bifurcation point.     

Photon-assisted transport through a quantum dot coupled to superconductors

Photon-assisted electron transport for resonant tunneling has been investigated by using a current formula developed based on the nonequilibrium Green’s function technique. We have studied the external frequency dependence as well as the energy level position dependence for the resonant ac tunneling through the quantum dot coupled to two superconducting reservoirs.     

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.     

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     

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.     

Investigation of the quantum dot infrared photodetectors dark current

Quantum dot infrared photodetectors (QDIPs) are more efficient than other types of semiconductor based photodetectors; so it has become an actively developed field of research. In this paper quantum dot infrared photodetector dark current is evaluated theoretically. This evaluation is based on the model that was developed by Ryzhii et al. Here it is assumed that both thermionic emission and field-assisted tunneling mechanisms determine the dark current of QDIPs; moreover we have considered Richardson effect, which has not been taken into account in previous research. Then a new formula for estimating average number of electrons in a quantum dot infrared photodetector is derived. Considering the Richardson effect and field-assisted tunneling mechanisms in the dark current improves the accuracy of algorithm and causes the theoretical data to fit better in the experiment. The QDIPs dark current temperature and biasing voltage dependency, contribution of thermionic emission and field-assisted tunneling at various temperatures and biasing voltage in the QDIPs dark current are investigated. Moreover, the other parameter effects like quantum dot (QD) density and QD size effect on the QDIPs dark current are investigated.     

Laser location and manipulation of a single quantum tunneling channel in an InAs quantum dot

We use a femtowatt focused laser beam to locate and manipulate a single quantum tunneling channel associated with an individual InAs quantum dot within an ensemble of dots. The intensity of the directed laser beam tunes the tunneling current through the targeted dot with an effective optical gain of 10(7) and modifies the curvature of the dot's confining potential and the spatial extent of its ground state electron eigenfunction. These observations are explained by the effect of photocreated hole charges which become bound close to the targeted dot, thus acting as an optically induced gate electrode.     

Electroluminescence spectra of an STM-tip-induced quantum dot

We analyze the electroluminescence spectrum of an STM-tip-induced quantum dot in a GaAs surface layer. A flexible model has been developed, that combines analytical and numerical methods and describes the key features of many-particle states in the STM-tip-induced quantum dot. The dot is characterized by its depth and lateral width, which are experimentally controlled by the bias and the tunneling current. We find, in agreement with experiment, that increasing voltage on the STM-tip results in a red shift of the electroluminescence peaks, while the peak positions as a function of the electron tunneling current through the STM-tip reveal a blue shift.     

Fano-type spin-flip mesoscopic transport through an Aharonov-Bohm interferometer

We have investigated the mesoscopic transport properties of a quantum dot embedded Aharonov-Bohm (AB) interferometer applied with a rotating magnetic field. The spin-flip effect is induced by the rotating magnetic field, and the tunneling current is sensitive to the spin-flip effect. The spin-flipped electrons tunneling from the direct channel and the resonant channel interfere with each other to form spin-polarized tunneling current components. The non-resonant tunneling (direct transmission) strength and the AB phase φ play important roles. When the non-resonant tunneling (background transmission) exists, the spin and charge currents form asymmetric peaks and valleys, which exhibit Fano-type line shapes by varying the source-drain bias voltage, or gate voltage. The AB oscillations of the spin and charge currents exhibit distinct dependence on the magnetic flux and direct tunneling strength.     

Single electron charging of InAs quantum dots characterized by δ-doped channel conductivity

The electron tunneling through single self-assembled InAs dot in split-gate δ-doped channel transistor structure is reported for the first time. In the nearly pinch-off conditions, the channel current was found to manifest itself single-electron tunneling through a self-assembled InAs dot buried in adjacent to the channel. The line shape of the single-electron tunneling current through a single InAs dot is discussed.     

Temperature-controllable spin-polarized current and spin polarization in a Rashba three-terminal double-quantum-dot device

We propose a Rashba three-terminal double-quantum-dot device to generate a spin-polarized current and manipulate the electron spin in each quantum dot by utilizing the temperature gradient instead of the electric bias voltage. This device possesses a nonresonant tunneling channel and two resonant tunneling channels. The Keldysh nonequilibrium Green's function techniques are employed to determinate the spin-polarized current flowing from the electrodes and the spin accumulation in each quantum dot. We find that their signs and magnitudes are well controllable by the gate voltage or the temperature gradient. This result is attributed to the change in the slope of the transmission probability at the Fermi levels in the low-temperature region. Importantly, an obviously pure spin current can be injected into or extracted from one of the three electrodes by properly choosing the temperature gradient and the gate voltages. Therefore, the device can be used as an ideal thermal generator to produce a pure spin current and manipulate the electron spin in the quantum dot.     

Controlled dephasing of a quantum dot: from coherent to sequential tunneling

Resonant tunneling through two identical potential barriers renders them transparent, as particle trajectories interfere coherently. Here we realize resonant tunneling in a quantum dot (QD), and show that detection of electron trajectories renders the dot nearly insulating. Measurements were made in the integer quantum Hall regime, with the tunneling electrons in an inner edge channel coupled to detector electrons in a neighboring outer channel, which was partitioned. Quantitative analysis indicates that just a few detector electrons completely dephase the QD.     

Resonance breakdown of a Coulomb blockade due to the mechanical vibrations of a quantum dot

The effect of forced mechanical vibrations of a suspended single-electron transistor on Coulomb-blockade limited electron tunneling through a quantum dot has been studied. The mechanical vibrations of the quantum dot have been shown to result in the Coulomb blockade breakdown, which is manifested by narrow resonance peaks of the transistor conductance as a function of the excitation frequency at the frequencies corresponding to the eigenmodes of the mechanical vibrations. The mechanism of the observed effect presumably associated with the oscillations of the mutual electrical capacitances between the quantum dot and the surrounding electrodes is discussed.     

Resonant tunneling of electrons through single self-assembled InAs quantum dot studied by conductive atomic force microscopy

Resonant tunneling of electrons through a quantum level in single self-assembled InAs quantum dot (QD) embedded in thin AlAs barriers has been studied. The embedded InAs QDs are sandwiched by 1.7-nm-thick AlAs barriers, and surface InAs QDs, which are deposited on 8.3 nm-thick GaAs cap layer, are used as nano-scale electrodes. Since the surface InAs QD should be vertically aligned with a buried one, a current flowing via the buried QD can be measured with a conductive tip of an atomic force microscope (AFM) brought in contact with the surface QD-electrode. Negative differential resistance attributed to electron resonant tunneling through a quantized energy level in the buried QD is observed in the current–voltage characteristics at room temperature. The effect of Fermi level pinning around nano-scale QD-electrode on resonance voltage and the dependence of resonance voltage on the size of QD-electrodes are investigated, and it has been demonstrated that the distribution of the resonance voltages reflects the size variation of the embedded QDs.     

弱耦合GaAs/AlGaAs/InGaAs双势阱隧穿结构的磁隧穿特性研究

研究了低温(15 K)条件下弱耦合GaAs/AlGaAs/InGaAs双势阱结构的纵向磁隧穿特性. 研究表明,器件在零偏压下处于共振状态. 通过分析不同偏压下的磁电导振荡曲线,可以得到双量子阱中的基态束缚能级随偏压的变化规律,从而可以确定隧穿电流峰对应的隧穿机制. 所得结果可为弱耦合双量子点器件的制备提供基础. 关键词： 双量子阱 隧穿结构 磁电导振荡     

Adiabatic pumping in the mixed-valence and Kondo regimes

We investigate adiabatic pumping through a quantum dot with a single level in the mixed-valence and Kondo regimes using the slave boson mean field approximation. The pumped current is driven by a gauge potential due to time-dependent tunneling barriers as well as by the modulation of the Friedel phase. The sign of the former contribution depends on the strength of the Coulomb interaction. Under finite magnetic fields, the separation of the spin and charge currents peculiar to the Kondo effect occurs.     

The photocurrent of resonant tunneling diode controlled by the charging effects of quantum dots

We experimentally studied the photocurrent of AlAs/GaAs/AlAs double barrier resonant tunneling diode (RTD), which is composed of an InAs layer of self-assembled quantum-dots on top of AlAs barrier layer. It is found that the charging InAs quantum dots can effectively modulate the carrier transport properties of the RTD. Moreover, we also found that the resonant tunneling current through a single energy level of an individual quantum dot is extremely sensitive to the photo-excited holes bound nearby the dot, and the presence of the holes lowers the electrostatic energy of the quantum dot state. In addition, it is also observed that the photocurrent behaves like step way with the individual photon pulse excitation when the illumination is low enough. The experiment results well demonstrated the quantum amplified characteristics of the device.     

Room-temperature observation of current bistability and fine structures in germanium quantum dots/SiO2 resonant tunneling diodes

The steady-state and time-dependent current–voltage (I–V) characteristics are experimentally investigated in Ge quantum dot (QD)/SiO₂ resonant tunneling diodes (RTDs). Ge QDs embedded in a SiO₂ matrix are naturally formed by thermal oxidation of Si_0.9Ge_0.1 nanowires (30 nm×50 nm) on silicon-on-insulator substrates. The average dot size and spacing between dots are 9±1 and 25 nm, respectively, from TEM observations, which indicate that one or two QDs are embedded between SiO₂ tunneling barriers within the nanowires. Room-temperature resonant oscillation, negative differential conductance, bistability, and fine structures are observed in the steady-state tunneling current of Ge-QD/SiO₂ RTDs under light illumination. Time-dependent tunneling current characteristics display periodic seesaw features as the Ge-QDs RTD is biased within the voltage regime of the first resonance peak while they exhibit harmonic swing behaviors as the RTD is biased at the current valleys or higher-order current peaks. This possibly originates from the interplay of the random telegraph signals from traps at the QD/SiO₂ interface as well as the electron wave interference within a small QD due to substantial quantum mechanics effects.