嵌入T型耦合双量子点介观A-B环系统的显著Fano 效应

利用平均场近似理论,研究了一个嵌入T型弱耦合双量子点的介观环系统的基态性质. 结果表明,体系中复杂的基态性质源于Kondo效应与Fano效应相互竞争. 当介观环的尺寸达到足以产生完全Kondo共振时,随双量子点间耦合强度的增强,尖锐的持续电流峰出现了,且越发显著,这说明体系中存在着显著的Fano 效应. 但介观环的Kondo共振持续电流峰值却几乎不发生变化,这为测定Kondo 屏蔽云提供了一个新的可能模型. 关键词： 耦合量子点 持续电流 Kondo效应 Fano 效应     

Negative Coulomb drag in coupled wire and quantum dot system

We measure the Coulomb drag between parallel split-gate quantum wires with a quantum dot embedded in one of the two wires (drive wire). We observe negative Coulomb drag when a Coulomb oscillation peak appears in the drive wire and the conductance of the other wire (drag wire) is slightly below the first plateau. This indicates that correlation holes are dragged in the drag wire by single electron tunneling through the quantum dot in the drive wire. The drag is only promoted in the drag wire near the barrier regions of the dot, and low compressibility of the drag wire is necessary for the negative drag to occur.     

Quantum coherence in quantum dot—Aharonov–Bohm ring hybrid systems

We investigate coherent transport through hybrid systems of quantum dots and Aharonov–Bohm (AB) rings. Strong coherence over the entire system leads to the Fano effect, which originates from the interference and the phase shift caused by the discrete states in the dots. The high controllability of the system parameters reveals that the Fano effect in mesoscopic transport can be a powerful tool for detecting the phase shift of electrons. We apply it to detect electrostatic phase modulation and the phase shift in a quantum wire with a side-coupled dot. Finally, we provide an experimental answer to the problem of “neighboring in-phase Coulomb peaks”.     

Coulomb drag in the quantum Hall state: Role of disorder

We consider Coulomb drag between two layers of two-dimensional electron gases subject to a strong magnetic field, with the Landau level filling factor in each layer being . We find to be very large, as compared to the zero magnetic field case. We attribute this enhancement to the slow decay of density fluctuations in a strong magnetic field. For a clean system, the linear -dependence of the longitudinal conductivity, characteristic of the state, leads a unique temperature dependence – . Within a semiclassical approximation, disorder leads to a decrease of the transresistivity as compared to the clean case, and a temperature dependence of at low temperatures.     

A mean field approach to Coulomb blockade for a disordered assembly of quantum dots

The Coulomb blockade (CB) in quantum dots (QDs) is by now well documented. It has been used to guide the fabrication of single electron transistors. Even the most sophisticated techniques for synthesizing QDs (e.g. MOCVD/MBE) result in an assembly in which a certain amount of disorder is inevitable. On the other hand, theoretical approaches to CB limit themselves to an analysis of a single QD. In the present work we consider two types of disorders: (i) size disorder; e.g. QDs have a distribution of sizes which could be unimodal or bimodal in nature. (ii) Potential disorder with the confining potential assuming a variety of shapes depending on growth condition and external fields. We assume a Gaussian distribution in disorder in both size and potential and employ a simplified mean field theory. To do this we rely on the scaling laws for the CB (also termed as Hubbard U) obtained for an isolated QD [1]. We analyze the distribution in the Hubbard U as a consequence of disorder and observe that Coulomb blockade is partially suppressed by the disorder. Further, the distribution in U is a skewed Gaussian with enhanced broadening.      

Spin-flip effects in a parallel-coupled double quantum dot molecule

We investigate theoretically the electronic transport through a parallel-coupled double quantum dot (DQD) molecule attached to metallic electrodes, in which the spin-flip scattering on each quantum dot is considered. Special attention is paid to the effects of the intradot spin-flip processes on the linear conductance by using the equation of motion approach for Green’s functions. When a weak spin-flip scattering on each quantum dot is present, the single Fano peak splits into two Fano peaks, and the Breit–Wigner resonance may be suppressed slightly. When the spin-flip scattering strength on each quantum dot becomes strong, the linear conductance spectrum consists of two Breit–Wigner peaks and two Fano peaks due to the quantum interference effects. The positions and shapes of these resonant peaks can be controlled by using the magnetic flux through the quantum device.     

Spin effects in a quantum ring

Recent experiments that are reviewed explore the spin states of a ring-shaped many-electron quantum dot. Coulomb-blockade spectroscopy is used to access the spin degree of freedom. The Zeeman effect observed for states with successive electron number allows to select possible sequences of spin ground states of the ring. Spin-paired orbital levels can be identified by probing their response to magnetic fields normal to the plane of the ring and electric fields caused by suitable gate voltages. This narrows down the choice of ground-state spin sequences. A gate-controlled singlet–triplet transition is identified and the size of the exchange interaction matrix element is determined.     

Anomalous quantum Hall effect induced by nearby quantum dots

Transport measurements in high magnetic fields have been performed on two-dimensional electron system (2DES) separated by a thin barrier layer from a layer of InAs self-assembled quantum dots (QDs). Clear feature of quantum Hall effect was observed in spite of presence of QDs nearby 2DES. However, both magnetoresistance, ρ_xx, and Hall resistance, ρ_xy, are suppressed significantly only in the magnetic field range of filling factor in 2DES ν<1 and voltage applied on a front gate . The results indicate that the electron state in QDs induces spin-flip process in 2DES.     

Fano resonance and spectral compression in a ring resonator drop filter with feedback

We propose a ring resonator drop filter with feedback loop which can form the other ring cavity to produce an asymmetric Fano-resonance line shape and mode compression. By properly adjusting the feedback loop, the asymmetric line shapes of the transmission spectra can be controlled and selectively mode compression of the circuiting intensity in the feedback loop is achieved. Such characteristics are useful for applications in ring resonator-based photonic devices such as all-optical switches, sensors and intracavity frequency selection single-mode laser.     

Coulomb blockade in triangular lateral small-size quantum dots

An AlGaAs/GaAs lateral quantum dot of triangular shape with a characteristic size L<100 nm containing less than ten electrons was studied. Single-electron oscillations of the conductance G of this dot were measured at G<e²/h. When going from Ge²/h to G≈0.5e²/h, a decrease was found not only in the amplitude but also in the period of the oscillations. A calculation of the 3D-electrostatics demonstrated that this effect is due to a change in the dot size produced by control voltages.     

Tunneling of slow quantum packets through the high Coulomb barrier

We study the tunneling of slow quantum packets through a high Coulomb barrier. We show that the transmission coefficient can be quite different from the standard expression obtained in the plane wave (WKB) approximation (and larger by many orders of magnitude), even if the momentum dispersion is much smaller than the mean value of the momentum.     

Spacing and width of Coulomb blockade peaks in a silicon quantum dot

We present an experimental study of the fluctuations of Coulomb blockade peak positions of a quantum dot. The dot is defined by patterning the two-dimensional electron gas of a silicon MOSFET structure using stacked gates. The ratio of charging energy to single-particle energy is considerably larger than in comparable GaAs/AlGaAs quantum dots. The statistical distribution of the conductance peak spacings in the Coulomb blockade regime was found to be unimodal and does not follow the Wigner surmise. The fluctuations of the spacings are much larger than the typical single-particle level spacing and thus clearly contradict the expectation of random matrix theory. Measurements of the natural line width of a set of several adjacent conductance peaks suggest that all of the peaks in the set are dominated by electrons being transported through a single-broad energy level.     

Persistent currents in a mesoscopic ring with a side connected quantum dot

We study the persistent current circulating along a mesoscopic ring with a dot side-coupled to it when threaded by a magnetic field. A cluster including the dot and its vicinity is diagonalized and embedded into the rest of the system. The result is numerically exact. We show that in the Kondo regime, the current can be a smooth or a strongly dependent function of the gate potential according to the structure of occupation of the highest energetic electrons of the system.     

Complex-number asymmetry parameters of the optical Fano effect in ring resonators

We studied the sharp asymmetric Fano line shape in fiber ring resonator systems and provided an explicit expression for asymmetry parameters using the physical parameters of the system. The fiber ring system was controllable and reconfigurable, allowing us to produce a variety of Fano line shapes in different configurations. Experimentally observed asymmetric spectral structures were fully reproduced using the complex-number asymmetry parameters, validating the approximations used to reduce the analytical expression for the line shape to the phenomenological Fano formula. The results may be useful in the design of on-chip application systems.     

Thermoelectric properties of correlated double quantum dot system in Coulomb blockade regime

We theoretically study thermoelectric properties of a coupled double quantum dot (DQD) system coupled to normal leads using two impurity Anderson model with intra- as well as interdot Coulomb interactions. A generic formulation, which was earlier developed to study electronic properties (zero bias maximum of differential conductance and interesting partial swapping in Fano phenomena) of DQD system within Coulomb blockade regime for a non-magnetic case, is extended to investigate thermoelectric properties i.e. electrical conductance, thermoelectric power and thermal conductance of the same system, as a function of temperature by varying interdot Coulomb interaction and interdot tunneling. Interdot Coulomb interaction is found to trigger some novel features like crossover in thermoelectric power with temperature in all the configurations (series, parallel and T-shape) and a small peak in thermal conductance toward low temperatures, T≈Γ/10, in series and T-shape configurations, which is found to be missing in case of symmetric parallel configuration. The origin of these novel features is attributed to the interplay of renormalization of energy levels caused by the interdot Coulomb interaction which is interpreted in terms of local density of states and the asymmetry effects related to dot-lead couplings/interference effects.     

Fano effect of a parallel-coupled triple Rashba quantum dot system

Using the nonequilibrium Keldysh Green's function technique, the Fano effect of a parallel-coupled triple Rashba quantum dot system is investigated. The conductance as a function of electron energy is numerically calculated. Compared with the case of a parallel-coupled double quantum dot system, two additional Fano resonance peaks occur in the conductance spectrum. By adjusting the structural parameters, the two Fano resonance peaks may change into the resonance peaks. In addition, the influence of Rashba spin-orbit interaction on the conductance is studied.     

An effective screened Coulomb interaction in a quasi-one-dimensional system

In this paper, we derive an analytical expression for the screened Coulomb potential between charge carriers in quasi-one-dimensional (Q1D) semiconductor structures. As an application, this potential has been used to investigate the screening effect on the binding energy of a neutral donor (D⁰$D^0$) in quantum wires (QWRs). It is found that the screening effect decreases the neutral donor binding energy, and the screening effects are more obvious in wide QWRs than that in narrow ones. Dependence of screening length on temperature and carrier concentration has also been discussed.     

AC transport through a quantum dot: from Kondo to Coulomb-blockade behaviour

We analyze the conductance ( ) of a quantum dot (QD) in an AC potential at finite temperature. The Friedel–Langreth sum rule (FLSR) is generalized to include the effect of an AC potential and finite T. We have solved the Anderson Hamiltonian by means of a self-consistent procedure which fulfills the generalized FLSR. New features are found in the density of states (DOS) and in when an AC voltage is applied. Our model describes the effect of an AC potential on the transition from Kondo regime to a Coulomb-blockade behaviour as T increases.     

Fano effect and Andreev bound states in T-shape double quantum dots

In this Letter, we investigate the transport through a T-shaped double quantum dot coupled to two normal metal leads left and right and a superconducting lead. Analytical expressions of Andreev transmission and local density of states of the system at zero temperature have been obtained. We study the role of the superconducting lead in the quantum interferometric features of the double quantum dot. We report for first time the Fano effect produced by Andreev bound states in a side quantum dot. Our results show that as a consequence of quantum interference and proximity effect, the transmission from normal to normal lead exhibits Fano resonances due to Andreev bound states. We find that this interference effect allows us to study the Andreev bound states in the changes in the conductance between two normal leads.