Inter-dot coupling effects on transport through correlated parallel coupled quantum dots

Transport through symmetric parallel coupled quantum dot system has been studied, using non-equilibrium Green function formalism. The inter-dot tunnelling with on-dot and inter-dot Coulomb repulsion is included. The transmission coefficient and Landaur-Buttiker like current formula are shown in terms of internal states of quantum dots. The effect of inter-dot tunnelling on transport properties has been explored. Results, in intermediate inter-dot coupling regime show signatures of merger of two dots to form a single composite dot and in strong coupling regime the behaviour of the system resembles the two decoupled dots.      

玻璃中的半导体量子点

介绍了玻璃中的半导体量子点。对玻璃中半导体量子点的生长过程、量子的电子态，量子尺寸效应、库仑阻塞效应及介电效应，做了比较全面的介绍。讨论了量子点的应用及发展前景。     

Linear and nonlinear transports of coupled quantum dots

Series of double quantum dots each with a size around 400 × 400nm² have been realized by delineating a 2DEG in modulation-doped AlGaAs/GaAs with 100 nm wide Schottky split gates fabricated by an electron-beam lithography and a lift-off technique. The split gate in the middle of the double dot allows us to control interdot coupling widely. The charging diagram obtained from linear transports in the Coulomb blockade regime shows that the isolated dots merge into a single composite dot with increase of interdot coupling. A clear Coulomb staircase has been observed in the double-dot system at a limited high-bias condition.     

Probing the coherent transport through coupled quantum dots

We investigate the photoconductance through a double quantum dot or ‘artificial molecule’ induced by a broadband millimeter wave source. This source functions as a heterodyne interferometer, consisting of two nonlinear transmission lines generating harmonic output in the range of 2–400 GHz, and, being coherent, allows tracking of the induced current of the sample in both magnitude and phase. This enables us to monitor effects related to coherent electronic transport through these ‘artificial molecules’.     

Coulomb blockade in molecular quantum dots

The rate-equation approach is used to describe sequential tunneling through a molecular junction in the Coulomb blockade regime. Such device is composed of molecular quantum dot (with discrete energy levels) coupled with two metallic electrodes via potential barriers. Based on this model, we calculate nonlinear transport characteristics (conductance-voltage and current-voltage dependences) and compare them with the results obtained within a self-consistent field approach. It is shown that the shape of transport characteristics is determined by the combined effect of the electronic structure of molecular quantum dots and by the Coulomb blockade. In particular, the following phenomena are discussed in detail: the suppression of the current at higher voltages, the charging-induced rectification effect, the charging-generated changes of conductance gap and the temperature-induced as well as broadening-generated smoothing of current steps.     

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-polarized transport through the T-shaped double quantum dots

Using the Keldysh nonequilibrium Green function and equation-of-motion technique, this paper investigates the spin-polarized transport properties of the T-shaped double quantum dots （DQD） coupled to two ferromagnetic leads. There are both Fano effect and Kondo effect in the system, and due to their mutual interaction, the density of states, the current, and the differential conductance of the system depend sensitively on the spin-polarized strength. Thus the obtained results show that this system is provided with excellent spin filtering property, which indicates that this system may be a candidate for spin valve transistors in the spintronics.     

Transport properties of quantum dots

Linear and nonlinear transport through a quantum dot that is weakly coupled to ideal quantum leads is investigated in the parameter regime where charging and geometrical quantization effects coexist. The exact eigenstates and spins of a finite number of correlated electrons confined within the dot are combined with a rate equation. The current is calculated in the regime of sequential tunneling. The analytic solution for an Anderson impurity is given. The phenomenological charging model is compared with the quantum mechanical model for interacting electrons. The current-voltage characteristics show Coulomb blockade. The excited states lead to additional fine-structure in the current voltage characteristics. Asymmetry in the coupling between the quantum dot and the leads causes asymmetry in the conductance peaks which is reversed with the bias voltage. The spin selection rules can cause a ‘spin blockade’ which decreases the current when certain excited states become involved in the transport. In two-dimensional dots, peaks in the linear conductance can be suppressed at low temperatures, when the total spins of the corresponding ground states differ by more than 1/2. In a magnetic field, an electron number parity effect due to the different spins of the many-electron ground states is predicted in addition to the vanishing of the spin blockade effect. All of the predicted features are consistent with recent experiments.     

Quantum phase transition and Coulomb blockade effect in triangular quantum dots with interdot capacitive and tunnel couplings

The quantum phase transition and the electronic transport in a triangular quantum dot system are investigated using the numerical renormalization group method.We concentrate on the interplay between the interdot capacitive coupling V and the interdot tunnel coupling t.For small t,three dots form a local spin doublet.As t increases,due to the competition between V and t,there exist two first-order transitions with phase sequence spin-doublet-magnetic frustration phase-orbital spin singlet.When t is absent,the evolutions of the total charge on the dots and the linear conductance are of the typical Coulomb-blockade features with increasing gate voltage.While for sufficient t,the antiferromagnetic spin correlation between dots is enhanced,and the conductance is strongly suppressed for the bonding state is almost doubly occupied.     

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.     

Lateral electron transport through single InAs quantum dots grown by molecular beam epitaxy

The transport properties of single InAs quantum dots (QDs) grown by molecular beam epitaxy have been investigated by metallic leads with nanogaps. It was found that the uncapped InAs QDs grown on the GaAs surfaces show metallic conductivities, indicating that even the exposed QDs are not depleted. On the contrary, it was found that no current flows through the exposed wetting layers. For the case of the QDs covered with GaAs capping layers, clear Coulomb gaps and Coulomb staircases have been observed at 4.2 K.     

Quantum fluctuations and the Kondo effect in small quantum dots

We consider electron transport through quantum dots with large level spacing and charging energy. At low temperature and strong coupling to the leads, quantum fluctuations and the Kondo effect become important. They show up, e.g., as zero-bias anomalies in the current–voltage characteristics. We use a recently developed diagrammatic technique as well as a new real-time renormalization-group approach to describe charge and spin fluctuations. The latter gives rise to a Kondo-assisted enhancement of the current through the dot as seen in experiments.     

碳纳米管量子点的电子输运

在紧束缚近似下,利用常量相互作用模型和Landauer-Bütticker公式,计算了扶手椅型和金属锯齿型碳纳米管量子点的电导。发现,根据碳纳米管量子点的长度的不同,扶手椅型碳纳米管量子点的电导可以具有两电子或四电子的壳层结构。而锯齿型碳纳米管量子点的电导却仅有四电子的壳层结构,与长度无关;这些理论结果与之前的实验结果符合的很好。     

Transport in split-gate silicon quantum dots

We report on the transport properties of novel Si quantum dot structures with controllable electron number through both top and side gates. Quantum dots were fabricated by a split-gate technique within a standard MOSFET process. Four-terminal dc electrical measurements were performed at 4.2 K in a liquid helium cryostat. Strong oscillations in the conductance through the dot are observed as a function of both the top gate bias and of the plunger bias. An overall monotonic and quasi-periodic movement of the peak conductance is observed which is believed to be associated with the bare level structure of the electronic states in the dot coupled with the Coulomb charging energy. Crossing behavior is observed as well, suggestive of either many-body effects or symmetry breaking of the dot states by the applied bias.     

Nonlinear transport in p-type SiGe quantum well structure containing Ge quantum dots

We use magneto-transport spectroscopy to study a dramatic instability between a low and high conductivity mode in Si/SiGe-based resonant tunneling diodes with an embedded layer of self-assembled Ge hut cluster quantum dots in the Si barrier. In the low current regime a simple activation-type behavior with an astonishingly low activation energy in the order of 0.1 meV is determined. In the high current regime a region of negative differential conduction can be observed. We discuss the influence of different layer structures and magnetic fields.     

Carrier tunneling in asymmetric coupled quantum dots

Exciton spin relaxation at low temperatures in InAlAs–InGaAs asymmetric double quantum dots embedded in AlGaAs layers has been investigated as a function of the barrier thickness by the time-resolved photoluminescence measurements. With decreasing the thickness of the AlGaAs layer between the dots, the spin relaxation time change from 3 ns to less than 500 ps. The reduction in the spin relaxation time was considered to originate from the spin-flip tunneling between the ground state in InAlAs dot and the excited states in InGaAs dot, and the resultant tunneling leads to the spin depolarization of the ground state in InGaAs dot.     

Carbon nanotubes as building blocks of quantum dots

Quantum dots have been fabricated with single-wall carbon nanotubes (SWCNTs), and their transport properties have been measured at low temperatures. The single-electron transport measurements revealed the artificial atom characteristics with a shell structure and the Zeeman splitting of single particle states. They have been observed with the metallic SWCNT that includes many electrons, in striking contrast to the case of semiconductor artificial atoms that have a few electrons. The unique features in the SWCNT artificial atom are discussed in terms of the energy scales associated with the quantum dot.