Nonequilibrium Kondo effect and Andreev reflection through double quantum dots with spin-flip scattering

The Kondo effect and the Andreev reflection tunneling through a normal (ferromagnet)-double quantum dots-superconductor hybrid system is examined in the low temperature by using the nonequilibrium Green's function technique in combination with the slave-boson mean-field theory. The interplay of the Kondo physics and the Andreev bound state physics can be controlled by varying the interdot hopping strength. The Andreev differential conductance is mainly determined by the competition between Kondo states and Andreev states. The spin-polarization of the ferromagnetic electrode increases the zero-bias Kondo peak. The spin-flip scattering influences the Kondo effect and the Andreev reflection in a nontrivial way. For the ferromagnetic electrode with sufficiently large spin polarization, the negative Andreev differential conductance is found when the spin flip strength in the double quantum dots is sufficiently strong.     

Single-photon detection mechanism in a quantum dot transistor

We study the transport mechanisms in a quantum dot MODFET by tuning the localization induced by charge stored on the quantum dots with light. The temperature dependence of the resistivity of a macroscopic sample reveals a hopping transport when the dots contain an excess of electrons. The resistance of a mesoscopic sample however, which is capable of detecting single photons, exhibits a much weaker dependence upon temperature. This points towards source-drain tunnelling as a transport mechanism and is confirmed by a statistical analysis of the single-photon-induced conductance steps. The complexity of the conducting paths increases as the average hopping length reduces.     

Observation of hopping transitions for delocalized electrons by temperature-dependent conductance in silicon junctionless nanowire transistors

We demonstrate transitions of hopping behaviors for delocalized electrons through the discrete dopant-induced quantum dots in n-doped silicon junctionless nanowire transistors by the temperature-dependent conductance characteristics.There are two obvious transition platforms within the critical temperature regimes for the experimental conductance data,which are extracted from the unified transfer characteristics for different temperatures at the gate voltage positions of the initial transconductance gm peak in V_(g1) and valley in V_(g2). The crossover temperatures of the electron hopping behaviors are analytically determined by the temperature-dependent conductance at the gate voltages V_(g1) and V_(g2). This finding provides essential evidence for the hopping electron behaviors under the influence of thermal activation and long-range Coulomb interaction.     

Quantum Phase Transition and Ferromagnetic Spin Correlation in Parallel Double Quantum Dots

We investigate electronic transport through a parallel double quantum dot （DQD） system with strong on-site Coulomb interaction, as well as the interdot tunnelling. By applying numerical renormalization group method, the ground state of the system and the transmission probability at zero temperature are obtained. For a system of quantum dots with degenerate energy levels and small interdot tunnel coupling, the spin correlations between the DQDs is ferromagnetic, and the ground state of the system is a spin-1 triplet state. The linear conductance will reach the unitary limit （2e^2/h） due to the Kondo effect at low temperature. As the interdot tunnel coupling increases, there is a quantum phase transition from ferromagnetic to anti-ferromagnetic spin correlation in DQDs and the linear conductance is strongly suppressed.     

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     

Phononless hopping conduction in two-dimensional layers of quantum dots

Regularities are studied in charge transport due to the hopping conduction of holes along two-dimensional layers of Ge quantum dots in Si. It is shown that the temperature dependence of the conductivity obeys the Efros-Shklovskii law. It is found that the effective localization radius of charge carriers in quantum dots varies nonmonotonically upon filling quantum dots with holes, which is explained by the successive filling of electron shells. The preexponential factor of the hopping conductivity ceases to depend on temperature at low temperatures (T<10 K) and oscillates as the degree of filling quantum dots with holes varies, assuming values divisible by the conductance quantum e²/h. The results obtained indicate that a transition from phonon-assisted hopping conduction to phononless charge transfer occurs as the temperature decreases. The Coulomb interaction of localized charge carriers has a dominant role in these phononless processes.     

The role of the indirect tunneling processes and asymmetry in couplings in orbital Kondo transport through double quantum dots

A system of two quantum dots attached to external electrodes is considered theoretically in the orbital Kondo regime. In general, the double dot system is coupled via both Coulomb interaction and direct hopping. Moreover, the indirect hopping processes between the dots (through the leads) are also taken into account. To investigate the system's electronic properties we apply the slave-boson mean field (SBMF) technique. With the help of the SBMF approach the local density of states for both dots and the transmission (as well as linear and differential conductance) is calculated. We show that Dicke- and Fano-like line shapes may emerge in the transport characteristics of the double dot system. Moreover, we observed that these modified Kondo resonances are very susceptible to the change of the indirect coupling's strength. We have also shown that the Kondo temperature becomes suppressed with increasing asymmetry in the dot-lead couplings when there is no indirect coupling. Moreover, when the indirect coupling is turned on the Kondo temperature becomes suppressed. By allowing a relative sign of the nondiagonal elements of the coupling matrix with left and right electrodes, we extend our investigations to become more generic. Finally, we have also included the level renormalization effects due to indirect tunneling, which are mostly neglected.     

Conductance peak splitting in charge polarized coupled quantum dots

We report a measurement of linear conductance through a series double dot as a function of the total double dot charge and the charge difference for interdot tunnel conductances between zero and one mode. The dots are defined by ten independently tunable electrostatic gates on the surface of a GaAs/AlGaAs heterostructure to allow separate adjustment of dot charge and interdot conductance. For weak interdot tunneling the measured double dot conductance agrees with a transport model in which each dot is individually governed by Coulomb blockade theory. As interdot tunnel conductance increases toward one mode, the measured conductance peak positions and shapes indicate both a relaxation of the charge quantization condition for individual dots and quantum mechanical charge sharing between dots. The results are in quantitative agreement with many body charge fluctuation theory.     

Hopping conductivity and Coulomb correlations in 2D arrays of Ge/Si quantum dots

The temperature and magnetic-field dependences of the conductivity associated with hopping transport of holes over a 2D array of Ge/Si(001) quantum dots with various filling factors are studied experimentally. A transition from the Éfros-Shklovski? law for the temperature dependence of hopping conductivity to the Arrhenius law with an activation energy equal to 1.0–1.2 meV is observed upon a decrease in temperature. The activation energy for the low-temperature conductivity increases with the magnetic field and attains saturation in fields exceeding 4 T. It is found that the magnetoresistance in layers of quantum dots is essentially anisotropic: the conductivity decreases in an increasing magnetic field oriented perpendicularly to a quantum dot layer and increases in a magnetic field whose vector lies in the plane of the sample. The absolute values of magnetoresistance for transverse and longitudinal field orientations differ by two orders of magnitude. The experimental results are interpreted using the model of many-particle correlations of holes localized in quantum dots, which lead to the formation of electron polarons in a 2D disordered system.     

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.     

Negative magnetoresistance in the regime of hopping conduction through <Emphasis Type="Italic">p</Emphasis> states at quantum dots

The magnetoresistance in the system of quantum dots with hopping conduction and filling factor 2 < ν < 3 in the limit of small quantum dots has been considered. In this case, hopping conduction is determined by p states. It has been shown that the system exhibits negative magnetoresistance associated with a change in the wavefunctions of p states in a magnetic field. This mechanism of magnetoresistance is linear in magnetic field in a certain range of fields and can compete with the known interference mechanism of magnetoresistance. The magnitude of this magnetoresistance is independent of the temperature at fairly low temperatures and increases with a decrease in the size of a quantum dot.     

Single electron charging in parallel coupled quantum dots

We report low-temperature conductance measurements in the Coulomb blockade regime on two nominally identical tunnel-coupled quantum dots in parallel defined electrostatically in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure. At low interdot tunnel coupling we find that the conductance measured through one dot is sensitive to the charge state of the neighboring dot. At larger interdot coupling the conductance data reflect the role of quantum charge fluctuations between the dots. As the interdot conductance approaches 2e²/h, the coupled dots behave as a single large dot.     

Nonlinear high-frequency hopping conduction in two-dimensional arrays of Ge-in-Si quantum dots: Acoustic methods

Using acoustic methods we have measured nonlinear AC conductance in 2D arrays of Ge-in-Si quantum dots. The combination of experimental results and modeling of AC conductance of a dense lattice of localized states leads us to the conclusion that the main mechanism of AC conduction in hopping systems with large localization length is due to the charge transfer within large clusters, while the main mechanism behind its non-Ohmic behavior is charge heating by absorbed power.     

Kondo effect for electron transport through an artificial quantum dot

We have calculated the transport properties of electron through an artificial quantum dot by using the numerical renormalization group technique in this paper. We obtain the conductance for the system of a quantum dot which is embedded in a one-dimensional chain in zero and finite temperature cases. The external magnetic field gives rise to a negative magnetoconductance in the zero temperature case. It increases as the external magnetic field increases. We obtain the relation between the coupling coefficient and conductance. If the interaction is big enough to prevent conduction electrons from tunnelling through the dot, the dispersion effect is dominant in this case. In the Kondo temperature regime, we obtain the conductivity of a quantum dot system with Kondo correlation.     

Quantum dots as scatterers in electronic transport: interference and correlations

Conductance through a system consisting of a wire with side-attached quantum dots is calculated. Such geometry of the device allows to study the coexistence of quantum interference, electron correlations and their influence on conductance. We underline the differences between ‘classical’ Fano resonance in which the resonant channel is of single-particle nature and ‘many-body’ Fano resonance with the resonant channel formed by Kondo effect. The influence of electron-electron interactions on the Fano resonance shape is also analyzed.     

Creation of entangled states of excitons in coupled quantum dots

We present two methods for the creation of two-particle entangled states of excitons in a coupled quantum dot system. The system contains two identical quantum dots that are coupled by an inter-dot hopping process. The manipulation of the system is succeeded by proper application of an external laser field.     

Nonequilibrium transport through double quantum dots: Kondo effect versus antiferromagnetic coupling

We theoretically study the nonequilibrium transport properties of double quantum dots, in both series and parallel configurations. Our results lead to novel experimental predictions that unambiguously signal the transition from a Kondo state to an antiferromagnetic spin-singlet state, directly reflecting the physics of the two-impurity Kondo problem. We prove that the nonlinear conductance through parallel dots directly measures the exchange constant J between the spins of the dots. In serial dots, the nonlinear conductance provides an upper bound on J.     

Zero temperature conductance of parallel T-shape double quantum dots

We analyze the zero temperature conductance of a parallel T-shaped double quantum dot system. We present an analytical expression for the conductance of the system in terms of the total number of electrons in both quantum dots. Our results confirm that the system's conductance is strongly influenced by the dot which is not directly connected to the leads. We discuss our results in connection with similar results reported in the literature.     

Negative differential conductance in InAs wire based double quantum dot induced by a charged AFM tip

We investigate the conductance of an InAs nanowire in the nonlinear regime in the case of low electron density where the wire is split into quantum dots connected in series. The negative differential conductance in the wire is initiated by means of a charged atomic force microscope tip adjusting the transparency of the tunneling barrier between two adjoining quantum dots. We confirm that the negative differential conductance arises due to the resonant tunneling between these two adjoining quantum dots. The influence of the transparency of the blocking barriers and the relative position of energy states in the adjoining dots on a decrease of the negative differential conductance is investigated in detail.     

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.