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.     

Tunneling through two single molecular magnets in series arrangement junction in parallel/antiparallel situations

We investigate the spin-dependent tunneling transport in a heterostructure with two single molecular magnets (SMMs). The tunneling magnetoresistance (TMR) and negative differential conductance due to the strong resonant tunneling in the junction are demonstrated by the master equation approach. At low bias voltage, the device presents low/high resistant states with the initial states of the single molecular magnets parallel/antiparallel. Strong Coulomb repulsive interaction suppresses the current greatly in antiparallel situation. At high voltage, the middle system containing two SMMs tends to be non-polarized, and acts like ordinary quantum dots.     

Phonon assisted resonant tunneling and its phonons control

We observe a series of sharp resonant features in the tunneling differential conductance of InAs quantum dots. We found that dissipative quantum tunneling has a strong influence on the operation of nanodevices. Because of such tunneling the current–voltage characteristics of tunnel contact created between atomic force microscope tip and a surface of InAs/GaAs quantum dots display many interesting peaks. We found that the number, position, and heights of these peaks are associated with the phonon modes involved. To describe the found effect we use a quasi-classical approximation. There the tunneling current is related to a creation of a dilute instanton–anti-instanton gas. Our experimental data are well described with exactly solvable model where one charged particle is weakly interacting with two promoting phonon modes associated with external medium. We conclude that the characteristics of the tunnel nanoelectronic devices can thus be controlled by a proper choice of phonons existing in materials, which are involved.     

Multi-mode transport through a quantum nanowire with two embedded dots

We investigate the conductance of a quantum wire with two embedded quantum dots using a T-matrix approach based on the Lippmann-Schwinger formalism. The quantum dots are represented by a quantum well with Gaussian shape and the wire is two-dimensional with parabolic confinement in the transverse direction. In a broad wire the transport can assume a strong nonadiabatic character and the conductance manifests effects caused by intertwined inter- and intra-dot processes that are identified by analysis of the “nearfield” probability distribution of the transported electrons.     

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     

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.     

Coulomb effects in artificial molecules

We study the capacitance spectra of artificial molecules consisting of two and three coupled quantum dots from an extended Hubbard Hamiltonian model that takes into account quantum confinement, intra- and inter-dot Coulomb interaction and tunneling coupling between all single particle states in nearest neighbor dots. We find that, for weak coupling, the interdot Coulomb interaction dominates the formation of a collective molecular state. We also calculate the effects of correlations on the tunneling probability through the evaluation of the spectral weights, and corroborate the importance of selection rules for understanding experimental conductance spectra.     

Tunneling current spectroscopy of a nanostructure junction involving multiple energy levels

A multilevel Anderson model is employed to simulate the system of a nanostructure tunnel junction with any number of one-particle energy levels. The tunneling current, including both shell-tunneling and shell-filling cases, is theoretically investigated via the nonequilibrium Green's function method. We obtain a closed form for the spectral function, which is used to analyze the complicated tunneling current spectra of a quantum dot or molecule embedded in a double-barrier junction. We also show that negative differential conductance can be observed in a quantum dot tunnel junction when the Coulomb interactions with neighboring quantum dots are taken into account.     

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.     

Distortions of the coulomb blockade conductance line in scanning gate measurements of inas nanowire based quantum dots

We performed measurements at helium temperatures of the electronic transport in the linear regime in an InAs quantum wire in the presence of a charged tip of an atomic force microscope (AFM) at low electron concentration. We show that at certain concentration of electrons, only two closely placed quantum dots, both in the Coulomb blockade regime, govern conductance of the whole wire. Under this condition, two types of peculiarities—wobbling and splitting—arise in the behavior of the lines of the conductance peaks of Coulomb blockade. These peculiarities are measured in quantum-wire-based structures for the first time. We explain both peculiarities as an interplay of the conductance of two quantum dots present in the wire. Detailed modeling of wobbling behavior made in the framework of the orthodox theory of Coulomb blockade demonstrates good agreement with the obtained experimental data.     

Signatures of spin-charge separation in scanning probe microscopy

We analyze the effect of an auxiliary scatterer, such as the potential of a scanning tip, on the conductance of an interacting one-dimensional electron system. We find that the differential conductance for tunneling into the end of a semi-infinite quantum wire reflects the separation of the elementary excitations into spin and charge modes. The separation is revealed as a specific pattern in the dependence of the conductance on bias and on the position of the scatterer.     

Tunneling induced transparency and giant Kerr nonlinearity in multiple quantum dot molecules

The linear absorption and the Kerr nonlinearity of multiple quantum dots molecules controlled by the tunneling rather than the laser fields are investigated. The tunneling between the dots can induce multiple transparency windows. By varying the energy splitting of the excited states and the tunneling intensity, the width of the tunneling induced transparency windows can be narrowed. Within the narrowed transparency windows, the steep dispersion profile of the probe field makes it possible to enhance the Kerr nonlinearity. Therefore more than one probe fields with different frequencies can acquire giant Kerr accompanied by vanishing absorption simultaneously.     

STM tunneling through a quantum wire with a side-attached impurity

The STM tunneling through a quantum wire (QW) with a side-attached impurity (atom, island) is investigated using a tight-binding model and the non-equilibrium Keldysh Green function method. The impurity can be coupled to one or more QW atoms. The presence of the impurity strongly modifies the local density of states of the wire atoms, thus influences the STM tunneling through all the wire atoms. The transport properties of the impurity itself are also investigated mainly as a function of the wire length and the way it is coupled to the wire. It is shown that the properties of the impurity itself and the way it is coupled to the wire strongly influence the STM tunneling, the density of states and differential conductance.     

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.     

Conductance in an Aharonov-Bohm Interferometer with Parallel-Coupled Double Dots

We present a theoretical study of the conductance in an Aharonov-Bohm interferometer containing two coupled quantum dots. The interdot tunneling divides the interferometer into two coupled subrings, where opposite magnetic fluxes are threaded separately while the net flux is kept zero. Using the Green function technique we derive the expression of the linear conductance. It is found that the Aharonov-Bohm effect still exists, and when the level of each dot is aligned, the exchange of the Fano and Breit-Wigner resonances in the conductance can be achieved by tuning the magnetic flux. When the two levels are mismatched the exchange may not happen. Further, for some specific asymmetric systems where the coupling strengths between the two dots and the leads are not equal, the flux can change the Fano resonance into an antiresonance, which is absent in symmetric systems.     

Tunneling and nonhyperbolicity in quantum dots

We argue that many major features in electronic transport in realistic quantum dots are not explainable by the usual semiclassical approach, due to the contributions of the quantum-mechanical tunneling of the electrons through the Kolmogorov-Arnol'd-Moser islands. We show that dynamical tunneling gives rise to a set of resonances characterized by two quantum numbers, which leads to conductance oscillations and concentration of wave functions near stable and unstable periodic orbits. Experimental results agree very well with our theoretical predictions, indicating that tunneling has to be taken into account to understand the physics of transport in generic nanostructures.     

Tunneling exponents sensitive to impurity scattering in quantum wires

We show that the scaling exponent for tunneling into a quantum wire in the "Coulomb Tonks gas" regime of impenetrable, but otherwise free, electrons is affected by impurity scattering in the wire. The exponent for tunneling into such a wire thus depends on the conductance through the wire. This striking effect originates from a many-body scattering resonance reminiscent of the Kondo effect. The predicted anomalous scaling is stable against weak perturbations of the ideal Tonks gas limit at sufficiently high energies, similar to the phenomenology of a quantum critical point.     

Spin-dependent transport through quantum dot with inelastic interactions

Using the Keldysh nonequilibrium Green function method, we theoretically investigate the electron transport properties of a quantum dot coupled to two ferromagnetic electrodes, with inelastic electron-phonon interaction and spin flip scattering present in the quantum dot. It is found that the electron-phonon interaction reduces the current, induces new satellite polaronic peaks in the differential conductance spectrum, and at the same time leads to oscillatory tunneling magnetoresistance effect. Spin flip scattering suppresses the zero-bias conductance peak and splits it into two, with different behaviors for parallel and anti-parallel magnetic configuration of the two electrodes. Consequently, a negative tunneling magnetoresistance effect may occur in the resonant tunneling region, with increasing spin flip scattering rate.     

电子关联效应对平行双量子点系统磁输运性质的影响

从理论上研究了平行双量子点系统中的电子关联效应对该系统磁输运性质的影响. 基于广义主方程方法, 计算了通过此系统的电流、微分电导和隧穿磁阻. 计算结果表明: 电子自旋关联效应可以促发一个很大的隧穿磁阻, 而电子库仑关联效应不仅可以压制电子自旋关联效应, 还可以导致负隧穿磁阻和负微分电导的出现. 对相关的基本物理问题进行了讨论.     

Kondo-enhanced Andreev tunneling in InAs nanowire quantum dots

We report measurements of the nonlinear conductance of InAs nanowire quantum dots coupled to superconducting leads. We observe a clear alternation between odd and even occupation of the dot, with subgap peaks at |V(sd)| = Delta/e markedly stronger (weaker) than the quasiparticle tunneling peaks at |V(sd)| = 2Delta/e for odd (even) occupation. We attribute the enhanced Delta peak to an interplay between Kondo correlations and Andreev tunneling in dots with an odd number of spins, and we substantiate this interpretation by a poor man's scaling analysis.