Excited state spectroscopy in carbon nanotube double quantum dots

We report on low-temperature measurements in a fully tunable carbon nanotube double quantum dot. A new fabrication technique has been used for the top-gates in order to avoid covering the whole nanotube with an oxide layer as in previous experiments. The top-gates allow us to form single dots and control the coupling between them, and we observe 4-fold shell filling. We perform inelastic transport spectroscopy via the excited states in the double quantum dot, a necessary step toward the implementation of new microwave-based experiments.     

Electronic excited states in bilayer graphene double quantum dots

We report tunneling spectroscopy experiments on a bilayer graphene double quantum dot device that can be tuned by all-graphene lateral gates. The diameter of the two quantum dots are around 50 nm and the constrictions acting as tunneling barriers are 30 nm in width. The double quantum dot features additional energies on the order of 20 meV. Charge stability diagrams allow us to study the tunable interdot coupling energy as well as the spectrum of the electronic excited states on a number of individual triple points over a large energy range. The obtained constant level spacing of 1.75 meV over a wide energy range is in good agreement with the expected single-particle energy spacing in bilayer graphene quantum dots. Finally, we investigate the evolution of the electronic excited states in a parallel magnetic field.     

A triple quantum dot in a single-wall carbon nanotube

A top-gated single-wall carbon nanotube is used to define three coupled quantum dots in series between two electrodes. The additional electron number on each quantum dot is controlled by top-gate voltages allowing for current measurements of single, double, and triple quantum dot stability diagrams. Simulations using a capacitor model including tunnel coupling between neighboring dots captures the observed behavior with good agreement. Furthermore, anticrossings between indirectly coupled levels and higher order cotunneling are discussed.     

Two carriers in vertically coupled quantum dots: magnetic field effect

We study a two-charge-carrier (two holes or two electrons) quantum dot molecule in a magnetic field. In comparison with the electron states in the double quantum dot, the switching between the hole states is achieved by changing both the inter-dot distance and magnetic field. We use harmonic potentials to model the confining of two charge carriers and calculate the energy difference delta E between the two lowest energy states with the Hund-Mulliken technique, including the Coulomb interaction. Introducing the Zeeman effect, we note a ground-state crossing, which can be observed as a pronounced jump in the magnetization at a perpendicular magnetic field of a few Tesla. The ground states of the molecule provide a possible realization for a quantum gate.     

Spintronic Transport and Kondo Effect in Quantum Dots

We investigate the spin-dependent transport properties of quantum-dot based structures where Kondo correlations dominate the electronic dynamics. The coupling to ferromagnetic leads with parallel magnetizations is known to give rise to nontrivial effects in the local density of states of a single quantum dot. We show that this influence strongly depends on whether charge fluctuations are present or absent in the dot. This result is confirmed with numerical renormalization group calculations and perturbation theory in the on-site interaction. In the Fermi-liquid fixed point, we determine the correlations of the electric current at zero temperature (shot noise) and demonstrate that the Fano factor is suppressed below the Poissonian limit for the symmetric point of the Anderson Hamiltonian even for nonzero lead magnetizations. We discuss possible avenues of future research in this field: coupling to the low energy excitations of the ferromagnets (magnons), extension to double quantum dot systems with interdot antiferromagnetic interaction and effect of spin-polarized currents on higher symmetry Kondo states such as SU(4).     

Spintronic Transport and Kondo Effect in Quantum Dots

We investigate the spin-dependent transport properties of quantum-dot based structures where Kondo correlations dominate the electronic dynamics. The coupling to ferromagnetic leads with parallel magnetizations is known to give rise to nontrivial effects in the local density of states of a single quantum dot. We show that this influence strongly depends on whether charge fluctuations are present or absent in the dot. This result is confirmed with numerical renormalization group calculations and perturbation theory in the on-site interaction. In the Fermi-liquid fixed point, we determine the correlations of the electric current at zero temperature (shot noise) and demonstrate that the Fano factor is suppressed below the Poissonian limit for the symmetric point of the Anderson Hamiltonian even for nonzero lead magnetizations. We discuss possible avenues of future research in this field: coupling to the low energy excitations of the ferromagnets (magnons), extension to double quantum dot systems with interdot antiferromagnetic interaction and effect of spin-polarized currents on higher symmetry Kondo states such as SU(4).     

A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor

One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.     

Towards registered single quantum dot photonic devices

We have registered the position and wavelength of a single InGaAs quantum dot using an innovative cryogenic laser lithography technique. This approach provides accurate marking of the location of self-organized dots and is particularly important for realizing any solid-state cavity quantum electrodynamics scheme where the overlap of the spectral and spatial characteristics of an emitter and a cavity is essential. We demonstrate progress in two key areas towards efficient single quantum dot photonic device implementation. Firstly, we show the registration and reacquisition of a single quantum dot with 50 and 150?nm accuracy, respectively. Secondly, we present data on the successful fabrication of a photonic crystal L3 cavity following the registration process.     

Selecting single quantum dots from a bundle of single-wall carbon nanotubes using the large current flow process

We demonstrate selecting a Coulomb peak which originates from a single quantum dot, from a bundle of many single-wall carbon nanotubes. The method uses the previously reported current flowing process, by which the number of nanotubes can be reduced for the transport. By adjusting the gate voltage in an appropriate range, the single peak belonging to the single quantum dot has been selected. The effect of the high frequency application on the peak has been investigated, and it is shown that the basic response can be explained by the adiabatic response of the single dot to the high frequency signal.     

Voltage-Controlled Electron-Hole Interaction in a Single Quantum Dot

The ground state of neutral and negatively charged excitons confined to a single self-assembled InGaAs quantum dot is probed in a direct absorption experiment by high resolution laser spectroscopy. We show how the anisotropic electron-hole exchange interaction depends on the exciton charge and demonstrate how the interaction can be switched on and off with a small dc voltage. Furthermore, we report polarization sensitive analysis of the excitonic interband transition in a single quantum dot as a function of charge with and without magnetic field.     

Voltage-Controlled Electron-Hole Interaction in a Single Quantum Dot

The ground state of neutral and negatively charged excitons confined to a single self-assembled InGaAs quantum dot is probed in a direct absorption experiment by high resolution laser spectroscopy. We show how the anisotropic electron-hole exchange interaction depends on the exciton charge and demonstrate how the interaction can be switched on and off with a small dc voltage. Furthermore, we report polarization sensitive analysis of the excitonic interband transition in a single quantum dot as a function of charge with and without magnetic field.     

Control of electron localization in a coupled quantum dot structure

Abstract

We systematically investigate the dynamical behaviour of an electron in a double quantum dot system under the influence of an external AC field. It is assumed that the quantum dot confined structure exhibits a non-negligible Coulomb charging energy, inversely proportional to its small capacitance. The dynamic evolution of the system is obtained by numerically solving the coupled, nonlinear, equations derived from the time-dependent Schrödinger equation. We find cases where the electron is localized in the initially placed dot when both effects of the Coulomb charging energy and the external field are present, even though if either effect is absent the electron will tunnel between dots. We also show that we can pre-select the shape and rise time of a semi-infinite, pulsed, AC field in order to transfer an electron from the initially placed dot to the other dot and localize it there.     

喷墨打印输出设备墨量控制的研究

分别对混合通道、单通道各色设计不同网点面积率的色块,打印输出测量其密度值,然后使用多项式回归法建立了网点面积率与密度之间的函数关系,通过分析数据,找出了最大密度对应的网点面积率,并以其作为墨量的最大值,最后设计实验验证了这一最值的准确性。为研究喷墨打印输出设备的墨量控制提供了一种简便可行的方法,对辅助研究打印机的线性化及研发具有自主知识产权的线性化软件,具有重要的指导意义。     

Few electron double quantum dots in InAs/InP nanowire heterostructures

We report on fabrication of double quantum dots in catalytically grown InAs/InP nanowire heterostructures. In the few-electron regime, starting with both dots empty, our low-temperature transport measurements reveal a clear shell structure for sequential charging of the larger of the two dots with up to 12 electrons. The resonant current through the double dot is found to depend on the orbital coupling between states of different radial symmetry. The charging energies are well described by a capacitance model if next-neighbor capacitances are taken into account.     

Manipulating coupling between a single semiconductor quantum dot and single gold nanoparticle

Using atomic force microscopy nanomanipulation, we position a single Au nanoparticle near a CdSe/ZnS quantum dot to construct a hybrid nanostructure with variable geometry. The coupling between the two particles is varied in a systematic and reversible manner. The photoluminescence lifetime and blinking of the same quantum dot are measured before and after assembly of the structure. In some hybrid structures, the total lifetime is reduced from about 30 ns to well below 1 ns. This dramatic change in lifetime is accompanied by the disappearance of blinking as the nonradiative energy transfer from the CdSe/ZnS quantum dot to the Au nanoparticle becomes the dominant decay channel. Both total lifetime and photoluminescence intensity changes are well described by simple analytical calculations.     

Blinking suppression and intensity recurrences in single CdSe-oligo(phenylene vinylene) nanostructures: experiment and kinetic model

We report time-resolved single molecule fluorescence imaging of individual CdSe quantum dots that are functionalized with oligomeric conjugated organic ligands. The fluorescence intensity trajectories from these composite nanostructures display both a strong degree of blinking suppression and intensity fluctuations with characteristic recurrence times on the order of 10-60?s. In addition, fluorescence decay rate measurements of individual hybrid nanostructures indicate significantly modified non-radiative quantum dot decay rates relative to conventional ZnS-capped CdSe quantum dots. We show that a modified diffusive reaction coordinate model with slow fluctuations in quantum dot electron energies (1S(e), 1P(e)) can reproduce the experimentally observed behaviour.     

Nano patterning of cone dots and nano constrictions of negative e-beam resist for single electron transistor fabrication

We present an optimization of nano dot of negative tone e-beam resist which is a very important step in single electron transistor fabrication process. The optimum design of dot and nano constriction plays a significant role in determining optimum etching resolution and single electron transistor performance. In this research, we have optimized nano dot and nano constriction dimensions of resist by controlling some parameters, such as e-beam dose, spin speed, pre-bake time and image development time. However, a nano constriction design variety of 120–200 nm in width was carried out to reach the optimum design. In this paper, the fabrication process of cone nano dots using e-beam lithography with considering proximity effect is reported. As nano constriction design decreased, cone nano dot changed to pyramid nano dot and the compression effect on the dot also significantly increased as well.     

Magnetic field control of exchange and noise immunity in double quantum dots

We employ density functional calculated eigenstates as a basis for exact diagonalization studies of semiconductor double quantum dots, with two electrons, through the transition from the symmetric bias regime to the regime where both electrons occupy the same dot. We calculate the singlet-triplet splitting J(epsilon) as a function of bias detuning epsilon and explain its functional shape with a simple, double anticrossing model. A voltage noise suppression "sweet spot," where d J(epsilon)/d(epsilon) = 0 with nonzero J(epsilon), is predicted and shown to be tunable with a magnetic field B.     

Large array of single, site-controlled InAs quantum dots fabricated by UV-nanoimprint lithography and molecular beam epitaxy

We present the growth of single, site-controlled InAs quantum dots on GaAs templates using UV-nanoimprint lithography and molecular beam epitaxy. A large quantum dot array with a period of 1.5 μm was achieved. Single quantum dots were studied by steady-state and time-resolved micro-photoluminescence experiments. We obtained single exciton emission with a linewidth of 45 μeV. In time-resolved experiments, we observed decay times of about 670 ps. Our results underline the potential of nanoimprint lithography and molecular beam epitaxy to create large-scale, single quantum dot arrays.     

Spin Filter Effect in a Parallel Double Quantum Dot

We present a theoretical study of the spectral and the spin-dependent transport properties of a few electron semiconductor parallel double quantum dot (DQD) in the presence of local induced Zeeman splittings at the quantum dots. Working in an extended Hubbard model and treating the coupled QD as a single coherent system, the linear response spin-dependent conductance is calculated at low temperatures. We analyze the conditions such that the device would operate as a bipolar spin filter by only varying the incident electron Fermi energy from non-magnetic leads.