One-dimensional Quantum Dot Research Articles

Correlation effects on electron tunneling through doubly occupied quantum dots: A study beyond phenomenological models

Electron transport through doubly occupied quantum dots is studied theoretically. Beyond the semi-classic framework of phenomenological models, a quantum mechanical solution for the cotunneling and sequential tunneling of electrons through a one-dimensional multi-level quantum dot is obtained. Correlation effects are shown to play an important role in inelastic sequential tunneling through the doubly occupied system, which accommodates four single-particle levels. It is revealed that the cotunneling conductance exhibits strong dependence on the spin configuration of the electrons confined inside the dot. Especially for the triplet configuration, the conductance shows an obvious deviation from the well-known quadratic dependence on the applied bias voltage.

Electron–electron interaction effect on the singlet–triplet transitions in one-dimensionalquantum dots

We have studied singlet–triplet transitions (STTs) in a system of two interacting electronsin a one-dimensional quantum dot in the presence of the spin–orbit interaction (SOI) and amagnetic field. The STTs are shown to occur owing only to a combined action of the SOIand magnetic field. The energy gap at the STT point depends on the relative direction ofthe external magnetic field and an effective magnetic field of the SOI. An interesting effectis produced by the electron–electron interaction that can be controlled by nearby gates.The increase of the interaction strength results in the significant reduction of the criticalmagnetic field, under which the STT occurs, and lowering of the energy gap.

Electron cotunneling through doubly occupied quantum dots: effect of spin configuration

A microscopic theory is presented for electron cotunneling through doubly occupied quantum dots in the Coulomb blockade regime. Beyond the semiclassic framework of phenomenological models, a fully quantum mechanical solution for cotunneling of electrons through a one-dimensional quantum dot is obtained using a quantum transmitting boundary method without any fitting parameters. It is revealed that the cotunneling conductance exhibits strong dependence on the spin configuration of the electrons confined inside the dot. Especially for the triplet configuration, the conductance shows an obvious deviation from the well-known quadratic dependence on the applied bias voltage. Furthermore, it is found that the cotunneling conductance reveals more sensitive dependence on the barrier width than the height.

Pumped double quantum dot with spin-orbit coupling.

We study driven by an external electric field quantum orbital and spin dynamics of electron in a one-dimensional double quantum dot with spin-orbit coupling. Two types of external perturbation are considered: a periodic field at the Zeeman frequency and a single half-period pulse. Spin-orbit coupling leads to a nontrivial evolution in the spin and orbital channels and to a strongly spin- dependent probability density distribution. Both the interdot tunneling and the driven motion contribute into the spin evolution. These results can be important for the design of the spin manipulation schemes in semiconductor nanostructures.PACS numbers: 73.63.Kv,72.25.Dc,72.25.Pn

Approximation of the entanglement in quantum dot chains using Hubbard models

We investigate the Hubbard model, and the extended Hubbard model with and without correlated hopping, as an approximation to the average single-site (or local) entanglement of two-electron one-dimensional quantum dot chains with long range interactions. We focus on the effect of the chain length on the approximation accuracy. We also consider the particle-particle spatial entanglement of the three Hubbard models using the technique of Ref. [3] and compare the accuracy of this approximation to the numerically exact results of the nanostructure system.

The nonlinear optical rectification of a confined exciton in a quantum dot

An exciton in a disc-like quantum dot (QD) with the parabolic confinement, under applied electric field, is studied within the framework of the effective-mass approximation. The nonlinear optical rectification between the ground and the first-excited states has been examined through the computed energies and wave functions in details for the excitons. The results show that the optical rectification susceptibility obtained in a disc-like QD reach the magnitude of 10−2m/V, which is 3–4 orders of magnitude higher than in one-dimensional QDs. It is found that the second-order nonlinear optical properties of exciton states in a QD are strongly affected by the confinement strength and the electric field.

Anisotropic Confinement, Electronic Coupling and Strain Induced Effects Detected by Valence-Band Anisotropy in Self-Assembled Quantum Dots.

A method to determine the effects of the geometry and lateral ordering on the electronic properties of an array of one-dimensional self-assembled quantum dots is discussed. A model that takes into account the valence-band anisotropic effective masses and strain effects must be used to describe the behavior of the photoluminescence emission, proposed as a clean tool for the characterization of dot anisotropy and/or inter-dot coupling. Under special growth conditions, such as substrate temperature and Arsenic background, 1D chains of In0.4Ga0.6 As quantum dots were grown by molecular beam epitaxy. Grazing-incidence X-ray diffraction measurements directly evidence the strong strain anisotropy due to the formation of quantum dot chains, probed by polarization-resolved low-temperature photoluminescence. The results are in fair good agreement with the proposed model.

Excitonic effects on the nonlinear intersubband optical properties of a semi-parabolic one-dimensional quantum dot

In this work the excitonic effects on the linear and nonlinear optical properties of semi-parabolic one-dimensional quantum dots are investigated in all confinement regimes. The Energy eigenvalues and eigenfunctions of these systems are calculated numerically, and optical properties are obtained using the compact density matrix approach. Our results show that the intersubband energy intervals and the dipole matrix elements in the presence of excitonic effects are larger than those without these effects. The calculations reveal that the resonant peak values of the optical properties are greater and occur at higher values of incident photon energy when excitonic effects are considered.

Microscopic theory of electron cotunneling through quantum dots

A microscopic theory is presented for electron cotunneling through quantum dots in the Coulomb blockade regime. Beyond the semiclassic framework of phenomenological models, a fully quantum mechanical solution for cotunneling of electrons through a one-dimensional quantum dot is obtained by using a quantum transmitting boundary method without any fitting parameters. Elastic and inelastic cotunneling conductance is calculated as a function of the energy of the incident electron. It is revealed that the cotunneling current contains a significant term proportional to V2 (V being the bias voltage) in additional to the well-known V3 term. The result also indicates that the cotunneling conductance exhibits little dependence on the spin configuration of the incident and confined electrons.

Spin phases and currents in ring shaped one-dimensional quantum dot arrays

The electronic states of an array of Nd quantum dots (QDs) patterned in an isolated quantum ring are calculated. An external magnetic field B perpendicular to the ring’s plane is considered. The single particle spectra show the presence of an insulating pseudoband which can be totally filled by 2Nd electrons. A large gap is present between the insulating pseudoband and the metallic one. The damping of Aharonov–Bohm oscillations reflects the localization of the quantum states and can be justified in terms of metallic-insulating transition induced by the barriers. The energy spectra of low-lying states of Ne≤2Nd interacting electrons in the QDs array as a function of B are obtained. Phase diagrams are presented for the insulating band from half to integer filling. The diagrams indicate a rich variety of ground states revealing transitions in which the total spin changes.

Imaging and manipulating electrons in a one-dimensional quantum dot with Coulomb blockade microscopy

Motivated by recent experiments by the Westervelt group, which used a mobile tip to probe the electronic state of a segmented nanowire, we calculate shifts in Coulomb blockade peak positions, as a function of tip location, which we term ``Coulomb blockade microscopy.'' We show that if the tip can be brought sufficiently close to the nanowire, one can distinguish a high-density electronic liquid state from a Wigner-crystal state by microscopy with a weak-tip potential. In the opposite limit of a strongly negative tip potential, the potential depletes the electronic density under it and divides the quantum wire into two partitions. There the tip can push individual electrons from one partition to the other and the Coulomb blockade micrograph can clearly track such transitions. We show that this phenomenon can be used to qualitatively estimate the relative importance of the electron interaction compared to one-particle potential and kinetic energies. Finally, we propose that a weak-tip Coulomb blockade micrograph focusing on the transition between electron number $N=0$ and $N=1$ states may be used to experimentally map the one-particle potential landscape produced by impurities and inhomogeneities.

Strong electron-photon coupling in a one-dimensional quantum dot chain: Rabi waves and Rabi wave packets

We predict and theoretically investigate the new coherent effect of nonlinear quantum optics -- spatial propagation of Rabi oscillations (Rabi waves) in one-dimensional quantum dot (QD) chain. QD-chain is modeled by the set of two-level quantum systems with tunnel coupling between neighboring QDs. The space propagation of Rabi waves in the form of traveling waves and wave packets is considered. It is shown, that traveling Rabi waves are quantum states of QD-chain dressed by radiation. The dispersion characteristics of traveling Rabi waves are investigated and their dependence on average number of photons in wave is demonstrated. The propagation of Rabi wave packets is accompanied by the transfer of the inversion and quantum correlations along the QD-chain and by the transformation of quantum light statistics. The conditions of experimental observability are analyzed. The effect can find practical use in quantum computing and quantum informatics.

Quantum computation with graphene nanoribbon

We propose a scheme to implement quantum computation in graphene nanoribbon (GNR). It is shown that an electron or hole can be naturally localized in each zigzag region for a GNR with a sequence of Z-shaped structures, without using confined gates. A one-dimensional graphene quantum dot chain is formed in such a GNR, where an electron or hole spin can be used as a qubit. The coupling interaction between neighboring qubits is found to be of the always-on Heisenberg type. By exploiting the bang–bang control strategy and the decoherence-free subspaces encoding method, universal quantum gates are argued to be realizable with the present techniques.

Hole transport in one-dimensional aligned GeSi quantum dots at low temperatures

One-dimensional (1D) aligned GeSi quantum dots (QDs) along 1D trenches have been prepared on the Si(0 0 1) substrates. The resistance values along the transverse and the longitudinal direction are significantly different at low temperatures. Two conductive layers are suggested to explain the difference. One is the boron doped layer which is at 10 nm underneath the QDs layer, the other is the QDs layer which is effective only for the longitudinal transport. Fitting the resistance as a function of temperature indicates that the conduction mechanism in both layers is two-dimensional Mott variable-range hopping. The magnetoresistance (MR) for the longitudinal transport at low temperatures shows a positive MR at low magnetic fields and a large negative MR at high fields. This behaviour is tentatively explained provided that the two conductive layers have different MR mechanisms.

Wave propagation of Rabi oscillations in one-dimensional quantum dot chain

Interaction of traveling wave of classic light with 1D-chain of coupled quantum dots (QDs) in the strong coupling regime is theoretically considered. The effect of space propagation of Rabi oscillations in the form of traveling waves and wave packets is predicted introducing thus a new family of quasi-particles. Physical interpretation of the effect is given and principles of its experimental observation are discussed.

Single-Gated Single-Electron Transfer in Nonuniform Arrays of Quantum Dots

Single-electron transfer in single-gated one-dimensional quantum dot arrays is investigated statistically from the viewpoint of robustness against parameter fluctuations. We have found numerically that inhomogeneous quantum dot arrays formed in doped nanowires exhibit single-electron transfer over a wide range of parameters. This confirms our frequent experimental observation of single-electron transfer in doped-nanowire field-effect transistors. The most important result in this work is that three-dot arrays with small-large-small dot size distribution always allow single-electron transfer even if dot sizes fluctuate. This structure is, we believe, most promising for fabricating devices with high immunity against structural fluctuations on the nanometer scale. On the basis of these findings, we propose methods to fabricate high-yield single-electron transfer devices.

Simulation of the electrical characteristics of a one-dimensional quantum dot array

A quantum dot array, consisting of Au dots, was prepared by the linear aggregation technique and assembled between two electrodes. We study the voltage–current characteristic of the quantum dot array, using a Non-Equilibrium Green’s Function (NEGF) model based on the Keldysh formalism. The results of our simulation and experimental data are compared. The simulated voltage–current curve is a reasonable fit with the measured data. It shows that the present model can be used to study quantum dot arrays. Furthermore, our results indicate that the electrical characteristics of an Au dot array are directly related to the coupling parameters.

High-density one-dimensional well-aligned germanium quantum dots on a nanoridge array

The selective growth of high-density one-dimensional well-aligned Ge quantum dots (QDs) on the top of nanoridges patterned on Si substrate is reported. The period of ridge array is 150 nm, the width of each ridge is 80 nm, and the depth of the trench is 20 nm. The areal density of QDs is about 5.4×109 cm−2. Simulations of the chemical potential show that a proper distribution of the surface curvature may give rise to a suitable chemical potential minimum helping positioning the QDs. These ridges can also be used to control the shape and the uniformity of QDs.

Electrodeposition of photoactive 1D gallium selenide quantum dots

One-dimensional (1D) quantum dots of gallium selenide have been obtained by cathodic electrodeposition onto the tin doped indium oxide (ITO) glass substrates from aqueous acidic solutions at room temperature. Characterizations of the as-deposited films by energy dispersive X-ray (EDX) spectroscopy confirm a selenium rich chemistry, X-ray diffraction (XRD) shows that mixture of phases like GaSe/Ga 2Se 3, and optical spectroscopy shows a direct optical band gap of 2.85 eV with intermediate transition energy at 1.9 eV. From transmission electron microscopy (TEM), the films show the one-dimensional quantum dots chains in grains. Scanning electron microscopy (SEM) images indicate dimorphous placement of nanoparticles. The elementals surface analysis of the core-shell nanoparticles determined by X-ray photoelectron spectroscopy (XPS) supported the EDX results and confirmed the chemical nature of the material. The photoelectrochemical (PEC) studies of gallium selenide films were carried out and the nanocrystalline gallium selenide films were found to be photoactive in aqueous sodium thiosulphate solution.

Swap action in a solid-state controllable anisotropic Heisenberg model

Correct swap action can be realized via the control of the anisotropic Heisenberg interaction in solid-state quantum systems. The conditions of performing a swap are derived by the dynamics of arbitrary bipartite pure state. It is found that swap errors can be eliminated in the presence of symmetric anisotropy. In realistic quantum computers with unavoidable fluctuations, the gate fidelity of swap action is estimated. The scheme of quantum computation via the anisotropic Heisenberg interaction is implemented in a one-dimensional quantum dots. The slanting and static magnetic field can be used to adjust the anisotropy.