Parallel-coupled Quantum Dot Research Articles

Fano and Dicke Effects in Parallel-Coupled Quantum Dots Embedded Between Two Leads

In this article, electronic transport through a double quantum dot attached in parallel to a metal lead and with the presence of a magnetic flux is studied with the help of the Green-function formalism and the equation of motion approach. A detail formula is obtained for the density of states and the conductance. The Fano and Dicke effects in the conductance spectra of the double quantum dots system DQD is found and controlled by tuning the coupling (Γ) of the two dots to right (left) Leads and by changing the external magnetic flux.

Superior thermoelectric properties through triangular triple quantum dots (TTQD) attached to one metallic and one superconducting lead.

We theoretically investigate the thermoelectric transport properties of triangular triple quantum dots (TTQD) with the central quantum dot coupled to one metallic and one superconducting lead. The system shows significantly superior thermoelectric performance over parallel coupled triple quantum dots and those coupled to two conventional metallic leads. The thermoelectric coefficients strongly depend on the ratio of superconducting gap to interdot coupling, as well as asymmetry and interference effects. The thermopower exhibits single-platform and double-platform structures for different ratios of superconducting gap to interdot coupling. The thermopower and figure of merit achieve quite remarkable values near the superconducting gap edges where the single-particle tunnelling occurs. For symmetric coupling, the maximal figure of merit might reach the order of 102 when the superconducting gap is about half that of the interdot coupling. Moreover, the figure of merit can be further greatly enhanced by appropriately matching the electrode coupling asymmetry and interdot coupling asymmetry.

Selective tuning of spin-orbital Kondo contributions in parallel-coupled quantum dots

We use co-tunneling spectroscopy to investigate spin-, orbital-, and spin-orbital Kondo transport in a strongly confined system of InAs double quantum dots (QDs) parallel-coupled to source and drain. In the one-electron transport regime, the higher symmetry spin-orbital Kondo effect manifests at orbital degeneracy and no external magnetic field. We then proceed to show that the individual Kondo contributions can be isolated and studied separately; either by orbital detuning in the case of spin-Kondo transport, or by spin splitting in the case of orbital Kondo transport. By varying the inter-dot tunnel coupling, we show that lifting of the spin degeneracy is key to confirming the presence of an orbital degeneracy, and to detecting a small orbital hybridization gap. Finally, in the two-electron regime, we show that the presence of a spin-triplet ground state results in spin-Kondo transport at zero magnetic field.

Andreev tunneling assisted suppression of Fano resonance in asymmetric parallel quantum dot system coupled to superconducting leads

Electronic transport through asymmetrically parallel coupled quantum dot system hybridized between conventional superconducting leads has been investigated theoretically in the Coulomb blockade regime by using Non-Equilibrium Green Function formalism. For resonant Cooper pair tunneling, the Andreev transmission probability and Josephson supercurrent (I−Vcharacteristics) have been presented for each configuration of coupled quantum dot system and for transition of coupled quantum dot system from series to symmetric parallel configuration. Superconducting order parameter regulates the Josephson Cooper pair tunneling in such a way that it initially enhances and then suppresses the resonant Cooper pair tunneling when system is tuned from series to symmetric parallel configuration. Furthermore, it has been found that the Fano effect, reported in asymmetric parallel configuration of CQD system, gets completely suppressed by Andreev tunneling for a particular value of superconducting order parameter and lead-dot coupling strength.

Low bias negative differential conductance and reversal of current in coupled quantum dots in different topological configurations

Electronic transport through asymmetric parallel coupled quantum dot system hybridized between normal leads has been investigated theoretically in the Coulomb blockade regime by using Non-Equilibrium Green Function formalism. A new decoupling scheme proposed by Rabani and his co-workers has been adopted to close the chain of higher order Green's functions appearing in the equations of motion. For resonant tunneling case; the calculations of current and differential conductance have been presented during transition of coupled quantum dot system from series to symmetric parallel configuration. It has been found that during this transition, increase in current and differential conductance of the system occurs. Furthermore, clear signatures of negative differential conductance and negative current appear in series case, both of which disappear when topology of system is tuned to asymmetric parallel configuration.

Parallel-Coupled Quantum Dots in InAs Nanowires.

We use crystal-phase tuning during epitaxial growth of InAs nanowires to create quantum dots with very strong confinement. A set of gate electrodes are used to reproducibly split the quantum dots into even smaller pairs for which we can control the populations down to the last electron. The double quantum dots, which are parallel-coupled to source and drain, show clear and stable odd-even level pairing due to spin degeneracy and the strong confinement. The combination of hard-wall barriers to source and drain, shallow interdot tunnel barriers, and very high single-particle excitation energies allow an order of magnitude tuning of the strength for the first intramolecular bond. We show examples for nanowires with different facet orientations, and suggest possible mechanisms behind the reproducible double-dot formation.

Spin-orbital and spin Kondo effects in parallel coupled quantum dots

Strong electron correlations and interference effects are discussed in parallel-coupled single-level and orbitally doubly degenerate quantum dots. The finite-U mean-field slave boson approach is used to study many-body effects. The analysis is carried out in a wide range of parameter space including both atomic-like and molecular-like Kondo regimes and taking into account various perturbations, like interdot tunneling, interdot interaction, mixing of the electrode channels and exchange interaction. We also discuss the influence of singularities of electronic structure and the impact of polarization of electrodes. Special attention is paid to potential spintronic applications of these systems showing how current polarization can be controlled by adjusting interference conditions and correlations by gate voltage. Simple proposals of double dot spin valve and bipolar electrically tunable spin filter are presented.

Phase controlled swapping effect in electron transport through asymmetric parallel coupled quantum dot system

We present a theoretical study of the role of asymmetry and magnetic flux on electronic transport through various configurations of coupled quantum dot system, by using Non-Equilibrium Green Function formalism. Transport properties (Transmission Probability, Current–Voltage Characteristics and Differential Conductance) of the different configurations of coupled quantum dot system have been studied by self-consistent approach, in the presence of on-dot Coulomb interaction. Fano effect, appearing in Transmission probability, has been explored during transition of the system from series to symmetric parallel configuration and in response to the variation in magnetic flux threading the system. The results show Fano peaks for asymmetric and symmetric parallel configurations. By adjusting the magnetic flux, swapping effect in Fano peaks appears due to the exchange of states, which sustains despite strong Coulomb blockade effect. The transmission probability spectrum shows mirror symmetry whenever the sum of two values of magnetic flux threading the system is 2π.

Spin accumulation in parallel-coupled quantum dots driven by a symmetric dipolar spin battery

By applying a symmetric dipolar spin battery in a parallel-coupled quantum-dot (QD) structure, the spin accumulation in the QDs is investigated. We find that the spin accumulation can be achieved via electrically adjusting the left-right antisymmetry of the QD-lead couplings or introducing different magnetic fluxes through the sub-rings of this system. And, the spin accumulation properties are closely dependent on the number of QDs in this structure. Compared with the electrical method, the magnetic method is more efficient to manipulate the spin accumulation. When the intradot Coulomb interaction is considered, the electrically induced spin accumulation is somewhat suppressed, but in the magnetic method the spin accumulation is efficiently enhanced. We believe that the results can be observed in the experiment of spintronics, which provides an alternative scheme for spin manipulation.

Single-electron switching effect in graphene parallel-coupled double quantum dots

We have fabricated parallel-coupled quantum dots on single-layer graphene. The tunnel coupling between the quantum dots can be tuned by a graphene in-plane gate. Owing to the tunnel coupling, the Coulomb blockade oscillation peaks exhibit periodic shifts as the number of electron in the non-conducting side-coupled QD is changed. The result suggests the observation of the single electron switching effect, which is a prerequisite for a single photon detection scheme using parallel-coupled quantum dots.

Fano-effect and negative differential conductance in asymmetric parallel coupled quantum dots

Transport through coupled quantum dot system has been studied, using non-equilibrium Green function formalism. Inter-dot tunneling with on-dot Coulomb repulsion is included while inter-dot Coulomb interaction is neglected. Behavior of system in transition from series configuration to symmetric parallel and in T-shaped coupled quantum dot system has been explored. Transmission coefficient as a function of Fermi energy shows Fano effect during transition. The system shows negative differential conductance and reverse current at low bias in series. It has been shown that the existence of negative differential conductance during transition from series to asymmetric parallel configuration is regulated by a competition between inter-dot tunneling and dot-lead coupling. Effect of inter-dot tunneling on negative differential conductance and on transport properties of T-shaped double dot has been studied.

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.

Nonequilibrium spin-polarized transport through two parallel-coupled quantum dots

Spin-polarized transport through an Aharonov–Bohm ring containing two quantum dots (QDs) in each of its arms is studied by using the nonequilibrium Green’s function technique. We take both the Rashba spin-orbit interaction that exists in one of the QDs, and an inhomogeneous magnetic flux penetrating through the ring, into consideration. It is found that a 100% spin-polarized current can be driven out of the QDs ring, and both the spin directions and the magnitude of the outgoing current can be controlled. The origin of the pure spin-up or spin-down current is interpreted in terms of the spin accumulation in the QDs. This device is realizable by presently available technologies and can be used as a spin filter.

Magnetic-Flux-Controlled Giant Fano Factor for Coherent Tunneling Through a Parallel Double-Quantum-Dot

We report our studies of zero-frequency shot noise in tunneling through a parallel-coupled quantum dot interferometer by employing number-resolved quantum rate equations. We show that the combination of quantum interference effect between two pathways and strong Coulomb repulsion could result in a giant Fano factor, which is controllable by tuning the enclosed magnetic flux.

Spin accumulation in coupled quantum dots with ferromagnetic and superconducting electrodes

We theoretically investigate the spin accumulation in two parallel coupled quantum dots (QDs) with ferromagnetic and superconducting electrodes. Due to the ferromagnetic lead, the spin accumulation appears on the resonance of Andreev reflection. The spin accumulation in each of the two QDs can be controlled by the gate voltage. The sign of the spin accumulation is also controllable by tuning the bias. Furthermore, tuning the magnetic flux can exchange the amplitude of the spin accumulation in the two QDs. The Aharonov–Bohm oscillation effects also provides a way to control the spin accumulation of each QD.

Thermopower and thermal conductance through parallel coupled quantum dots

We study the thermoelectric transport properties through two parallel coupled, gate-defined quantum dots (QDs), in the framework of the X-boson treatment for the impurity Anderson model. We compute the thermopower S, the thermal conductance κ, the electrical conductance G, and the product of the thermoelectric figure of merit and the temperature ZT, as function of the dot energy. We concentrate the calculations on ZT, that is, a measure of the usefulness of materials or devices as thermopower generators or cooling systems. If the coupling between the QDs is weak, ZT is greater than 1 when T≃Δ (Δ is the mixing width between the QD and the leads) but when the system is coupled, the second dot can tune the temperature region where ZT>1. This result increases the possibilities of practical application of the system in mesoscopic cooling process.

First-order coherent resonant tunneling through an interacting coupled-quantum-dot interferometer: Generic quantum rate equations and current noise

We carry out a detailed analysis of coherent resonant tunneling through two coupled quantum dots (CQDs) in a parallel arrangement in the weak-tunneling limit. We establish a set of quantum rate equations in terms of the eigenstate representation by means of a generic quantum Langevin equation approach, which is valid for arbitrary bias voltage, temperature, and interdot hopping strength. Based on linear-response theory, we further derive the current and frequency-independent shot noise formulas. Our results reveal that a previously used formula for evaluating Schottky-type noise of a ``classical'' single-electron transistor is a direct result of linear-response theory, and it remains applicable for small quantum devices with internal coupling. Our numerical calculations show some interesting transport features (i) for a series CQD: the appearance of a negative differential conductance due to the bias-voltage-induced shifting of bare levels or a finite interdot Coulomb repulsion, and (ii) for a parallel CQD in strong interdot Coulomb repulsion regime: finite-bias-induced Aharonov-Bohm oscillations of current, and magnetic-flux-controllable negative differential conductance and a huge Fano factor.

Controlling spin-dependent transport via inhomogeneous magnetic flux in double-dot Aharonov–Bohm interferometer

We study the spin-dependent electron transport through parallel coupled quantum dots (QDs) embedded in an Aharonov–Bohm (AB) interferometer connected asymmetrically to leads. Both the Rashba spin–orbit interaction (RSOI) inside one of the QDs, which acquires a spin-dependent phase factor in the tunnel-coupling strengths when the electrons flow through this arm of the AB ring, and an inhomogeneous magnetic flux penetrating the structure are taken into account. Due to the existence of the RSOI induced phase factor, magnetic flux and the interdot coupling, a spin-dependent Fano effect will arise. We pay special attention on the properties of the local density of states and the conductance when the electron phase factor is close to integer multiplies of a quantum of flux. It is shown that the roles and lifetimes of the bonding and antibonding states of the two spin components are very sensitive to the phase factor and can be well controlled accordingly. This manipulation of the spin degree of freedom relies on the existence of RSOI but can be fulfilled even when its strength is very weak. The proposed structure can be easily realized with present technology and might be of practical applications in spintronics devices and quantum computing.

Linear conductance through parallel coupled quantum dots

We study the electronic transport through two parallel coupled quantum dots (QDs), employing the X-boson treatment for the single impurity Anderson model. We compute the linear conductance (LC) and transmission coefficient for different regimes of the system, as function of the QDs energy; our results show a suppression of the linear conductance at low temperatures; when the coupling between the QDs is significant, a drop in the transmission coefficient is evident, at the energy value of the side-coupled QD. We also obtain the temperature dependence of the LC, for different hybridizations between the QDs and the energy of one of them. Our results are consistent with those obtained by other theoretical treatments and recovers what is expected when the coupling between the QDs is weak.

Evolution of Coulomb blockade spectra 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 gas of a GaAs/AlGaAs heterostructure. We find that the Coulomb blockade spectra of such devices exhibit two distinct sets of peaks, each of which behaves differently with varying interdot tunnel conductance and with temperature. The results conform to recent theories regarding the role of interdot quantum charge fluctuations, and provide evidence for the possible role of inelastic cotunneling between dots at finite interdot conductances.