Parallel-coupled Double Quantum Dot Research Articles

Electronic correlations in parallel-coupled double quantum dot system: An exact analytical approach

Exact eigenstates of the parallel coupled double quantum dots attached to the non-interacting leads taken in the zero-bandwidth limit (effectively quadruple quantum dots) are analytically obtained in each particle and spin subspace. The ground state of the half-filled system is determined within a four-dimensional subspace of the twenty-dimensional Hilbert space. The effects of tunable parameters, such as quantum dot energy levels, interdot tunneling, ondot and interdot Coulomb interactions, on spin-spin correlation and dot occupancies are analyzed. In the parameter space defined by interdot tunneling and ondot Coulomb interaction, the tunnel-coupled dots exhibit both ferromagnetic and antiferromagnetic correlations, suggesting suitability for singlet-triplet qubits in quantum computing. A critical dependency of interdot tunneling on ondot Coulomb interaction leads to a transition from ferromagnetic to antiferromagnetic correlation as interdot tunneling increases. These correlations persist even without interdot tunneling via indirect exchange through the leads, with the interdot Coulomb interaction significantly influencing this dependency.

Photon-assisted destructive interference through a parallel-coupled double quantum dots

In this paper, we establish a model to realize the photon-assisted destructive interference phenomena, the model consisting of parallel-coupled double quantum dots and Luttinger liquid leads. By applying the canonical transformation and the nonequilibrium Green’s function technique, the general time-dependent current formula is developed. The joint effects of the intralead Coulomb interaction and an external ac field on the quantum interference are investigated. For weak intralead interactions, besides the usual photon-assisted tunneling resonance, there are two types of photon-assisted tunneling destructive interference: one is related to interdot coupling and the other to intralead interactions. For moderately strong intralead interaction, the photon-assisted sidebands are suppressed and the differential conductance scales as a power law in bias voltage. In particular, we show the multiphoton absorption and emission up to four photons in an electron tunneling process. We believe that the photo-assisted destructive interference results obtained in this paper are instructive for future experiments.

Thermoelectric enhancement of a parallel double quantum dot system connecting to Luttinger liquid leads

In this work, we study the thermoelectric properties of the parallel-coupled double quantum dot connected to Luttinger liquid (LL) leads. By applying nonequilibrium Green's function methods, the expressions of the electron current, heat currents, and transport coefficients are derived. We mainly focus on the effect of intralead Coulomb interaction and the spatial asymmetry of the system. Various causes for the enhancement of the thermoelectric efficiency are investigated. We find that Coulomb interactions in the LL not only enhance quantum interference but also considerably enhance thermoelectric effects. For moderately strong intralead interaction, the figure of merit can be considerably high and reach over 60 in the case of weakly coupled to LL leads and detuning of the energy levels, while in the case of strongly coupled to LL leads and the two aligned levels, the figure of merit approaches 8. The quantum interference induced by the Coulomb interaction yields a very high thermoelectric efficiency when the intralead interaction is very strong. These results may provide a design rule for the development of high-efficiency thermoelectric devices and facilitate optimization of device performance.

Inverse current induced thermoelectric conversion in a parallel-coupled double quantum dot system

We investigate the thermoelectric transport and conversion of a parallel-coupled double quantum dot system, which consisting of two capacitively coupled quantum dots in the Coulomb-blockade regime. We found that the system exhibits an unconventional thermoelectric conversion process induced by the inverse current effect, which is attributed to the increased Coulombic interaction between quantum dots, resulting in strong asymmetry in the system. We study the transport properties of steady-state particle current and heat current, and analyze the influence of Coulomb interaction on the thermodynamic characteristics of unconventional thermoelectric heat engines and refrigerators.

Magnetic Field Dependent Kondo Transport through Double Quantum Dots System

AbstractThe Kondo transport associated with the magnetic field in parallel‐coupled double quantum dots (PDQDs) is theoretically investigated using the hierarchical‐equation‐of‐motion approach (HEOM). In this system, the interdot tunneling induces an effective antiferromagnetic interaction; thus, its ground state is an orbital singlet associated with approximately zero differential conductance. When an appropriate large magnetic field is applied, the two spins of the two dots have a possibility with a parallel arrangement, and thus the singlet and one of the triplets are degenerated. At the degenerate point, a distinct zero‐energy resonance peak appears, associated with significant differential conductance. The Kondo scale behavior at the degenerate point and the total and spin resolved spectral functions around the degenerate point are examined. The results confirm the spin‐1/2 Kondo resonance near the singlet‐triplet transition point. The differential conductance phase diagram, as well as thermoelectric transport, is also quantitatively presented.

Antiresonance and negative differential conductance in parallel-coupled vibrating molecular dots coupled to Luttinger liquid leads

We theoretically study the interplays of electron–phonon interaction, inter-dot coupling tunneling and intralead electron interaction on transport properties of parallel-coupled double quantum dots connected to Luttinger liquid leads. The nonequilibrium Green’s function method is used. Numerical results show several intriguing physic phenomena. In the weak intralead electron interaction, there is a zero-bias antiresonance dip followed by phonon-assisted satellite peaks for strong electron–phonon interaction and inter-dot tunneling. With decreasing inter-dot tunneling, there is an evolution from satellite peaks to dips, which signifies the phase transition from phonon-assisted single-channel to two-channel Kondo physics. When the dot-lead coupling is asymmetric, in the case of moderately strong intralead interaction, strong electron–phonon coupling and weak inter-dot tunneling, there may appear the pronounced negative differential conductance and Fano resonance, while there does not when the coupling is symmetric. These results provide a convenient way to probe destructive quantum interference and extract information of the electron–phonon interaction strength, which is crucial for designing molecular switches and thermoelectric devices. • We study the transport properties of parallel coupled double quantum dots connected to Luttinger liquid leads. • We have theoretically studied a series of problems such as electron phonon interaction . • The method used is nonequilibrium Green’s function. • The phenomena of resonance and peak of satellite are found. • Negative differential conductance and Fano resonance are found.

Coexistence of parallel and series current paths in parallel-coupled double quantum dots in nonlinear transport regime

We investigated the electron transport properties of parallel-coupled double quantum dot (DQD) devices under magnetic fields. When a low magnetic field was applied, electron tunneling through parallel-coupled DQDs was observed. Under a high magnetic field, we observed both electron tunneling through parallel- and series-coupled DQDs under nonlinear transport conditions. In addition, the Pauli spin blockade was observed, indicating tunneling through the series-coupled DQDs. We attribute these behaviors to the magnetic-field-induced changes in the tunnel-couplings that allow the coexistence of the current paths of the parallel and series configurations.

Thermoelectric transport through strongly correlated double quantum dots with Kondo resonance

We theoretically study the thermoelectric transport properties of a parallel-coupled double quantum dots (PDQDs) with Kondo effect, using the hierarchical–equation–of–motion approach. At weak interdot tunneling, the thermocurrent as a function of energy level of individual QD gives rise to a continuous transition from negative to positive value. However, the thermocurrent gives rise to a sign reversal from positive to negative value and shows a sawtooth line-shape structure at strong interdot tunneling. The above charge polarity reversal behavior of thermocurrent can be illustrated via the competition between the Kondo singlet state of individual QD at weak interdot tunneling and the spin singlet state forming between the two QDs at strong interdot tunneling. To understand the physical insight, we also examine the thermocurrent and corresponding spectral functions with their dependence on the interdot tunneling strength and temperature. Moreover, the observed transition behavior of thermocurrent will be subdued by high temperature.

Quantum Interference Effects in Quantum Dot Molecular With Majorana Bound States

Non-invasive detection of the Majorana bound state (MBSs), a kind of quasiparticle without charge and mass, is one of the core issues in current condensed matter physics. Here we study in theory the quantum interference effect in parallel-coupled double quantum dots which are connected either by Majorana bound states (MBSs) or regular fermions. We find that the zero-energy conductance develops a sharp peak when the dots are connected by the MBSs, whereas that in the case of the dots are coupled via regular fermions shows a valley. By varying the coupling strength between the dots and the electron reservoirs, the conductance in the two structures changes in different ways. By comparing the properties of the linear conductance in the two systems, the information of the MBSs formed at the two ends of a topological superconductor nanowire then can be inferred. We also find that the MBSs in the present structure also induces the Fano effect, and is favorable in quantum information processing.

Fano resonance and zero-bias anomaly in parallel double quantum dots coupled to Luttinger liquid leads

We study the nonequilibrium transport through a parallel-coupled double quantum dot coupled to Luttinger liquid leads using the nonequilibrium Green's function method. A current formula is derived, where the inter-dot tunneling, the intralead interaction, and asymmetric dot-lead coupling are taken into account. The interplay of these interactions on the differential conductance is investigated. The differential conductance of the double-dot system exhibits the coexistence of zero-bias dip and Fano resonance for asymmetric coupling. The zero-bias dip of differential conductance scales onto a single universal curve for different temperature with the same scaling exponent as a system of Kondo dot. As temperature increases, the zero-bias minimum is gradually lifted.

Tuning the Two-Electron Hybridization and Spin States in Parallel-Coupled InAs Quantum Dots.

We study spin transport in the one- and two-electron regimes of parallel-coupled double quantum dots (DQDs). The DQDs are formed in InAs nanowires by a combination of crystal-phase engineering and electrostatic gating, with an interdot tunnel coupling (t) tunable by one order of magnitude. Large single-particle energy separations (up to 10meV) and |g^{*}| factors (∼10) enable detailed studies of the B-field-induced transition from a singlet-to-triplet ground state as a function of t. In particular, we investigate how the magnitude of the spin-orbit-induced singlet-triplet anticrossing depends on t. For cases of strong coupling, we find values of 230 μeV for the anticrossing using excited-state spectroscopy. Experimental results are reproduced by calculations based on rate equations and a DQD model including a single orbital in each dot.

Fano Effect and Anti-Resonance Band in a Parallel-Coupled Double Quantum Dot System with Two Multi-Quantum Dot Chains**Supported by the National Natural Science Foundation of China under Grant Nos 11447132 and 11504042, the Chongqing Science and Technology Commission Project under Grant Nos cstc2014jcyjA00032 and cstc2016jcyjA1158, and the Scientific Research Project for Advanced Talents of Yangtze Normal University under Grant No 2017KYQD09.

A parallel-coupled double quantum dot (PCDQD) system with two multi-quantum dot chains is designed. Conductance versus Fermi energy level is investigated utilizing the non-equilibrium Green’s function approach. If two quantum dots are added on each side of the PCDQD system, additional Breit–Wigner and Fano resonances occur in the conductance spectra. If quantum dots are added on one side of the system, small Fano resonances can be observed in the conductance spectra. Adjusting the number of side-coupled quantum dots, the anti-resonance bands emerge at different positions, which makes the system applicable as a quantum switching device. Moreover, the I–V characteristic curve presents the step characteristic and the width of the step decreases with increasing the number of side-coupled quantum dots.

Quantum entanglement of parallel-coupled double quantum dots: A theoretical study using the hierarchical equations of motion approach

Quantum dots comprise a type of quantum impurity system. The entanglement and coherence of quantum states are significantly influenced by the strong electron-electron interactions among impurities and their dissipative coupling with the surrounding environment. Competition between many-body effects and transfer couplings plays an important role in determining the entanglement among localized impurity spins. In this work, we employ the hierarchical-equations-of-motion approach to explore the entanglement of a strongly correlated double quantum dots system. The relation between the total system entropy and those of subsystems is also investigated.

The influence of side-coupled quantum dots on thermoelectric effect of parallel-coupled double quantum dot system

The thermoelectric transport properties of a parallel-coupled double quantum dot (PCDQD) system with side-coupled quantum dots (QDs) is investigated by using the Keldysh non-equilibrium Green's function technique. The thermoelectric quantities, including the thermal conductance, thermopower, and thermoelectric figure of merit denoted by ZT, are sensitive to the inter-dot coupling strength. With the help of side-coupled QD, unusual double Fano resonances are created in the conductance spectra to largely enhance the thermoelectric effect at low-temperature. Benefited from the coexistence of local bipolar effect and Fano resonance, the ZT can be improved by one-fold higher than that of original PCDQD system. Moreover, when the asymmetry parameter α, which indicates the geometric arrangement of coupled QDs with a given lead, takes appropriate value, the optimization of ZT can be achieved at high temperature. Our work suggests that the side-coupled QDs scheme holds promise for the designing of high-efficiency thermoelectric conversion devices.

Conductance through a parallel-coupled double quantum dot with a side-coupled quantum dot system

Using the non-equilibrium Green’s function technique, conductance through a parallel-coupled double quantum dot (PCDQD) with a side-coupled quantum dot system is investigated. The evolution of the conductance strongly depends on the coupling between the side-coupled quantum dot and PCDQD. Moreover, the conductance as a function of the level of side-couple quantum dot is investigated. Numerical results indicate the lineshape of Fano resonance can be modulated by adjusting the interdot coupling strength.

Kondo effect in double quantum dots with ferromagnetic RKKY interaction

We study Kondo effect in parallel-coupled double quantum dots with ferromagnetic Ruderman–Kittel–Kasuya–Yoshida (RKKY) interaction, by using the exact and nonperturbative hierarchical equations of motion approach. We construct the phase diagram in the parameter plane of the inter-dot coupling t and the ferromagnetic exchange interaction J (J − 4t2/U plane). Three different ground states, the Kondo singlet, spin singlet and S = 1 Kondo, are determined in the diagram. We find the ferromagnetic coupling will raise the Kondo peak in the area of finite hopping t which is called ‘J-enhanced Kondo effect’. Another enhancement of the Kondo effect by t (‘t-enhanced Kondo effect’) at J = 0 is also presented. By checking the electron–electron interaction self-energy and magnetic susceptibility, we verify the ground state at large J is an under-screening Kondo state composed of a Fermi liquid with a residual spin 1/2, which is consistent with the ‘singular Fermi liquid state’ in the literature.

Interactions and thermoelectric effects in a parallel-coupled double quantum dot

We investigate the nonequilibrium transport properties of a double quantum dot system connected in parallel to two leads, including intradot electron-electron interaction. In the absence of interactions the system supports a bound state in the continuum. This state is revealed as a Fano antiresonance in the transmission when the energy levels of the dots are detuned. Using the Keldysh nonequilibrium Green's function formalism, we find that the occurrence of the Fano antiresonance survives in the presence of Coulomb repulsion. We give precise predictions for the experimental detection of bound states in the continuum. First, we calculate the differential conductance as a function of the applied voltage and the dot level detuning and find that crossing points in the diamond structure are revealed as minima due to the transmission antiresonances. Second, we determine the thermoelectric current in response to an applied temperature bias. In the linear regime, quantum interference gives rise to sharp peaks in the thermoelectric conductance. Remarkably, we find interaction induced strong current nonlinearities for large thermal gradients that may lead to several nontrivial zeros in the thermocurrent. The latter property is especially attractive for thermoelectric applications.

Majorana modes in a triple-terminal Josephson junction with embedded parallel-coupled double quantum dots

We investigate the Josephson effect in one triple-terminal junction with embedded parallel-coupled double quantum dots. It is found that the inter-superconductor supercurrent has opportunities to oscillate in period 4π, with the adjustment of the phase differences among the superconductors. What is notable is that such a result is robust and independent of fermion parities, intradot Coulomb strength, and the dot-superconductor coupling manner. By introducing the concept of spinful many-particle Majorana modes, we present the analytical definition of the Majorana operator via superposing electron and hole operators. It can be believed that this work provide a simple but feasible proposal for the realization of Majorana modes in a nonmagnetic system.

Spin accumulation in the parallel-coupled double quantum dots with Rashba spin-orbit interaction connected with ferromagnetic and superconducting electrodes

Using the standard nonequilibrium Green’s function techniques, we investigate the effect of Rashba spin-orbit interaction (RSOI) and ferromagnetic electrode on the spin accumulation in the parallel-coupled double quantum dots coupled with a ferromagnetic and a superconducting electrode. It is demonstrated that FM electrode cannot induce the spin polarization of Andreev reflection (AR) current, but can induce the spin accumulation in the QDs. However, RSOI can lead to the spin polarization of AR current as well as the spin accumulation in the QDs. In the existence of RSOI, complete spin-polarized QD can be achieved with negative bias voltage [Formula: see text], which is the most significant advantage of our device. When energy levels [Formula: see text] and the interdot coupling strength [Formula: see text], the maximum value of spin accumulation in this paper is obtained as 0.7. The results may be useful on the design of spintronic devices.

Exact many-electron scattering states in a parallel-coupled double quantum-dot system

We study a parallel-coupled double quantum-dot (PDQD) system with both intradot and interdot Coulomb interactions by constructing many-electron scattering eigenstates explicitly. We obtain exact solutions of the two-electron scattering eigenstates whose incident states are plane-wave states. We find that, as a result of the Coulomb interactions, many-body bound states appear in the scattering eigenstates. The binding strength of the many-body bound states is shown to depend on the difference of gate energies of the two quantum dots, which is interpreted as an effect of geometrical interference.