Dot-lead Coupling Research Articles

Majorana-Free Bound State in the Continuum in the Double-Channel Fano–Anderson Structure

Abstract We investigate the electron transport through a double-channel Fano–Anderson structure, by considering the side-coupling of Majorana bound states (MBSs). It is found that the bound state in the continuum (BIC) phenomenon can be induced when interchannel dot–lead couplings are identical to the intrachannel dot–lead couplings, whose signature is manifested as a halved quantum transmission ability. If the balance between the dot–lead couplings is broken, the BIC phenomenon will also be destroyed, leading to further suppression of the transmission ability. Next, when MBSs are introduced to this system, the BIC phenomenon can be modified efficiently. For the case of two MBSs side-coupled to the quantum dots in a symmetric way, level degeneracy causes a new BIC phenomenon. As only one MBS is incorporated, it suppresses the electron transmission by inducing new transmission dips. When the interdot Coulomb interaction is considered, the BIC phenomena are still robust despite the complicated changes of the transmission ability spectra. One can then understand the realization of BICs in coupled quantum dots and their variation due to the presence of side-coupled MBSs.

Controllable antiresonance and low-bias negative differential conductance in T-shaped double dots with electron–phonon interaction

Controlling the quantum interference in nanoscaled systems has potential application for the realization of high-performance functional devices. In this work, the quantum interference of a T-shaped double-quantum-dot system is studied in detail by evaluation of differential conductance by means of nonequilibrium Green’s function method. The factors of Coulomb interaction in Luttinger liquid leads, electron–phonon interaction, interdot tunneling, and dot-lead coupling are taken into account. In the differential conductance curve, there appear a series of antiresonance dips due to phonon-assisted destructive interference when the interdot tunneling is weak, and they can coexist with zero-bias antiresonance dip. When the dot-lead couplings are asymmetric, both the antiresonance dip and Fano line shape dip can occur for strong dot-lead coupling. Our results also show the coexistence of low-bias negative differential conductance and Fano antiresonance, as a manifestation of the interplay between strong intralead Coulomb interaction and weak dot-lead coupling. These interference phenomena emerged in a relatively simple model promises helpful guidance for controlling over the electrical performance of interference-based molecular devices.

Manipulation of internal blockage in triangular triple quantum dot

We utilize the calculation of hierarchical equations of motion to demonstrate that the spin-dependent properties between adjacent quantum dots (QDs) can be changed by breaking the internal symmetry configuration, corresponding to the inversion of dominant chiral states. In the linear triple quantum dots (LTQDs) connected to two electron reservoirs, we can observe that the blockage appears at the triangle triple quantum dots (TTQDs) by gradually increasing the coupling strength between next-nearest double QDs. When the initial coupling between LTQDs has altered, the internal chiral circulation also undergoes the corresponding transform, thus achieving qualitative regulation and detection of the blocking region. We also investigate the response of the chiral circulation to the dot–lead coupling strength, indicating the overall robust chiral circulation of the TTQDs frustration.

Josephson current across a Double Quantum Dot Josephson junction in T-Shape Configuration

By implementing the Keldysh non-equilibrium Green's function equation of motion approach, Josephson current has been examined across a T-shaped uncorrelated double quantum dot Josephson junction. The behavior of the Josephson current as a function of the main quantum dot energy level for varied interdot tunneling and different dot-lead coupling strengths is examined. With this configuration, we illustrate that the side-attached quantum dot offers an alternative route for electron transmission, which modifies the Josephson current by varying interdot tunneling. Further, we also investigate how the dot-lead coupling strengths affect the Josephson current.

Electron transport properties through a benzene-type quantum dots molecular system

A benzene-type quantum dots molecular model is designed, and the conductance and time-average current through the system are obtained using the Green's function. The results show that one single resonance peak changes to three resonance peaks with increasing the inter-dot coupling strength. The intensities of the twin peaks in the conductance spectrum are sensitive to the dot-lead coupling strength. An anti-resonance band emerges in the conductance spectrum as the dot-lead coupling strength is strong enough. When an external magnetic field is introduced, the original three conductance resonance peaks evolves into six resonance peaks and meanwhile two anti-resonance bands occur. The widths of the new anti-resonance bands can be tuned by adjusting the magnetic flux. Moreover, a photon-assisted tunneling phenomenon can be observed when the system is irradiated by a time-dependent external field. These results provide insights into the design and development of efficient molecular electronics.

Two-Channel Charge-Kondo Physics in Graphene Quantum Dots.

Nanoelectronic quantum dot devices exploiting the charge-Kondo paradigm have been established as versatile and accurate analogue quantum simulators of fundamental quantum impurity models. In particular, hybrid metal–semiconductor dots connected to two metallic leads realize the two-channel Kondo (2CK) model, in which Kondo screening of the dot charge pseudospin is frustrated. In this article, a two-channel charge-Kondo device made instead from graphene components is considered, realizing a pseudogapped version of the 2CK model. The model is solved using Wilson’s Numerical Renormalization Group method, uncovering a rich phase diagram as a function of dot–lead coupling strength, channel asymmetry, and potential scattering. The complex physics of this system is explored through its thermodynamic properties, scattering T-matrix, and experimentally measurable conductance. The strong coupling pseudogap Kondo phase is found to persist in the channel-asymmetric two-channel context, while in the channel-symmetric case, frustration results in a novel quantum phase transition. Remarkably, despite the vanishing density of states in the graphene leads at low energies, a finite linear conductance is found at zero temperature at the frustrated critical point, which is of a non-Fermi liquid type. Our results suggest that the graphene charge-Kondo platform offers a unique possibility to access multichannel pseudogap Kondo physics.

Peltier effect in noninteracting double quantum dots

Peltier effect is an important thermoelectric phenomenon which stands for the generation of temperature gradient of the interface between two dissimilar conductors by the electric current flowing through them. In this work, we investigate the Peltier effect in serially coupled noninteracting double quantum dot system under a bias voltage. By means of an accurate hierarchical equations of local temperatures of the two dots differ from each other by applying an electric current through the dots. We then analyze theoretically the influence of interdot and dot-lead coupling on the thermopower and electric current. Finally, we elucidate the variation of Peltier heat and Joule heat with the interdot coupling and dot-lead coupling, which leads to the changes in the local temperature of the quantum dots.

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.

Effect of magnetic fields on the shot noise of a quantum dot with Luttinger leads

We study the shot noise of a strongly correlated quantum dot system in the Kondo regime, concentrating on the joint effect of a magnetic field B and intralead Coulomb interaction. A shot noise formula is derived. The shot noise is suppressed by the magnetic field when the intralead interaction is weak, but improved by the field when the interaction is strong. The differential shot noise has a peak or valley at eV = ±gμBB for weak or moderately strong intralead interaction. As the interactions increase, these singular behaviours gradually disappear. In the strong interaction limit, the shot noise's curve has a power-law dependence on the voltage. The intralead Coulomb interaction also modulates the Fano line shape of shot noise. The Fano factor is enhanced by the magnetic field and dot-lead asymmetric coupling and is always sub-Poissonian noise.

Tunable Valley Splitting and Bipolar Operation in Graphene Quantum Dots.

Quantum states in graphene are 2-fold degenerate in spins, and 2-fold in valleys. Both degrees of freedom can be utilized for qubit preparations. In our bilayer graphene quantum dots, we demonstrate that the valley g-factor gv, defined analogously to the spin g-factor gs for valley splitting in a perpendicular magnetic field, is tunable by over a factor of 4 from 20 to 90, by gate voltage adjustments only. Larger gv results from larger electronic dot sizes, determined from the charging energy. On our versatile device, bipolar operation, charging our quantum dot with charge carriers of the same or the opposite polarity as the leads, can be performed. Dots of both polarities are tunable to the first charge carrier, such that the transition from an electron to a hole dot by the action of the plunger gate can be observed. Addition of gates easily extends the system to host tunable double dots.

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.

Kondo resonance assisted thermoelectric transport through strongly correlated quantum dots

We theoretically studied the thermoelectric transport propertiesof a strongly correlated quantum dot system in the presence of the Kondo effect based on accurate numerical evaluations using the hierarchical equations of motion approach.The thermocurrent versus gate voltage shows a distinct sawtooth line-shape at high temperatures.In particular, the current changes from positive (hole charge) to negative (particle charge) inthe electron number $N~=~1$ region due to the Coulomb blockade effect. However, at low temperatures, where the Kondo effect occurs, the thermocurrents charge polarity reverses,along witha significantly enhanced magnitude.As anticipated, the current sign can be analyzed by the occupation difference between particle and hole. Moreover, the characteristic turnover temperature can be further definedat which the influences of the Coulomb blockade and Kondo resonance are in an effective balance.Remarkably, the identified characteristic turnover temperature, as a function of the Coulomb interaction and dot-lead coupling, possessed a much higher value than the Kondo temperature. When a magnetic field is applied, a spin-polarized thermocurrent can be obtained, which could be tested in future experiments.

Long-range overlapping of Kondo clouds in open triple quantum dots

We study the phenomena of overlapping of Kondo clouds in an open triple quantum dots (OTQDs) system by using the dissipaton equation of motion (DEOM) theory. Motivated by the long-rang interaction of the TQDs system demonstrated in Cheng et al (2017 Phys. Rev. B 95 155417), we present a comprehensive picture of the long-range overlapping behavior of Kondo clouds via investigation of the spectral functions, spin–spin correlation, dot occupancies and susceptibility. For the configuration , a conduction electron peak occurs in the spectral function of intermediate QD in the Kondo regime. This peak results from the overlapping of the two Kondo clouds forming from between the two peripheral QDs and leads, enhances with decreasing temperature and increasing dot-lead coupling. Both the spin–spin correlations between the two adjacent QDs and the two peripheral QDs own negative values. It also confirms the physical picture of the overlapping between left and right Kondo clouds via the intermediate QD. To understand the physical insight, we examine also the electron occupacies and the spectral functions, with their dependence on the temperature and dot-lead coupling. In addition, a distinct nonmonotonic behavior of the susceptibility associated with the Kondo clouds is characterized.

Spin-Polarized Transport and Spin Seebeck Effect in Triple Quantum Dots with Spin-Dependent Interdot Couplings

We study the spin-dependent electronic and thermoelectric transport through a structure composed of triple quantum dots (TQDs) coupled to two metallic leads in the presence of spin-dependent interdot couplings, which is reliable by applying a static magnetic field on the tunnel junctions between different dots. When the TQDs are serially connected, a 100 % spin-polarized conductance and thermopower emerge even for very small spin-polarization of the interdot coupling as the dots are weakly coupled to each other. Whereas if the TQDs are connected in a ring shape, the Fano antiresonance will result in sharp peaks in the conductance and thermopower. In the presence of spin-dependent interdot couplings, the peaks of the spin-up and spin-down thermopowers will shift to opposite directions in the dot level regime, resulting large either 100 % spin-polarized or pure spin thermopowers. The latter generally arises at low temperatures and is robust against the level detuning, the dot-lead coupling, and the system equilibrium temperature.

Quenched Fano effect due to one Majorana zero mode coupled to the Fano interferometer

We investigate the change of the Fano effect by considering one Majorana zero mode to couple laterally to the single-dot Fano interferometer. It is found that the Majorana zero mode quenches the Fano effect thoroughly and causes the conductance to be independent of the dot level, the dot-lead coupling, and the increase of the Majorana-dot coupling. As a result, the linear conductance becomes only related to the interlead coupling and the magnetic-flux phase factor. These results can be helpful for the detection of Majorana zero mode.

Thermal rectification in a double quantum dots system with a polaron effect

We investigate the rectification of heat current carried by electrons through a double quantum dot (DQD) system under a temperature bias. The DQD can be realized by molecules such as suspended carbon nanotube and be described by the Anderson-Holstein model in presence of electron-phonon interaction. Strong electron-phonon interaction can lead to formation of polaronic states in which electronic states are dressed by phonon cloud. Dressed tunneling approximation (DTA), which is nonperturbative in dealing with strong electron-phonon interaction, is employed to obtain the heat current expression. In DTA, self-energies are dressed by phonon cloud operator and are temperature dependent. The temperature dependency of imaginary part of dressed retarded self-energy gives rise to the asymmetry of the system and is the necessary condition of thermal rectification. On top of this, one can either tune DQD effective energy levels such that $|\bar{\epsilon}_1|\neq |\bar{\epsilon}_2|$ or have asymmetric dot-lead couplings to achieve thermal rectification. We numerically find that increasing electron-phonon coupling and reducing inter dot coupling can both improve thermal rectification effect, while the electronic heat current is reduced.

Effect of PT symmetry in a parallel double-quantum-dot structure

We present a comprehensive analysis about the effect of $\mathcal{PT}$-symmetric complex potentials on quantum transport through one parallel coupled double-quantum-dot structure. It is found that such potentials are able to eliminate the decoupling phenomenon while inducing the Fano effect, even under the situation of uniform dot-lead coupling. Moreover, the Fano line shape in the transmission function spectrum is opposite to that that arises from the detuning of the dot levels. We ascertain that this work can be helpful in understanding the quantum transport properties in the quantum systems with $\mathcal{PT}$ symmetry.

Transport properties in a non-Hermitian triple-quantum-dot structure

In this work, we study the effect of $\mathcal{PT}$-symmetric complex potentials on the transport properties of one non-Hermitian system, which is formed by the coupling between a triple-quantum-dot molecule and two semi-infinite leads. As a result, it is found that the $\mathcal{PT}$-symmetric imaginary potentials take pronounced effects on transport properties of such a system, including changes from antiresonance to resonance, shift of antiresonance, and occurrence of new antiresonance, which are determined by the interdot and dot-lead coupling manners. This study can be helpful in understanding the quantum transport behaviors modified by the $\cal PT$ symmetry in non-Hermitian discrete systems.

Functional renormalization group study of parallel double quantum dots: Effects of asymmetric dot-lead couplings

We explore the effects of asymmetry of hopping parameters between double parallel quantum dots and the leads on the conductance and a possibility of local magnetic moment formation in this system using functional renormalization group approach with the counterterm. We demonstrate a possibility of a quantum phase transition to a local moment regime (so called singular Fermi liquid (SFL) state) for various types of hopping asymmetries and discuss respective gate voltage dependences of the conductance. It is shown, that depending on the type of the asymmetry, the system can demonstrate either a first order quantum phase transition to SFL state, accompanied by a discontinuous change of the conductance, similarly to the symmetric case, or the second order quantum phase transition, in which the conductance is continuous and exhibits Fano-type asymmetric resonance near the transition point. A semi-analytical explanation of these different types of conductance behavior is presented.

Anomalous Fano interference induced by Majorana zero mode

We investigate the Fano effect in electron transport through a parallel mesoscopic circuit, whose arms are embedded by one quantum dot and one Majorana zero mode (MZM), respectively. It is found that by adjusting the dot-lead and MZM-lead couplings, the Fano linshapes have opportunities to appear in the linear conductance spectra. However, they do not form at the cases of ϕ=nπ (ϕ is the phase factor of the local magnetic flux). It also shows that the Fano lineshapes can be reversed by changing the bias voltage. These results are important for describing the transport properties assisted by MZM.