T-shaped Double Quantum Dot Research Articles

Andreev reflection in a T-shaped double quantum-dot with coupled Majorana bound states

Owing to their potential applications in topological quantum computation and because of their fundamental interest, Majorana fermions are currently attracting increasing attention. Numerous theoretical and experimental studies exactly show that the quantum dot (QD) structure is a good candidate for the detection of Majorana bound state (MBSs). QD system has many unique transport properties and interesting quantum phenomena, such as quantum interference effect, Fano effect, etc. In addition, compared with a single QD, a coupled QD structure has many adjustable parameters, and thus has more important theoretical and practical value, which provides an excellent platform to detect MBSs. In addition, QD coupled with normal metallic conductor and with superconducting electrode structure exhibits interesting transport properties. One of these properties is the so-called Andreev reflection (AR). Especially, in the subgap regime, the current almost entirely originates from the anomalous Andreev channel; such spectroscopy can thus directly probe any in-gap state. In the present paper, we consider a T-shaped double QD structure with side-coupled to MBSs and investigate the transport properties through the system by adding a normal and a superconducting lead. We calculate the AR conductance through the system in the subgap transport. Here we focus on the effects of MBSs on AR through the system. We find that the AR conductance presents a resonant peak around zero Fermi energy when only one QD (QD1) connects to metal and superconducting leads. As a consequence of quantum interference, when using another QD2 side-attached to QD1, a pair of new Fano-type resonant peaks appear and is distributed aside the zero point and the Fano antiresonant point is at the energy level of the QD2. If an MBS is introduced to couple to QD2, the AR conductance shows several new features. First, a pair of new Fano-type resonance curves appears and the original ones also persist except for the position shifting. In addition, the AR conductance value at the zero Fermi energy point is exactly equal to 1/2G0(G0=2e2/h) in the presence of QD-MBS coupling and zero inter-MBS coupling, which is not dependent on the inert-dot coupling nor the energy levels of QD nor the strength of the QD-MBS coupling. This feature is different from which the T-shaped DQD structure side-coupled to a traditional fermions, showing the robust properties of the Majorana fermions. We also show that in the Andreev reflection conductance curves appear resonance zone changes into antiresonance near zero Fermi energy by adjusting the coupling strength between the double quantum dots in the system without MBSs, while the antiresonance disappears and new resonance peaks appear if an MBS is introduced to couple to QD2. We hope that these results will be helpful for understanding the quantum interference in MBS-assisted AR and may find significant applications, especially in quantum computation.

Majorana fermions in the interacting T-shaped double quantum dot

Non-equilibrium transport properties of the interacting T-shaped double quantum dot coupled with the topological superconductor are analyzed within the Keldysh Green’s function formalism. The low energy characteristics are found to be influenced by the interplay of quantum interference, electronic correlation, and the Majorana induced interaction such that the system can be driven onto either Kondo, Fano or Majorana dominated regime. It is demonstrated that the presence of the Majorana fermions at the edges of superconducting wire can be realized in the differential conductance and the zero-frequency shot noise when the system’s low energy is strongly influenced by the quantum interference effect.

Photon assisted electron transport through T-shaped double quantum dot molecule A–B interferometer

T-shape double quantum dot molecule A–B interference with the action of time-dependent external field is designed. Using the Keldysh non-equilibrium Green's function method, spin-dependent average current as a function of quantum dot energy is obtained. The magnitude and position of average current resonance peaks can be controlled by adjusting interdot coupling strength, frequency and amplitude of time-dependent electric field. The conversion between zero and non-zero value for average current can be realized under the condition with or without the action of time-dependent external field, respectively, which is the basis for the design of controlled quantum switch. Under the combined action of the external magnetic flux and Rashba spin–orbit coupling strength, the spin-polarized average current can be observed. Dc current–voltage characteristics are researched.

Qubit detection with a T-shaped double quantum dot detector

We propose to continuously monitor a charge qubit by utilizing a T-shaped double quantum dot detector, in which the qubit and double dot are arranged in such a unique way that the detector turns out to be particularly susceptible to the charge states of the qubit. Special attention is paid to the regime where acquisition of qubit information and backaction upon the measured system exhibit nontrivial correlation. The intrinsic dynamics of the qubit gives rise to dynamical blockade of tunneling events through the detector, resulting in a super-Poissonian noise. However, such a pronounced enhancement of the detector's shot noise does not necessarily produce a rising dephasing rate. In contrast, an inhibition of dephasing is entailed by the reduction of information acquisition in the dynamically blockaded regimes. We further reveal the important impact of the charge fluctuations on the measurement characteristics. Noticeably, under the condition of symmetric junction capacitances the noise pedestal of the circuit current is completely suppressed, leading to a divergent signal-to-noise ratio, and eventually to a violation of the Korotkov-Averin bound in quantum measurement. Our study offers the possibility for a double dot detector to reach the quantum limited effectiveness in a transparent manner.

Ferromagnets-induced splitting of molecular states of T-shaped double quantum dots

The exchange field for molecular states of double quantum dot, induced by two ferromagnets coupled to the device in T-shaped configuration, is defined and calculated. It is found, that in the regime of strong coupling between quantum dots, the dependence of the exchange field on this coupling becomes nontrivial. In particular, it changes the sign a few times to eventually vanish in the limit of infinite inter-dot coupling. The excitation energies of double quantum dot are calculated and the results used to predict the conditions for suppression of the two-stage Kondo effect in the considered nanostructure.

Two-stage Kondo effect in T-shaped double quantum dots with ferromagnetic leads

The linear-response transport properties of a T-shaped double quantum dot strongly coupled to external ferromagnetic leads are studied theoretically by using the numerical renormalization group method. It is shown that when each dot is occupied by a single electron, for antiparallel alignment of leads' magnetizations, the system exhibits the two-stage Kondo effect. For parallel alignment, however, the second stage of the Kondo effect becomes suppressed due to the presence of ferromagnetic-contact-induced exchange field. The difference between the two magnetic configurations results in highly nontrivial behavior of the tunnel magnetoresistance, which for some parameters can take giant values. In addition, the dependence of the linear conductance and tunnel magnetoresistance on external magnetic field, the double-dot levels' position, and the spin polarization of the leads is thoroughly analyzed. It is shown that the second stage of the Kondo effect can be restored by fine-tuning of the magnetic field or the dots' levels. The effect of spin-dependent tunneling on the low-temperature transition from the high to low conducting state of the system, which occurs when changing the hopping between the dots, is also discussed.

The Magnetic Field Effects on Spin Polarization of T-Shaped Double Quantum Dots Coupled to Ferromagnetic Leads

We analyze the spin-dependent conductance and spin polarization of a double quantum dot in a T-shape conguration coupled to ferromagnetic leads in the presence of external magnetic eld. The calculations are performed with the aid of the numerical renormalization group method. We show that in the antiparallel con guration, nite magnetic eld can give rise to the full spin polarization of the current, which can be controlled by tuning the dots' levels. On the other hand, for parallel con guration enhanced spin polarization can be generated by an exchange eld due to the presence of ferromagnetic leads and can be also tuned by changing level position or external magnetic eld. The magnetic eld can be thus used to improve the spin-resolved properties of the system.

Thermoelectric transport properties of a T-shaped double quantum dot system in the Coulomb blockade regime

We investigate the thermoelectric properties of a T-shaped double quantum dot system described by a generalized Anderson Hamiltonian. The system’s electrical conduction (G) and the fundamental thermoelectric parameters such as the Seebeck coefficient (S) and the thermal conductivity (κ), along with the system’s thermoelectric figure of merit (ZT) are numerically estimated based on a Green’s function formalism that includes contributions up to the Hartree-Fock level. Our results account for finite on-site Coulomb interaction terms in both component quantum dots and discuss various ways leading to an enhanced thermoelectric figure of merit for the system. We demonstrate that the presence of Fano resonances in the Coulomb blockade regime is responsible for a strong violation of the Wiedemann-Franz law and a considerable enhancement of the system’s figure of merit (ZT).

An equation of motion analysis of the two stage Kondo effect in T-shaped double-quantum-dot systems

We use the equation of motion method in connection with a generalized Anderson Hamiltonian to evaluate the electronic transmission in a T-shaped double quantum dot system. We consider the strong Coulomb interaction regime (U1→∞, U2→∞) using a decoupling procedure that includes terms beyond the standard Hartree–Fock approximation. The typical manifestation of the two stage Kondo effect is reflected in system's electronic transmission coefficient that presents both a large Lorentzian and a sharp dip around ω=0. We provide an analytical result for the first stage Kondo temperature that includes corrections due to the presence of an additional quantum dot in the system. For the second stage Kondo effect, we identify the logarithmic correction that is responsible for the additional dip in the system's electronic transmission coefficient.

Perfect spin polarization in T-shaped double quantum dots due to the spin-dependent Fano effect

We study the spin-resolved transport properties of T-shaped double quantum dots coupled to ferromagnetic leads. Using the numerical renormalization group method, we calculate the linear conductance and the spin polarization of the current for various model parameters and at different temperatures. We show that an effective exchange field due to the presence of ferromagnets results in different conditions for Fano destructive interference in each spin channel. This spin dependence of the Fano effect leads to perfect spin polarization, the sign of which can be changed by tuning the dots' levels. Large spin polarization occurs due to Coulomb correlations in the dot, which is not directly coupled to the leads, while finite correlations in the directly-coupled dot can further enhance this effect. Moreover, we complement accurate numerical results with a simple qualitative explanation based on analytical expressions for the zero-temperature conductance. The proposed device provides a prospective example of an electrically-controlled, fully spin-polarized current source, which operates without an external magnetic field.

Electron Transport in T-Shaped Double Quantum Dot System Using Non-Equilibrium Green's Function

Electron Transport in <i>T</i>-Shaped Double Quantum Dot System Using Non-Equilibrium Green's Function

Electron transport through T-shaped double quantum dot molecule Aharonov-Bohm interferometer

Using non-equilibrium Green's function method, the charge and spin transport properties through T-shape double quantum dot molecule Aharonov-Bohm (A-B) interference are theoretically investigated. Resonance or anti-resonance can occur at the same location in conductance spectrum by controlling coupling or uncoupling between two quantum dots in T-shape double quantum dot molecule, which is the basis for designing quantum switches. When two identical T-shaped double quantum dot molecules are embedded in two arms of A-B interferometer, respectively, totally destructive interference can appear by taking appropriate magnetic flux. Spin current through the system can be regulated by adjusting quantum dot level, bias between two electrodes and Rashba spin-orbit interaction.

The Electronic Transports Technology in a T-Shaped Double Quantum Dot

We study the phonon-assisted Fano interference of the linear conductance spectrum by taking into account the interdot-phonon exchange in a T-shaped double quantum dot (QD), where a central QD is coupled to a side QD and two nonmagnetic or ferromagnetic electrodes. Unlike the usual Fano interference between different elastic channels,this new-type Fano interference is shown to arise from electron waves tunneling coherently through phonon-assisted bonding and antibonding states.

Fano effect and Andreev bound states in T-shape double quantum dots

In this paper, we investigate the transport through a T-shaped double quantum dot coupled to two normal metal leads left and right and a superconducting lead. Analytical expressions of Andreev transmission and local Density of States of the system at zero temperature have been obtained. We study the role of the superconducting lead in the quantum interferometric features of the double quantum dot. We report for first time the Fano effect produced by Andreev bound states in a side quantum dot. Our results show that as a consequence of quantum interference and proximity effect, the transmission from normal to normal lead exhibits Fano resonances due to Andreev bound states. We find that this interference effect allows us to study the Andreev bound states in the changes in the conductance between two normal leads.

Josephson current through a T-shaped double quantum dots: π-junction transition and interdot antiferromagnetic correlations

By means of the exact diagonalization approach, the Josephson current in a T-shaped double quantum dot structure is theoretically investigated. The ground state is obtained within zero bandwidth approximation in which the superconductors are replaced by effective local pairing potentials. It is found that Josephson current can flow through this structure in the presence of various electron correlations. Furthermore, in the half-filled case, a novel 0 – π transition behavior is observed, which arises from the interplay of interdot antiferromagnetic coupling and electron correlations. ► By the exact diagonalization approach, Josephson current in a T-shaped quantum dot structure is investigated. ► The ground state is obtained within zero bandwidth approximation. ► Josephson current flows through this structure in the presence of various electron correlations. ► In the half-filled case, a novel 0 – π transition behavior is observed due to the interplay of interdot antiferromagnetic coupling and electron correlations.

Kondo—Fano Effect in T-shaped Double Quantum Dots Coupled to Ferromagnetic Leads

Using an equation-of-mot ion technique, we theoretically study the Kondo—Fano effect in the T-shaped double quantum dots coupled to two ferromagnetic leads by the Anderson Hamiltonian. We calculate the density of states in this system by solving Green function. Our results reveal that the density of states show some noticeable characteristics not only depending upon the interdot coupling tab, the energy level ∊d1 of the side coupled quantum dot QDb, and the relative angle θ of magnetic moment M, but also the asymmetry parameter α in ferromagnetic leads and so on. All these parameters greatly influence the density of states of the central quantum dot QDa. This system is a possible candidate for spin valve transistors and may have potential applications in the spintronics.

Tunable spin current through a T-shaped double quantum dot molecule

We present a spin current generator based on a T-shaped double quantum dot (TDQD) molecule connected with two leads, and the coherent spin-flip effect is taken into account within the TDQD. The spin current from the right output terminal is obtained, more importantly, the properties of the spin current are investigated in detail, these results offer us a way to manipulate the spin current with the system parameters.

Real-time counting of single electron tunneling through a T-shaped double quantum dot system

Real-time detection of single electron tunneling through a T-shaped double quantum dot is simulated, based on a Monte Carlo scheme. The double dot is embedded in a dissipative environment and the presence of electrons on the double dot is detected with a nearby quantum point contact. We demonstrate directly the bunching behavior in electron transport, which leads eventually to a super-Poissonian noise. Particularly, in the context of full counting statistics, we investigate the essential difference between the dephasing mechanisms induced by the quantum point contact detection and the coupling to the external phonon bath. A number of intriguing noise features associated with various transport mechanisms are revealed.

Spin accumulation and spin-dependent Andreev reflection current in a T-shaped double quantum dots

By means of the nonequilibrium Green's function (NGF) approach, the spin accumulation and spin-dependent Andreev reflection current in a T-shaped DQD coupled to ferromagnetic and superconducting leads in the presence of spin-flip effect are theoretically investigated. The spin accumulation of quantum dots appears due to the ferromagnetic lead and the Andreev reflection, it is found that the magnitude and the sign of the spin accumulation can be controlled by changing the system parameters such as the bias voltage, the dot level and the spin-flip strength. If the spin-flip effect is taken into account within the QDs, a spin-polarized Andreev reflection current can be generated. Moreover, the spin-polarized Andreev reflection current can be manipulated by tuning the system parameters.

Inelastic Fano interference induced by interdot-phonon exchange in a T-shaped double quantum dot

We study the phonon-assisted Fano interference of the linear conductance spectrum by taking into account the interdot-phonon exchange in a T-shaped double quantum dot (QD), where a central QD is coupled to a side QD and two nonmagnetic or ferromagnetic electrodes. Unlike the usual Fano interference between different elastic channels, this new-type Fano interference is shown to arise from electron waves tunneling coherently through phonon-assisted bonding and antibonding states. The inelastic Fano effect, characterized by a significant resonance and a zero-value antiresonance of conductances due to destructive quantum interference, is strongly dependent on the interdot coupling and interdot electron–phonon interactions (EPI) strength. We also examine that the introduction of ferromagnetic leads does not destroy either the elastic Fano interference or the inelastic one but richens their lineshapes.