T-shaped Double Quantum Dot Research Articles

Time-dependent quantum transport behavior through T-shaped double quantumdots

The time-dependent current flowing through a T-shaped double quantum dot istheoretically studied via nonequilibrium Green’s function methods. Quantum coherentringing or beats can appear in the transient current after a bias voltage is turned on or off,and their periods can be tuned by the pulse bias or the interdot coupling strength. In theweak interdot coupling case no quantum beat is observed and only quantumringing appears in the current, the frequency of which depends on the pulse bias.In the strong interdot coupling case, quantum beats appear in the current andtheir frequency depends on the interdot coupling strength. Quantum beats aresuppressed greatly by the large energy difference between the two quantum dots. Inaddition, quantum ringing and beats tend to disappear with increasing temperature.

Controlled π-junctions in a T-shaped double quantum dot system

The π-junction transition of a T-shaped double quantum dot system is investigated theoretically by using the nonequilibrium Green’s function method. It is found that the π-junction transition can occur with increasing the spin-flip strength. Furthermore, the π-junction in the system can be controlled by tuning the system parameters, such as the two quantum dot energy levels and the interdot coupling. These controlled π-junction transitions are interpreted in the picture of current-carrying density of states. When the main contributions to supercurrent is changed between the positive discrete spectrum and the negative continuous spectrum, the π-junction transitions can happen.

Electrically detected phonon spectroscopy in double quantum dots

We theoretically study the dephasing effect by the phonon emission on nonequilibrium transport at temperature T = 0, in T-shaped double quantum dots in which one of the dots is connected to external leads (dot 1) and the other is disconnected (dot 2). The differential conductance shows a dip of transmission zero, which is sensitive to the dephasing by the phonon emission. We show that (i) the phonon emission from dot 1 is dephasing-free; it does not affect the conductance dip. (ii) The conductance dip changes as a function of bias voltage, reflecting a product of the density of states for phonons and strength of electron-phonon coupling in dot 2. This implies the electrically detected phonon spectroscopy.

Fano–Kondo effect in the T-shaped double quantum dots coupled to ferromagnetic leads

Using an equation-of-motion technique, we theoretically study the Fano–Kondo 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 with both parallel and antiparallel lead-polarization alignments, and our results reveal that the interdot coupling, the spin-polarized strength and the energy level of the side coupled quantum dot greatly influence the density of states of the central quantum dot. This system is a possible candidate for spin valve transistors and may have potential applications in the spintronics.

Josephson current transport through T-shaped double quantum dots

The Josephson current through T-shaped double quantum dots (TDQD) has beeninvestigated theoretically using the non-equilibrium Green’s function method.Hybridization of energy levels of the central quantum dot (QD) and side QDresults in the renormalization of Andreev bound states, which dominates theJosephson current. Consequently, Josephson critical current can be modulated tointerference construction and destruction by adjusting the energy levels of two QDsε1,ε2, and interdotcoupling tc. Indetail, when ε1ε2 = tc2 is fulfilled, interference construction occurs and whenε2 = 0, interference destruction happens. These results are similar to the interferencebehavior of linear conductance in normal TDQD devices. In addition,Josephson critical current also shows Fano characteristics with variation ofε2 and the resonance line shape is determined byε1. Finally, the Josephson current has a symmetric relation ofI(ε1,ε2) = I(−ε1,−ε2) due to electron–hole symmetry.

Effect of a side-connected dot in a T-shaped double quantum dot interferometer

In this study we discuss the effect of a side-connected dot in a T-shaped double quantumdot (DQD) system under an infinitely large Coulomb interaction.Applying the non-crossing approximation scheme to a T-shaped DQD interferometer, theeffect of a side-connected dot on the Aharonov–Bohm (AB) oscillation is discussed. The ABoscillation in zero-bias conductance becomes large at an optimal interdot coupling. Under afinite bias voltage, the AB phase symmetry is broken in the transmission probability anddifferential conductance. The Fano interference depends on direct tunneling, temperature,and interdot coupling, and the amplitude of the AB oscillation is affected by thesefactors.

Spin-polarized transport through the T-shaped double quantum dots

Using the Keldysh nonequilibrium Green function and equation-of-motion technique, this paper investigates the spin-polarized transport properties of the T-shaped double quantum dots (DQD) coupled to two ferromagnetic leads. There are both Fano effect and Kondo effect in the system, and due to their mutual interaction, the density of states, the current, and the differential conductance of the system depend sensitively on the spin-polarized strength. Thus the obtained results show that this system is provided with excellent spin filtering property, which indicates that this system may be a candidate for spin valve transistors in the spintronics.

Fano Effect in T-Shaped Double Quantum Dot Structure with Decoherence Effect

By introducing a floating lead to mimic the decoherence mechanism, the Fano effect in the linear conductance spectrum of a T-shaped double quantum dot structure is studied. We find that even in the case that the self-energy arising from the decoherence mechanism is much smaller than the coupling strength between the connected quantum dot and the conducting lead, the Fano lineshape can be largely destroyed. In addition, the decoherence renders the high-order electron transmission paths unimportant to contributing to the Fano lineshape.

Fano effect in a T-shaped double quantum dot structure in the presence of Rashba spin–orbit coupling

The influence of Rashba spin–orbit coupling on the Fano lineshape of the conductance spectrum in a T-shaped double quantum dot structure is theoretically studied. By second-quantizing the electron Hamiltonian in this structure, it is found that the Rashba interaction brings about a spin-flip interdot hopping term. With the enhancement of the Rashba interaction, this term separates the two resonant peaks in the conductance spectrum from each other. More importantly, it causes the broadening of the narrow Fano peak, and the narrowing of the broader peak. Finally, the asymmetric Fano lineshape changes into a symmetric profile in the global conductance spectrum.

Nonequilibrium transport in T-shaped double quantum dots: Phonon emission without dephasing

We theoretically study the effect of electron–phonon interaction on transport properties in T-shaped double quantum dots, in which one of the dots is connected to the external leads (dot 1) and the other is disconnected (dot 2). Nonequilibrium transport under finite bias is calculated using Keldysh Green function method. The differential conductance shows a dip due to the destructive interference between an electron wave passing by the interdot bonding orbital and that passing by the interdot antibonding orbital. The phonon emission from dot 2 breaks the interference and reduces the dip in size. The phonon emission from dot 1, on the other hand, does not affect the interference at all. An experiment to observe this dephasing-free phonon emission is proposed.

Coherent and incoherent transport through T-shaped double quantum dots

We investigate the measurement induced dephasing of the Fano effect in the electronic transport through a double quantum dot mesoscopic interferometer coupled to a charge detector. The current and the differential conductance are computed within the Keldysh formalism, taking into account of the inelastic processes due to the dot–detector interaction. We show that the visibility of the Fano lineshape is reduced by applying a finite bias on the charge detector.

Spin-Polarized Transport through the T-Shaped Double QuantumDots with Fano–Kondo Interaction

We theoretically investigate the spin-polarized transport properties ofthe T-shaped double quantum dots coupled to two ferromagnetic leads bythe Anderson Hamiltonian. The Hamiltonian is solved by means of theslave-boson mean-field theory. We calculate the density of states andthe liner conductance in this system with both parallel andantiparallel lead-polarization alignments, and our results show thatthe transport properties of this system depend on both the tunnellingstrength between the two dots and the spin-polarized strength p.This system is a possible candidate for spin valve transistors in thespintronics.

Andreev reflection with spin-flip scattering through a T-shaped double quantum dot

The spin-dependent Andreev reflection (AR) tunneling through a ferromagnet double-quantum-dot superconductor (F-DQD-S) is studied using the nonequilibrium Green’s function method. The F-DQD-S is a T-shaped hybrid structure where one of the dots is coupled to the F and S electrodes and the other dot is coupled to the first dot only. On each dot the intradot spin-flip process is present. The behavior of the F-DQD-S tunneling conductance is determined by the competition among the interdot coupling, the intradot spin-flip process, and the coupling of the first dot to the electrodes. When the interdot coupling is present, the quantum interference effects between the two dots cause secondary peaks in the tunneling conductance besides the primary one, indicating a structure in the AR spectrum. On the other hand, the spin-flip scattering process suppresses the AR peaks. With the spin-flip scattering strength increasing, the resonance peaks are split.

Zero temperature conductance of parallel T-shape double quantum dots

We analyze the zero temperature conductance of a parallel T-shaped double quantum dot system. We present an analytical expression for the conductance of the system in terms of the total number of electrons in both quantum dots. Our results confirm that the system's conductance is strongly influenced by the dot which is not directly connected to the leads. We discuss our results in connection with similar results reported in the literature.

Fano Interference versus Kondo Effect in Strongly CorrelatedT-Shaped Quantum Dots Embedded in an Aharonov–Bohm Ring

We theoretically investigate the properties of the ground state of thestrongly correlated T-shaped double quantum dots embedded in anAharonov–Bohm ring in the Kondo regime by means of the one-impurityAnderson Hamiltonian. It is found that in this system, the persistentcurrent depends sensitively on the parity and size of the ring. With theincrease of interdot coupling, the persistent current is suppressed dueto the enhancing Fano interference weakening the Kondo effect. Moreover,when the spin of quantum dot embedded in the Aharonov–Bohm ring isscreened, the persistent current peak is not affected by interdotcoupling. Thus this model may be a new candidate for detecting Kondoscreening cloud.

Interference of conductance and shot noise properties of photon-assisted transport through a T-shaped double quantum dot

We have investigated the time-modulated quantum transport properties of a T-shaped double quantum dot. Transport properties of the model device are strongly influenced by the interplay of the quantum interference due to the T-shape arrangement and the inelastic scatterings due to photon-assisted tunneling processes. Electrons can absorb or emit one or multiple photons to reach the new path introduced by the side dot and therefore induce the quantum interference effect with the resonant or photon-assisted resonant path through the central dot. As a result, asymmetric line shapes due to Fano interference appear at the satellite peaks due to the (multi)photon absorption or emission processes. We further investigated the zero dc bias shot noise properties of the T-shaped device. At moderate ac field strength, local minima of the zero dc bias shot noise can be found at the frequency corresponding to multiphoton assisted transport. The phase properties of the elastic transmission amplitude and local density of states are investigated too. It is demonstrated that the abrupt phase drop by $\ensuremath{\pi}$ due to Fano interference can be greatly modified by the coherent photon-assisted transport.

Transport through a T-shaped double quantum dot with Fano–Kondo interaction

We present transport properties of a strongly correlated quantum dot attached to two leads with a side coupled non-interacting quantum dot. Transport properties are analyzed using the slave boson mean field theory which is reliable in the zero temperature and low bias regime. It is found that the transport properties are determined by the interplay of two fundamental physical phenomena, i.e. the Kondo effects where the conductance can reach the unitary limit and the Fano interference which leads to an asymmetric shape of the conductance as function of the gate voltage. It is found that the Kondo effects will be suppressed by the Fano interference. The linear conductance will depart from the unitary limit and the zero bias anomaly will be suppressed in the presence of interdot coupling. If the spin degeneracy of the side coupled dot is lifted, for example, by external magnetic field, the linear conductance of electrons with different spin displays dips at different position. This result might be useful for the design of spin filters.

Aharonov–Bohm Effect for Parallel and T-shaped Double Quantum Dots

We investigate the Aharonov-Bohm (AB) effect for the double quantum dots in the Kondo regime using the slave-boson mean-field approximation. In contrast to the non-interacting case, where the AB oscillation generally has the period of 4$\pi$ when the two-subring structure is formed via the interdot tunneling $t_c$, we find that the AB oscillation has the period of 2$\pi$ in the Kondo regime. Such effects appear for the double quantum dots close to the T-shaped geometry even in the charge-fluctuation regime. These results follow from the fact that the Kondo resonance is always fixed to the Fermi level irrespective of the detailed structure of the bare dot-levels.