T-shaped Quantum Research Articles

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.

Fano-type resonances induced by a boson mode in Andreev conductance**Project supported by the National Center of Science (Grant No. NN202 263138).

We study spectroscopic signatures of a monochromatic boson mode interacting with a T-shape double quantum dot coupled between the metallic and superconducting leads. Focusing on a weak interdot coupling, we find that the proximity effect together with the bosonic mode are responsible for the series of Fano-type resonances appearing simultaneously at negative and positive energies. We investigate these interferometric features and discuss their influence on the subgap Andreev conductance taking into account the correlation effects driven by the Coulomb repulsion.

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

Anomalous excitation-power-dependent photoluminescence of InGaAsN/GaAs T-shaped quantum wire

The selected InGaAsN/GaAs T-shaped quantum wire (T-QWR) fabricated by metal organic vapor phase epitaxy has been investigated by microphotoluminescence (µ-PL) and excitation-power-dependent µ-PL. The optical characteristics of one-dimensional structure were taken at low-temperature (4 K) and room temperature (RT) to clarify the intersection of two familiar quantum wells (QWs) in the [001] and [110] directions, named QW1 and QW2, respectively. For the excitation-power-dependent measurement, the intensity of the excitation source was used in the range of 0.001I0 to I0. The result shows that all of emissions related to QW1 and QWR peaks have a nonsymmetric line shape as evidenced by the tailing on the lower-energy side. All peaks shift to higher-energy side (blueshift) with the increase of the excitation power intensity. The blueshift and the low-energy tailing of PL peaks are attributed to the alloying effect. However, the emission peak related to QWR region shows a larger blueshift rate than that of QW1 on increasing of the excitation power intensity. This is an anomalous characteristic for the low-dimensional structure, affected by the large fluctuation state in the QWR region. This fluctuation state is combined of both edges of QWs (QW1 and QW2).

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.

Thermoelectric Properties of a T-Shaped Quantum Dot with Relatively Strong On-Site Coulomb Interaction

The thermoelectric properties of a correlated T-shaped quantum dot system symmetrically coupled to two metallic leads are studied. This system is described by a generalized Anderson model and an approximation of correlation dynamics is employed to derive the corresponding Green’s functions. The properties are investigated by calculating the electrical conductance G, thermal conductance ¬, thermopower S and figure of merit ZT for different gate voltages and temperatures. At last but not least, our numerical calculation results show that the thermoelectric properties of the T-shaped quantum dot with relatively strong on-site Coulomb interaction are mainly influenced by the bipolar effect, which is consistent with some theoretical and experimental results. [doi:10.2320/matertrans.M2013460]

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.

Refractive index change and absorption coefficient of T shaped quantum wires: comparing with experimental results

In this work, we have studied the optical properties of a GaAs/Al $$_{x}$$ Ga $$_{1-x}$$ As T-shaped quantum wire. In this regard, we have obtained the refractive index change and absorption coefficient. We have also studied the temperature effect on absorption coefficient. The results show that the refractive index change and absorption coefficient decrease and shift towards higher energies when temperature increases. Finally, to check our results, we have compared the absorption coefficient obtained from this work with experimental data at 5 K. Our theoretical prediction has some deviations with experimental data.

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.

Acoustic phonon transport and thermal conductance in quantum waveguide with abrupt quantum junctions modulated with double T-shapedquantum structure

By using the scattering matrix method, the transmission coefficient and thermal conductance of acoustic phonon through a quantum waveguide with abrupt quantum junctions modulated with double T-shaped quantum structure at low temperatures are studied. The results show that at very low temperatures, the double T-shaped quantum structure can enhance low-temperature thermal conductance; contrarily, at higher temperatures, the double T-shaped quantum structure can reduce low-temperature thermal conductance. However, in the whole low-temperature region, the low-temperature thermal conductance can be enhanced by adding the narrowest width c in the scattering region. Moreover, it is found that both the transmission coefficient and thermal conductance can be adjusted by changing the structural parameters of the the scattering region.

Optical study of GaAsN/GaAs and InGaAsN/GaAs T-shaped quantum wires grown by MOVPE

GaAsN/GaAs and InGaAsN/GaAs T-shaped quantum wires (T-QWRs) grown by metal-organic vapor phase epitaxy have been investigated by low-temperature photoluminescence (LT-PL) and temperature dependent PL to verify their optical properties. The T-QWR structure was clearly observed at the intersection of their familiar quantum wells (QWs), namely QW1 and QW2. LT-PL spectra show the related QWR feature located at the lower energy side of their quantum wells related peaks. It is found that the quantum energy state in the T-QWR structure depends on the quantum energy states in QW1 and QW2. It is interpreted that they act as the second confinement barrier of QWR, which is occurred by the effect of the lateral confinement energy. In our case, the lateral confinement energy larger than 100meV was observed for all samples. This is expected to be very useful for carrier confinement at higher temperature. PL spectrum of InGaAsN/GaAs T-QWR shows smaller peak-width and higher intense than that of GaAsN/GaAs T-QWRs. It may be explained by the strain compensation from an In atom incorporation, reducing the lattice distortion and improving the alloy fluctuation in the wire region.

Decoherence effect on Fano line shapes in double quantum dots coupled between normal and superconducting leads

We investigate the Fano-type spectroscopic line shapes of the T-shape double quantum dot coupled between the conducting and superconducting electrodes and analyze their stability on a decoherence. Because of the proximity effect the quantum interference patterns appear simultaneously at $\ifmmode\pm\else\textpm\fi{}{\ensuremath{\varepsilon}}_{2}$, where ${\ensuremath{\varepsilon}}_{2}$ is an energy of the side-attached quantum dot. We find that decoherence gradually suppresses both such interferometric structures. We also show that at low temperatures another tiny Fano-type structure can be induced upon forming the Kondo state on the side-coupled quantum dot due to its coupling to the floating lead.

Quantitative absorption spectra of quantum wires measured by analysis of attenuated internal emissions

An absorption-spectroscopy method that utilizes internal emissions as the source of the probe light was used to measure the absorption spectra of quasi-one-dimensional (q-1D) excitons in T-shaped quantum wires embedded in an optical waveguide. The modal absorption area of the 1D ground-state excitons was estimated to be 0.39 eV cm−1 and was almost independent of temperature in the range 4–150 K. Quantitative evaluation using the absorption spectra revealed that the absorption cross-section per unit length at resonance peak and the spectrally integrated absorption cross-section area per unit length of the 1D ground-state excitons were 1.0 nm and 2.5 × 10−3 eV nm, respectively.

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.

THE COMPARING OF ACOUSTIC PHONON TRANSPORT ABOUT MONOCHROMATIC MODE AND MIXING MODE THROUGH A DOUBLE T-SHAPED QUANTUM WAVEGUIDE

We compare the thermal conductance of SH mode and mixing mode of acoustic phonons in a double T-shaped quantum waveguide at low enough temperatures by using the scattering-matrix method, and analyze the influence from mode conversion. Our results show that the small changes of the geometric parameters can induce different variation on the thermal conductivity, where the effect of the stub height of SH wave is the strongest. Moreover, the mode conversion plays an important role on the thermal conductivity, especially at the higher temperatures, and the coupling effect between two stubs at mixing mode is larger than that at single SH mode.