Shot Noise in a Mesoscopic Interferometer

The charge conductance and the shot noise in an Aharonov--Bohm interferometer with double quantum dots embedded and coupled to each other by a capacity are studied in the framework of the equation of motion of Green’s function. From the impurity Anderson model Hamiltonian, the equations of motion of nonequilibrium Green functions are derived and solved including the effects of two body correlations under Lacroix’s approximation. Our results show that the conductance, the shot noise, and the Fano factor (the ratio of the shot noise to the Poisson noise) as functions of the magnetic flux oscillate with the period of h/e, and their oscillation behaviour is similar to the results of the experiment replacing the capacitive coupling by tunnelling between the two dots. The experiment is suggested to test the results.     

Fano--Kondo Effect in a Triple Quantum Dots Coupled to Ferromagnetic Leads

Using the Keldysh nonequilibrium Green function and equation-of-motion technique, we have qualitatively studied the spin-dependent transport of a triple-QD system in the Kondo regime. It is shown that the Kondo resonance and Fang interference coexist, and in this system the Fang Kondo effect shows dip behaviours richer than that in the T-shaped QDs. The interdot coupling, the energy level of the side coupled QDs and the spin polarization strength greatly influence the DOS of the central quantum dot QDo. Either the increase of the coupling strength between the two QDs or that of the energy levels of the side coupled QDs enhances the Kondo resonance. Especially, the Kondo resonance is strengthened greatly when the side dot energy is fixed at the Fermi energy. Meanwhile, the Kondo resonance splits for the spin-up and spin-down configurations due to the polarization： the down-spin resonance is enhanced, and the up-spin resonance is suppressed.     

Spin-Polarized Transport through Parallel Double Quantum Dots Coupled to Ferromagnetic Leads

We theoretically study the spin-polarized transport phenomena of the parallel double quantum dots coupled to two ferromagnetic leads by the Anderson Hamiltonian. The Hamiltonian is solved by means of the equation-of-motionapproach. We analyse the transmission probability of this system in both the equilibrium and nonequilibrium cases, and our results reveal that the transport properties show some noticeable characteristics depending upon both the spin-polarized strength p and the value of the magnetic flux Φ. Moreover, in the parallel configuration, the position of the Kondo peak shifts while it remains unchanged for the antiparallel configuration. These effects might have some potential applications in spintronics.     

Spin Pumping from a Quantum Dot in the Presence of Decoherence

We study the pumped spin current of an interacting quantum dot tunnel coupled to a single lead in the presence of electron spin resonance （ESR） field. The spin decoherence in the dot is included by the Bffttiker approach. Using the nonequilibrium Green＇s function technique, we show that ESR-induced spin flip can generate finite spin current with no charge transport. Both the Coulomb interaction and spin decoherence decrease the amplitude of spin current. The dependence of pumped spin current on the intensity and frequency of ESR field, and the spin decoherence is discussed.     

Kondo and Coulomb Interaction Effects in Spin-Polarized Transport through Double Quantum Dots

By means of the slave-boson mean-field approximation, we theoretically investigate the Kondo and Coulomb interaction effects in spin-polarized transport through two coupled quantum dots coupled to two ferromagnetic leads by the Anderson Hamiltonian. The density of states is calculated in the Kondo regime for the effect of the interdot Coulomb repulsion with both parallel and antiparallel lead-polarization alignments. Our results reveal that the interdot Coulomb interaction between quantum dots greatly influence the density of states of the dots.     

Multiterminal Conductance and Decoherence Effect of a Three-Terminal Kondo Dot

A three-terminal Kondo dot modelled by the Anderson Hamiltonian is investigated. In the strong correlation limit, we calculate the multiterminal conductance and the voltage-induced characteristic splitting of the nonequilibrium Kondo resonance by using the equation of motion approach from viewpoint of the correlation dynamics. A qualitative and reasonable agreement with a recently reported experiment is obtained. We also simulate phenomenologically the decoherence of the Kondo-coherent state formed in the two-terminal setup in the framework of our three-terminal model.     

Intradot spin-flip Andreev reflection tunneling through a ferromagnet-quantum dot-superconductor system with ac field

We investigate Andreev reflection (AR) tunneling through a ferromagnet-quantum dot-superconductor (F-QD-S) system in the presence of an external ac field. The intradot spin-flip scattering in the QD is involved. Using the nonequilibrium Green function and BCS quasiparticle spectrum for superconductor, time-averaged AR conductance is formulated. The competition between the intradot spin-flip scattering and photon-assisted tunneling dominates the resonant behaviors of the time-averaged AR conductance. For weak intradot spin-flip scattering strengths, the AR conductance shows a series of equal interval resonant levels. However, the single-peak at main resonant level develops into a well-resolved double-peak resonance at a strong intradot spin-flip scattering strength. Remarkable, multiple-photon-assisted tunneling that generates photonic sideband peaks with a variable interval has been found. In addition, the AR conductance-bias voltage characteristic shows a transition between the single-peak to double-peak resonance as the ratio of the two tunneling strengths varies.     

Spin-polarized current through a lateral double quantum dot with spin–orbit interaction

We study the spin-polarized current through a vertical double quantum dot scheme. Both the Rashba spin–orbit (RSO) interaction inside one of the quantum dots and the strong intradot Coulomb interactions on the two dots are taken into account by using the second-quantized form of the Hamiltonian. Due to the existence of the RSO interaction, spin-up and spin-down electrons couple to the external leads with different strengths, and then a spin polarized current can be driven out of the middle lead by controlling a set of structure parameters and the external bias voltage. Moreover, by properly adjusting the dot levels and the external bias voltages, a pure spin current with no accompanying charge current can be generated in the weak coupling regime. We show that the difference between the intradot Coulomb interactions strongly influences the spin-polarized currents flowing through the middle lead and is undesirable in the generation of the net spin current. Based on the RSO interaction, the structure we propose can efficiently polarize the electron spin without the usage of any magnetic field or ferromagnetic material. This device can be used as a spin-battery and is realizable using the present available technologies.     

Voltage-controllable spin-polarized transport through parallel-coupled double quantum dots with Rashba spin-orbit interaction

A spin device, consisting of parallel-coupled double quantum dots and three normal metal leads, is proposed to realize spin-polarized current without the help of magnetic field and magnetic material. Based on the Keldysh nonequilibrium Green function technique and equation of motion method, the spin-dependent current formula in each lead is derived. It is shown that not only a fully polarized current but also a tunable pure spin current can be obtained by modulating the structure parameters, strength of Rashba spin-orbit interaction and bias voltages properly. It further demonstrates the dependence of the spin-polarized current on the strength of the Rashba spin-orbit interaction.     

Thermal Rectification Effect of an Interacting Quantum Dot

We investigate the nonlinear thermal transport properties of a single interacting quantum dot with two energy levels tunnel-coupled to two electrodes using nonequilibrium Green function method and Hartree-Fock decoupling approximation. In the case of asymmetric tunnel-couplings to two electrodes, for example, when the upper level of the quantum dot is open for transport, whereas the lower level is blocked, our calculations predict a strong asymmetry for the heat （energy） current, which shows that the quantum dot system may act as a thermal rectifier in this specific situation.     

Influence of Strain-Reducing Layer on Strain Distribution of Self-Organized InAs/GaAs Quantum Dot and Redshift of Photoluminescence Wavelength

A systematic investigation about the strain distributions around the InAs/GaAs quantum dots using the finite element method is presented. A special attention is paid to influence of an Ino.2 Gao.sAs strain reducing layer. The numerical results show that the horizontal- and vertical-strain components and the biaz~ial strain are reinforced in the InAs quantum dot due to the strain-reducing layer. However, the hydrostatic strain in the quantum dot is reduced. In the framework of eight-band k · p theory, we study the band edge modifications due to the presence of a strain reducing layer. The results demonstrate that the strain reducing layer yields the decreasing band gap, i.e., the redshift phenomenon is observed in experiments. Our calculated results show that degree of the redshift will increase with the increasing thickness of the strain-reducing layer. The calculated results can explain the experimental results in the literature, and further confirm that the long wavelength emission used for optical fibre communication is realizable by adjusting the dependent parameters. However, based on the calculated electronic and heavy-hole wave function distributions, we find that the intensity of photoluminescence will exhibits some variations with the increasing thickness of the strain-reducing layer.     

Triple Quantum Dot Molecule as a Spin-Splitter

We propose a spin-splitter composed of triple quantum dots that works due to the Coulomb blockade effect and the charge and spin biases applied on external electron source and drains. The spin biases are applied only on the two drains and give their spin-dependent chemical potentials, which act as the driving forces for electron spin-polarized transport. By tuning the biases and the dots＇ levels, spin-up and spin-down electrons can be simultaneously split or separated from the source into two different drains. We show that such a tunneling process is detectable in terms of the spin accumulations on the dots or the currents flowing through the external leads. The present device is quite simple and realizable within currently existing technologies.     

ac field-induced Fano resonances in a parallel-coupled double quantum dot system

The effects of an ac electric field on the Fano resonance in a parallel-coupled double quantum dot system are investigated theoretically. The field can induce the photon-assisted Fano resonances for both symmetrical and asymmetrical parallel configurations. The magnitude and position of the photon-assisted Fano peak can be tuned by the ac field strength and frequency, respectively. Furthermore, the Fano resonance can appear with increasing the field frequency for both the symmetrical and asymmetrical configurations. This provides an efficient mechanism to control the Fano resonance. The photon-electron pumping effects for the symmetrical and asymmetrical cases are also studied in the weak- and strong-coupling regime.     

Transport through a quantum dot subject to spin and charge bias

Spin and charge transport through a quantum dot coupled to external nonmagnetic leads is analyzed theoretically in terms of the non-equilibrium Green function formalism based on the equation of motion method. The dot is assumed to be subject to spin and charge bias, and the considerations are focused on the Kondo effect in spin and charge transport. It is shown that the differential spin conductance as a function of spin bias reveals a typical zero-bias Kondo anomaly which becomes split when either magnetic field or charge bias are applied. Significantly different behavior is found for mixed charge/spin conductance. The influence of electron-phonon coupling in the dot on tunneling current as well as on both spin and charge conductance is also analyzed.     

Nonlocal Andreev reflection in a three-terminal Aharonov-Bohm interferometer

We theoretically report a nonlocal Andreev reflection in an Aharonov-Bohm interferometer, which is a three-terminal normal metal/superconductor (NS) mesoscopic hybrid system. It is found that this nonlocal Andreev reflection is sensitive to the systematic parameters, such as the bias voltages, the quantum dot levels, and the external magnetic flux. If we set the chemical potential of one normal metal lead equal to zero, the electronic current in the lead results from two competing processes: the quasiparticle transmission and nonlocal Andreev reflection. The appearance of zero electronic current signals unambiguously the existence of this nonlocal Andreev reflection.     

Charge dynamics effects in conductance through a large semi-open quantum dot

Fano lineshapes in resonant transmission in a quantum dot imply interference between localized and extended states. The influence of the charge accumulated at the localized levels, which screens the external gate voltage acting on the conduction channel is investigated. The modified Fano q parameter and the resonant conduction is derived starting from a microscopic Hamiltonian. The latest experiments on ‘charge sensing’ and ‘Coulomb modified Fano sensing’ compare well with the results of the present model.     

Thermoelectric Phenomena in a Quantum Dot Attached to Ferromagnetic Leads in Kondo Regime

We have studied the thermoelectric properties through ferromagnetic leads-QD coupled system (F-QD-F) in the Kondo regime by nonequilibrium Green's functions method. The spin-flip effect induced by ferromagnetic leads and Kondo effect influence the thermoelectric properties significantly. The peak-valley structure emerges at the low temperature due to Kondo resonance, and the peak-valley structure also relies on the polarization angle θ, the spin-dependent linewidth function Γ_γσ and the energy level of QD ε_d. Novel resonant peak also emerges in the curve of ZT_c versus polarization angle θ. The Kondo effect suppresses the figure of merit ZT_c and the spin-dependent figure of merit ZT_s. In addition, the spin-dependent figure of merit ZT_s is relate with the gap between Γ_γ↑ and Γ_γ↓.     

Thermoelectric Phenomena in a Quantum Dot Attached to Ferromagnetic Leads in Kondo Regime

We have studied the thermoelectric properties through ferromagnetic leads-QD coupled system(F-QD-F)in the Kondo regime by nonequilibrium Green's functions method. The spin-flip effect induced by ferromagnetic leads and Kondo effect influence the thermoelectric properties significantly. The peak-valley structure emerges at the low temperature due to Kondo resonance, and the peak-valley structure also relies on the polarization angle θ, the spindependent linewidth function Γγσ and the energy level of QD ?d. Novel resonant peak also emerges in the curve of ZTc versus polarization angle θ. The Kondo effect suppresses the figure of merit ZTc and the spin-dependent figure of merit ZTs. In addition, the spin-dependent figure of merit ZTs is relate with the gap between Γγ↑and Γγ↓.     

Conductance in an Aharonov-Bohm Interferometer with Parallel-Coupled Double Dots

We present a theoretical study of the conductance in an Aharonov-Bohm interferometer containing two coupled quantum dots. The interdot tunneling divides the interferometer into two coupled subrings, where opposite magnetic fluxes are threaded separately while the net flux is kept zero. Using the Green function technique we derive the expression of the linear conductance. It is found that the Aharonov-Bohm effect still exists, and when the level of each dot is aligned, the exchange of the Fano and Breit-Wigner resonances in the conductance can be achieved by tuning the magnetic flux. When the two levels are mismatched the exchange may not happen. Further, for some specific asymmetric systems where the coupling strengths between the two dots and the leads are not equal, the flux can change the Fano resonance into an antiresonance, which is absent in symmetric systems.     

Single Impurity Scattering in One-Channel Spinless Luttinger Liquid

Using bosonization and phase shift representation, we rigorously treat backward scattering of electrons on an impurity in a one-dimensional interacting electronic system, and demonstrate that correlation exponents of the system depend on a phase shift induced by the backward scattering, and usual exponent duality of the correlation functions between ultraviolet and infrared fixed points comes from the phase shift dependence of the correlation exponents. Finally, we study the tunnelling conductance of the system at zero temperature and obtain a modified Landauer-Bütiker formula.