The Electronic Transport Properties In The Coupled Quantum Dot Systems

Semiconductor quantum dot(QD) is a quasi-zero-dimensional mesoscopic structure, which presents discrete electron energy spectrum and strong electron interaction. Because of such electronic characteristics, a single QD is viewed as an artificial atom. Accordingly, a structure consisting of several coupled QDs confines electrons in a way as an artificial molecule. QDs can be incorporated into a mesoscopic circuit. Thus, the electron structure in QDs can be revealed by the observation of the electronic transport spectrum. Meanwhile, the controllable electronic transport properties through a variety of QD structures suggest many promising device applications. For example, some multiple QD structures are recently considered as the device prototype to realize the quantum computation. In contrast to a single QD, multiple QD structures possess more structural parameters to tune their electronic transport properties. Therefore, the investigation on the multiple QD structures are the current focus in the field of mesoscopic phyics.In this thesis we report our theoretical investigations about the elctronic transport through sveral typical multiple QD structures, by means of non-equilibrium Green function technique.It is well-known that quantum interference plays a dominant role in the electronic transport process through a mesoscopic structure. As a typical mesoscopic structure, coupled multiple QD structures provide a variety of Feynman paths to take part in the quantum interference. For instance, Feynman paths are allowed to include the Kondo level due to electron correlation, as well as the realistic single-particle levels in QDs. As a result, the quantum interference among these distinct Feynman paths brings about novel electronic transport properties. We will focus on the quantum interference which is the underlying mechanism for the electronic transport properties through several different multiple QD structures. Below we outline our works briefly.First, we studied systematically the Fano effect in the electron transport process through a parallel double QD structure. By establishing an expression of the linear conductance in the standard Fano form, we analyzed the presence or absence of the Fano antiresonance in various double QD structures. Then, we presented a detailed discussion on the adjustment of the applied fields on the Fano lineshape. In addition, we clarified the electron transmission paths to take part in the Fano resonance in the language of Feynman path. We found that there are generally infinite Feynman paths to contribute to the Fano interference. However, by tuning the magnetic field or the gate voltage, one can realize a very simple situation in which only two lowest-order paths remain. Only in such a simple situation, can the Fano resonance be understood as the quantum interference between two electron transmission paths, one of which is resonant and another nonresonant. Or to say, one is the 'more' resonant path and another the 'less' resonant path if two QD levels are simultaneously tuned by a gate voltage. On the contrary, in the absence of a magnetic field the high-order Feynman paths have nontrivial contributions to the Fano lineshape of the conductance spectrum.Secondly, by means of a generalized Anderson impurity Hamiltonian, we investigated the effect of e-p interaction on the Fano lineshape in the linear conductance spectrum in the parallel double QD structure. We considered the local and nonlocal phonon modes on an equal footing. We derived an expression about linear conductance in the presence of e-p interaction. And we found that the inelastic conductance vanishes in the zero bias and zero temperature case. Both phonon modes are typical in most QD structures. But they influence the Fano lineshape of the linear conductance spectrum distinctly: The nonlocal phonon can give rise to the multiple Fano peaks, since it can provide inter-dot electron tunneling path; Conversely, the local phonons mainly shift the single Fano peak without destroying its lineshape. For the case of finite temperature, these properties are kept even though the complex inelastic scattering process.Thirdly, Fano interference in the T-shaped double quantum dot structure affected by the Rashba spin-orbital (SO) coupling on QD was theoretically studied. By second-quantizing the electron Hamiltonian in this structure, it was found that the Rashba interaction brings about a spin-flip interdot hopping term. This term hybridizes the electron states in QDs with opposite spins to form the eigenstates of the QD molecule. As a result, the QD levels, the eigenenergies of the double-QD molecule and the couplings between these eigenstates and leads can be adjusted by the Rashba SO coupling. These results influence the Fano lineshape in the linear conductance spectrum. Moreover, the effects of Rashba interaction on the Fano lineshape remain even when the many-body effect is taken into account.Fourthly, we investigated the electronic transport through a laterally coupled double-QD chain. The calculated linear conductance spectrum exhibits a well-defined insulating band. To form such a very steep band does not require a very long chain, though its width coincides with the gap between the bonding and antibonding bands of an infinite double-QD chain. The underlying physics for the formation of the well-defined insulating band is the antiresonance effect arising from the existence of the attached QDs. Each double-QD molecule can be regarded as an antiresonance cell. The electron transmission in the antireso- nance valley is further suppressed whenever the electrons pass through one more antiresonance cell. As a result, the insulating band forms rapidly. When N=4, which is still a quite short chain, the band edge becomes very steep and almost fixed. Moreover, the influence of the many-body effect on the well-defined insulating band is discussed in the Hartree-Fock and second-order approximations. Our conclusion is that the many-body effect does not influence notably the shape of the insulating band. On the basis of this feature, we suggested a potential device application of such a structure as a spin filter. Because of the existence of the steep band edges, it is very easy to create a highly spin-polarized window when the electrons tunnel through this structure. In addition, selecting smaller QDs is advantageous for the realization of a spin filter, because the stronger electron interaction can enlarge the width of the spin-polarized window.Finally, the electron transport properties in a three-terminal triple-quantum-dot ring were theoretically studied with a focus on the spin dependent of the current flow in the respective channels. With the help of local Rashba SO coupling, an incident electron from one terminal can select a specific terminal to depart from the QD ring according to its spin state. Accordingly, spin polarization and spin separation can be simultaneously realized in this structure. Furthermore, with a specific bias, it is possible to obtain the tunable charge and spin currents. We find that a pure spin current without an accompanying charge current appears even at zero magnetic field case. The polarization direction of the spin current can be inverted by altering the bias voltage. In addition, by tuning the magnetic field strength, the charge and spin currents reach their respective peaks alternately.