Theoretical Study Of Transport Properties Of Coupled Quantum Dots Systems

Due to the tight confinement of charge carriers in all three spatial dimensions, quantum dots are significant for optical and electronic application, as well as attract more attention. In recent years, the experimental field of qubit based on semiconductor quantum dot has been greatly advanced. As a perfect fluorescence probe, quantum dots have great prospect of application in biomedicine domain. Semiconductor laser based on quantum dot also exhibit significantly performance. An essential reason of paying attention to quantum dot is its controllability, which is important for investigating quantum effect and performing quantum computation. Because quantum dots are semiconductors which are confined in all three spatial dimensions, the geometric structure of quantum dots systems can influence the transport properties due to the quantum coherent. On the other hand, electrons in the quantum dots also are affected by external magnetic field, electric field, microwave, and temperature. Consquently, with the control by geometric structure and external field, quantum dot can be reviewed as a laboratory for investigating various quantum effects and building quantum devices. Hence, the controll of the properties of quantum dot has become one of the most popular areas.Based on the nonequilibrium Green's function technique and quantum rate equation method, electronic transport properties of various coupled quantum dot systems are studied theoretically in this dissertation. Meanwhile, with geometric structure and external field, the transport properties are controlled and quantum dot devices are built. The dissertation consists of seven chapters:Chapter one is an introduction for mesoscopic systems and quantum interfere effects in quantum dots systems.Chapter two is an introduction for the nonequilibrium Green's function technique and quantum rate equation method, which are used in this dissertation.In chapter three, we have derived a modified rate equations for the TTQD system contacted with three metallic leads in the presence of a perpendicular magnetic field. The temporary evolutions of the electron-occupation probabilities and the currents following from source to drains are calculated. The results we have obtained show that with the inter-dot coupling be increased the "sticking" effect turns two QDs into a "big QD". The quantum interference results in a decrease of currents and localization of electronin QD1. We further investigate the effect of the multiterminal-interference effect from two output terminals in the period of the I-flux oscillation and find that the stationary currents show periodic oscillations with a period of 2??/?₂₃. And this multi-terminal-interference effect that stems from current correlation of output terminals should be considered in the design of multi-terminal QD devices. On the other hand, after introducing the time-dependent magnetic field, we find it causing a weakness of fundamental period oscillation in the I-flux with the increase of the magnetic field frequency.In chapter four, we have considered the Aharonov-Bohm ring where a quantum dot embedded in its arm with e-p interaction. Using Keldysh Green function method we have investigated the transport current, circulating current and the heat generation, particularly. In the mixed velance region, the circulating current shows strongly dependence on the occupation number, as well as the Fermi distributions. This means that the circulating current can be modulating by the transport current. The effects of the e-p interacton on the circulating current include two parts. The first part is the polaron effect which reduces the circulating current and the transport current. The second part is the slightly contribution due to the three-site hopping process. This contribution is clearly reflected in the thermal oscillation with magnetic field.In chapter five, we propose a closed twoterminal AB interferometer device which contains a DQD. Based on our calculations, we find that this scheme may be a useful way of detecting the formation of coherent molecular states in DQD. Moreover, our results give also a clear picture of the transition between the merged artificial molecular states and the nearly decoupled states. By comparison with an open multiterminal AB interferometer, one concludes that the behavior mentioned above is due to the phase locking effect in the closed two-terminal geometry. This scheme could be of practical use in future quantum detection and the operation of qubit states.In chapter six, we investigate the temporary evolutions of the current of the one dimension quantum dots array, as well as the transport current and persistent current of the two dimension quantum dot array in the magnetic field. We find that with the increase of the one dimension quantum dot array, the stationary currents decrease and the response time of current to voltage become longer. With the influence of the magnetic field, the transport current and the persistent current are coexist and interplay in the two dimension quantum dots array. And the transport current and the persistent current both oscillate with the magnetic flux. In the curve of the persistent current, there exist various oscillations with different periods, which arise from quantum interference for different paths.The last chapter presents a summary of this dissertation, and then gives some outlook for the investigation.