The Transportation Properties In Nano Quantum Dot Structure

In this thesis, we have studied three quantum-dot-related transportation models. Firstly, spin-polarized current is deduced by time-vary magnetic which localizes in the quantum dot, causing the spin-flip effect. The results show that the total charge current is magnetic independent, while spin-polarized current oscillates with external magnetic field under the large bias and zero temperature. Second model is the quantum dot and mesoscopic ring coupling system, the persistent current in the ring exists in equilibrium, when the quantum point contact measures electron occupation condition in quantum dot, the persistent current will be destroy to vanish no matter how weak coupling between dot and point contact. The result suggests that we can monitor the persistent current in the ring to analysis the coherence of the coupling system. The last model is the multi Aharonov-Bohm ring constructed by two parallel coupled quantum dot with normal metal lead and superconductor lead which pierced by magnetic flux. Using nonequlibrium Green function method, the formula of Andreev reflect current and transmitted current are derived, from which we analyzed the effect of inter dot coupling strength on the peaks of Andreev reflect current. And the new conclusion is that the period of the probability of Andreev reflect current oscillating with total magnetic flux is 2π in the one ring case (Ω = 0),while that is 2π(n +1) in the double rings case (Ω, 0), with n the ratio of two part magnetic fluxes.