Thermoelectric Effects In A Coupled Double Quantum Dot System

The electronic transport properties involving quantum dots are often employed to study the quantum coherence phenomenon. In recent years, with the remarkable advances and development of the complex compounds, mesoscopic scale equipment and nanostructures devices technology, thermoelectric properties of solid-state field, typically in the case of nanoscale systems, have been attracted much attention. The purpose is to improve the efficiency of thermoelectric conversion in mesoscopic and nanometer scale. Therefore, to study the thermoelectric properties and the effects with different parameters on the quantum dot systems are very necessary.In this thesis, within the framework of the non-equilibrium Green functions, thermoelectric effects in a parallel double quantum dot system are studied systematically. We investigate the differences between a parallel double quantum dot and a coupled double quantum dot, and the thermoelectric properties in systems. The works focus on analyzing and comparing electronic conductance, thermal conductance, thermopower and ZT, getting some meaningful results finally. Below we outline our works briefly from two aspects:On the one hand, we discuss the nonresonant and resonant channels for the Fano interference by adding magnetic fields in the QDs. It is found that at low temperature and local magnetic flux with its phase factor Φm=π, Fano line shapes appear in the electronic and thermal conductance spectra. The Fano effect contributes significantly to the enhancement of the thermoelectric efficiency. However, at the same temperature, compared with the case of Φm=0, the thermoelectric effect in the case of Φm=π is much more apparent. We analyze the differences between the quantum interferences in the case of Φm=0and Φm=π via using the concept of Feynman path. It is seen that in the case of Φm=0, Fano interference comes from the quantum interference among infinite order Feynman paths, but it occurs only between two lowest order Feynman paths in the case of Φm=π. The increase of temperature destroys the electron coherent transmission in each path. For this reason in the case of Φm=0, the thermoelectric effects assisted by the Fano interference is easy to be weaken by a little increase of temperature.On the other hand, we discuss the thermoelectric effects in a parallel double quantum dot system by considering the ferromagnetic leads. First of all, we pay attention to the case where the magnetic moment directions of the leads are in the same plane. It is found that at higher temperature, the thermoelectric effect in the system is more seriously restrained. At low temperature, with the increase of magnetic angle0from0to0.75n, spin polarizations can restrain the thermoelectric effect in the system when the spin polarization is changed from0.2to0.8. In the case of θ-π, spin polarizations can influence weakly the thermoelectric effect in the system. However, in the case of full spin polarization, some interesting phenomena emerge, i.e., the thermoelectric effect is apparently enhanced. In addition, we observe that when the system is in the typical energy regions, the spin polarizations can cause the relevant thermoelectric quantities to be independent of temperature increase. Second, we consider the case where the magnetic moments of the two ferromagnetic leads are in different planes. By denoting this angle as φ, we show the effect of the change of φ in the right lead on the thermoelectric effect. We find that the increase of φ can efficiently induce the conductance lineshapes to become sharp, and it also enhances the thermoelectric figure of merit. But the thermopower seems to be independent of the change of φ.