Thermoelectric Effect Of Quantum Dot System

In this thesis, we carried out theoretical investigations on thermoelectric effectof an interacting quantum dot with tunnel-coupling to two electrodes by means ofKeldysh's nonequilibrium Green function technique and the Anderson-type Hamil-tonian. Here our investigations focused on two kinds of systems. One is a singlemolecular quantum dot, and the other is a semiconductor two-level quantum dotsystem. In the first system we considered the electron-phonon interaction (EPI)between electrons and a single mode phonon. In the second system we consideredthe Coulomb interaction between electrons in the two energy levels. Our aim isto explore the thermoelectric effect of quantum dot system, and to obtain somevaluable insights in designing new nanoscale quantum thermoelectric devices withoptimal properties.In chapter 1, we introduced some basic knowledge of mesoscopic system and re-viewed the recent development of research on quantum dot system. In this chapter,we also brieffy introduced the main theoretical method used in this thesis, nonequi-librium Green function technique, and the main physical quantities used to describethermoelectric effect.In chapter 2, we investigated the nonlinear thermoelectric effect of a two-levelquantum dot system with inter-level Coulomb interaction. We calculated the nonlin-ear heat current through the quantum dot when a temperature difference is appliedbetween the two electrodes, and found that in some cases the heat current displaysa strong asymmetrical behavior, which indicates a thermal rectification effect. Weanalyzed further this effect in detail, and ascribed it to the asymmetrical couplingsbetween two electrodes and two levels: only when the upper level is open for trans-port and meanwhile the lower level is blocked, the rectification effect may occur. Besides, our numerical calculations shown that the strong Coulomb interaction isnecessary for the appearance of the rectification effect.In chapter 3, we investigated the thermoelectric effect of a single level quantumdot with EPI by examining conductance, thermopower and thermal conductance ofthe quantum dot in the linear regime. It is shown that the strong EPI obviouslydecreases the linear conductance and thermal conductance. We also calculatedthe nonlinear current when a bias voltage is applied between two electrodes, andfound that the tunneling current exhibits a stepwise behavior due to phonon-assistedtunneling. In the case of strong EPI, the current at weak bias-voltage range issignificantly suppressed owing to the Franck-Condon blockade.