Study Of The Thermoelectric Through Quantum Dots System

Thermoelectric material is a functional material which can convert thermal energy and electrical energy,which can act as refrigeration,cooling,or thermal energy sensors and be used in almost all fields such as military,aerospace,biology,medicine and industrial.In conventional solid material,the application range of the thermoelectric technology is limited for restriction of Wiedemann-Franz law and Mott relation.In recent years,the low dimensional thermoelectric material was widely used due to the high thermoelectric figure of merit.How to enhance performance of the thermoelectric device? How to manipulate the spin polarized parameter and influence the properties of thermoelectric transport? Thermoelectric effect in nanostructures has attracted much interest.In this thesis,based on the non-equilibrium Green function,we investigate the thermoelectric effect in QD systems.Rashba spin-orbit coupling effect,electron-electron interaction,electron-photon interaction and intradot spin-flip scattering are also discussed in this work.Following are the main content of the thesis:First,We theoretically investigate the thermoelectric transport properties through triple quantum dots attached to a metal and a superconducting electrodes in the linear response regime.We calculate thermoelectric quantities and analyze their dependence on the energy gap,interdot coupling and Coulomb interaction.Our results show that at low temperature the superconducting electrode suppresses the thermal conductance and enhances the thermopower outside the gap,which favors the improvement of figure of merit.The system considered in the work shows the rich physics in transport process,such as quantum interference,Coulomb blockade,Andreev reflection and bipolar effect,provide more ways for optimizing thermoelectric efficiency.The interdot tunneling coupling not only induces the interference effect,but also broadens the molecular-like level spacing and reduces the thermal conductance near the outer edge of the gap,as a result,the thermopower is greatly increased near outer edge of the gap.Coulomb interaction lowers effectively the thermal conductance near the thermopower peaks and improve figure of merit.In particular,when the interference effect and Coulomb blockade effect coexist,the values of figure of merit can be greatly enhanced.Then,we investigate the thermoelectric properties of a ferromagnet-quantum dot-superconductor hybrid system with the external microwave field.The intradot spin-flip scattering induces effective level splitting of the QD and the microwave field leads to the photon-assisted transport channels.Our results indicate that the increase of figure of merit outside the gap is significantly when the spin flip scattering strength increases.Choosing appropriate spin-flip scattering strength,microwave field strength and frequency can significantly enhances the figure of merit of thermoelectric conversion of the device,which can be used as a scheme improving thermoelectric efficiency using microwave frequency.Finally,We have studied spin-dependent thermoelectric transport through parallel triple quantum dots with Rashba spin–orbital interaction(RSOI)in the linear response regime.Rashba spin–orbital interaction can induce the spin-dependent phase factor,which has been used to produce the spin-dependent thermoelectric efficiency with magnetic flux phase.Under the appropriate configuration of magnetic flux phase and RSOI phase,the spin figure of merit can be enhanced and is even larger than the charge figure of merit.The contour graphs of spin-dependent thermopower present double-peak structures in the specific region.While the charge and the spin thermopowers present quadruple-peak structures.For some specific configuration of the two phases,the device can provide a mechanism that converts heat into a spin voltage when the charge thermopower vanishes while the spin thermopower is not zero,which is useful in realizing the thermal spin battery and inducing a pure spin current in the device.