Thermoelectric And Thermospin Effects Of Coupled Quantum Dots

Thermoelectric material is a functional material which can convert electricalenergy and thermal energy. In recent years, the low dimensional thermoelectricmaterial was widely used due to the high thermoelectric figure of merit. Theinvestigation of the spintronic in low dimensional material and system makes usknown that the thermoelectric effect is very sensitive to the spin polarizedtransportation in quantum dot system. Theoretical and experimental works havebeen devoted to investigate it. The most important issues in the field ofthermoelectric and thermospin effects are: manipulation of the spin polarizedparameter; how does spin polarization influence the thermal and electrical energyconversation; how to enhance performance of the thermoelectric device. In thisthesis, based on the non-equilibrium Green function and Dyson equation, weinvestigate the thermoelectric effect, thermospin effect and spin-dependentthermoelectric effect. Rashba spin-orbit coupling effect, Zeeman splitting effect andnon-uniform magnetic field effect are also discussed in this work.Firstly, we theoretically investigate the spin-dependent thermopower of theparallel coupled double quantum dots system in the presence of the Rashbaspin-orbit interaction. The results show that the coaction of the Rashba spin-orbitinteraction and the mangetic field flux is the main reason for tunning the magnitudeand sign of the thermopower. The intradot Coulomb interaction can make the sign ofthe thermopower stable in the whole period, and the type of the material carrier arealso can be tunned. In this case, the optimize work temperature for thermopower isapproach to the bandwidth function between dots and leads. It is found that thespin-dependent thermopower is different from the charge thermopower and the spinpolarization phenomenon occur due to the existence of the Rashba spin-orbitinteraction and the magnetic field flux.Secondly, we investigate the thermospin effect of an Aharonov-Bohm (AB)double quantum dot ring considering magnetic field flux, Zeeman splitting andRashba spin-orbit interaction. The results show that the coaction of these parametersmake the themoelectric and thermospin effect become obviously due to the Fanointerference. In the molecular reprsentation, the transmission resonance movetoward the bonding energy state and antibonding energy state due to the correctionof the Zeeman splitting and Coulomb interaction to the eigenstate of the quantumdot molecule states. With increasing temperature, the Seebeck coefficient signreversed when the chemical potential sweep the molecule state, it is related to the Fano resonance. The structure parameter also can influence the spin-dependenttransportation and the spin thermoelectric figure of merit. The results also show thatthe Zeeman splitting is one of the key factor to influence the thermoelectric figure ofmerit of the system.Thirdly, we theoretically study the non-uniform magnetic field effect oftemperature difference induced spin current in parallel coupled double quantum dotsytem. The influence of the spin-dependent interdot tunnel coupling strength,magnetic field flux difference and Zeeman splitting difference on the temperaturegradient induced spin polarized current was investigated by us. The magnetic fieldon the tunnel junction can induce spin flip. When the quantum dot energy is in thevicinity of Fermi energy, the temperature difference can induce spin current for while S=0in the case of=0and with increasing of δ, the charge Seebeckcoefficient oscillate with quantum dot energy. In the case of BRB≠0, the Spin Seebeckcoefficient oscillate with quantum dot energy and with increasing δ, the amplitudeand sign changed. At last, the result about the influence of Zeeman splittingdifference on the magnitude and sign of the spin-dependent Seebeck coefficient wasalso investigated.Finally, the magnetic field dependence of thermoelectric effect in the parallelcoupled three quantum dots molecular system was investigated. The results showthat the violation of the Wiedeman Franz law due to the coaction of the magneticflux and Zeeman splitting; it is very significant to find that the thermoelectric figureof merit decrease with increasing interdot tunnel coupling strength which means thatthe multi-channel system suppress the thermoelectric conversion efficiency;comparing with different three quantum dots system, the double coupled quantumdots system is found to be a good reference device for designing a betterthermoelectric conversion device.In conclusion, we systematically investigate the spin-dependent thermoelectrictransport properties for coupled quantum dots system and the parameters which cangenerate spin polarization on the system. It is clarified that these kind of parametersimprove the development of the enhancement of thermoelectric materialperformance. We anticipate that our research can provide the theoretical referencefor designing and manufacturing spin-polarization, thermoelectric generator andcooler devices.