【作者】 薛惠杰；

【导师】 吕天全；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2012， 博士

【摘要】 热电材料是能够将电能和热能相互转换的功能性材料，人们利用热电材料制造的设备能够实现发电和制冷的目的。近年来，由于低维材料结构具有高的能量转换效率而被广泛研究。关于低维结构中电子自旋极化输运问题的研究发现电子的自旋极化可以影响低维量子结构的热电效应，尤其是对量子点体系的自旋极化热电输运的影响极为明显。研究自旋极化对量子点系统热电效应和热自旋效应的影响，需要解决的问题是：有效控制产生自旋极化的参量；自旋极化对热和电输运的影响程度；如何更恰当的提高热电材料器件的性能。因此，论文利用非平衡态格林函数和Dyson方程方法，研究耦合量子点体系热电效应，自旋相关热电效应和热自旋效应等问题，并讨论了Rashba自旋轨道耦合效应、量子点中磁场塞曼劈裂和非均匀磁场效应等多种因素的作用下，平行耦合量子点体系的自旋相关热电传输特性。首先，研究了Rashba自旋轨道耦合影响平行耦合双量子点系统热电势的物理机制。研究结果表明，由于Rashba自旋轨道耦合的存在而产生自旋相关相位因子影响电子的输运特性。Rashba自旋轨道耦合和磁场通量共同作用是导致热电势的符号和大小改变的主要原因，而且在选择适当Rashba自旋轨道耦合诱生的相位因子和磁场通量相位因子情况下，对于某些确定的量子点内电子-电子库仑相互作用，可以得到符号不变的热电势,即材料导电类型相对固定；同时，这些参量对热电势出现绝对值最大值的工作温度也有影响，在温度近似等效于量子点和电极之间耦合的展宽能级时，热电势绝对值达到最大值。在研究中还发现，由于Rashba自旋轨道耦合的存在，Rashba自旋轨道耦合与磁场共同作用能够产生电子的自旋分离现象，从而可以产生自旋依赖的热电效应现象，并发现自旋依赖热电势和电荷热电势随量子点体系参量的变化规律明显不同。对镶嵌量子点的Aharonov-Bohm(AB)干涉系统的热自旋效应进行了研究。考虑磁场作用下量子点中电子能级的塞曼劈裂、Rashba自旋轨道耦合和磁场通量的共同作用下，耦合双量子点体系热电和热自旋现象呈现出新特征。研究结果表明由于塞曼劈裂和库仑相互作用，电子透射共振峰值分别向体系的分子成键态和反键态方向移动。当量子点分子态能级高于和低于电极中电子化学势的两种情形下，体系的塞贝克系数符号相反，系统塞贝克系数受电子透射谱中Fano共振的调制。而且量子点和电极之间的非对称耦合同样可以对系统热自旋输运特性产生影响，从而影响自旋热电优值。研究也表明塞曼劈裂是影响耦合双量子点体系热电优值的关键因素之一。对非均匀磁场作用下的平行耦合双量子点系统的微小温差产生自旋流的特性进行了研究。发现量子点之间隧穿结上施加的反向磁场能使自旋相关塞贝克系数产生自旋倒逆现象，从而可以通过改变量子点之间隧穿结上的磁场方向来改变自旋塞贝克系数的符号。如果将量子点能级调节到接近电极的费米能级附近某一区间内，两电极的温差可以产生自旋流；Rashba自旋轨道耦合引起的相位因子BRB=0时，自旋塞贝克系数为零，系统温差无产生自旋流的能力，随着两左右AB环中磁场通量差值引起的相位差值δ的增加，量子点能级的改变使电荷塞贝克系数随之发生振荡变化，当BRB≠0时，电荷塞贝克系数随着δ的增加，幅值和频率变化很小，而自旋塞贝克系数随量子点能级的变化振荡加剧；Rashba自旋轨道耦合强度对自旋塞贝克系数的影响表现为最大值和出现最大值的量子点能级位置发生改变；研究表明自旋相关的量子点间隧穿耦合强度是影响体系热电优值的另一个关键因素。最后，研究了磁场对平行耦合三量子点系统的热电和自旋相关热电效应的影响。结果表明，磁场通量和塞曼场共同作用使系统不再满足维德曼-弗朗兹定律。通过对多通道系统的分析发现，随着点间隧穿耦合强度的增加，系统热电优值降低，说明多通道系统抑制了系统热电优值的增加，这对设计更好的热电转换设备具有很重要的参考价值。通过与不同构型三量子点体系的计算结果对比发现，耦合双量子点结构是获得高热电优值较理想的体系。总之，论文考虑点间隧穿耦合、Rashba自旋轨道耦合、磁场通量、磁场塞曼劈裂以及量子点与电极之间隧穿耦合强度等参量的作用，深入研究了耦合量子点系统的热电和热自旋传输特性。也指出了如何通过调节体系的结构参量和外部条件来提高系统的热电效应和热自旋效应的途径。期望对具有广阔应用前景的自旋电子设备、热电制冷和发电设备的研制提供可供参考的有价值的理论依据。更多还原

【Abstract】 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.更多还原