【作者】 杨曦；

【导师】 郭永；

【作者基本信息】 清华大学 ， 物理学， 2015， 博士

【摘要】 本文基于介观量子点概念上设计了几种低维量子结构,采用非平衡态格林函数研究了其中的自旋热电输运性质,发现了一些新颖的自旋依赖的热电输运性质,并指出其潜在的应用价值,为新型热电器件的设计提供了物理模型和理论依据。提出了与普通金属电极弱耦合的AB环上嵌入了平行排列的双量子点系统,研究了其中的自旋热电效应,发现在不与端口直接耦合的量子点上施加一个自旋偏压,不借助任何外加磁场或引入其他磁性材料,两个量子点电子之间存在的Heisenberg交换作用将诱发显著的自旋热电效应,并且该效应可以通过磁通和Rashba自旋轨道耦合强度有效地进行调控,指出该系统能够获得较大的自旋热电优值。概念上设计了嵌入磁杂质量子点的AB环系统,研究表明磁性杂质与量子点内电子的交换作用使系统获得显著的自旋输出功和功率,其大小和方向均可通过门电压有效地调控。AB环中的干涉相位项,可以通过Rashba自旋轨道耦合强度和磁通调节,因此自旋热电输出效率呈现锯齿状振荡,从而可以方便地实现开/关的功能。设计了四端口量子点环系统,发现仅考虑Rashba自旋轨道耦合时,系统中可以观测到纯自旋Nernst效应;在特定的Rashba自旋轨道耦合强度和磁通情况下,系统中将观测到完全极化电流,其大小和极化方向都可以通过Rashba自旋轨道耦合强度和磁通调制。此外,还发现点内库仑排斥将显著增大,Nernst系数,其数值几乎是不考虑库仑排斥作用情况下的两倍。研究了四端口平行排列双量子点系统中的Nernst效应,研究表明端口中的磁化强度和方向能够有效的调节该效应。若磁矩反平行排列,系统中将会出现纯自旋Nernst效应;若金属端口变为铁磁端口,电荷和自旋Nernst效应共存。此外,点内和点间的库仑排斥势能显著增强Nernst效应,其数值甚至能达到没有库仑排斥作用时的100倍以上。更多还原

【Abstract】 In this paper, mesoscopic quantum dot(QD) systems are designed theoretically.And by using the non-equilibrium Green’s function, the spin thermoelectric properties were thoroughly studied in these structures. Some novel spin dependent transport properties and the potential application were found, which provides a physical the model and theoretical basis for the design of a new type of thermoelectric devices.In an AB ring QD system weakly coupled in metal electrodes, we found that by applying an external thermal bias to QD-1, the spin Seebeck e?ect can be induced due to the presence of Heisenberg exchange coupling without the need for any extra magnetic field or ferromagnetic electrodes, which can be manipulated e?ectively by the spin-orbit interaction(RSOI) and magnetic flux.In an AB ring with an embedded magnetic impurity quantum dot system, due to the exchange coupling between the impurity and the electrons in QD, a significant spin output power and e?ciency are obtained. Both the magnitude and polarization direction of that can be e?ectively modulated by adjusting the gate voltage. The interference term in the AB ring can be manipulated by the RSOI and magnetic flux, and thus the spin e?ciency shows a Zig-zag pattern, which can be used for an on/o? switch to the thermoelectric current.In a four-terminal four-QD ring structure, with the assistance of RSOI, the pure spin Nernst e?ect can occur by applying a thermal bias; Under certain RSOI and magnetic field strength, fully-polarized currents can be driven from terminal 2 or 4, of which the sign and the magnitude can be modulated by adjusting the RSOI-induced phase factor and the magnetic flux. Moreover, the magnitude of the Nernst e?ect can be remarkably enhanced by the intra-dot Coulomb blockade, the Nernst coe?cient is predicted to be more than two times larger than the case of zero Coulomb interaction.In a pair of vertically aligned quantum dots system attached to four leads, the sign and the magnitude of Nernst coe?cient can be modulated by the relative magnetic configuration and polarization strength of ferromagnetic(FM) electrodes. When the magnetic moments in the 1 and 3 electrodes are turned to antiparallel alignment, the pure spin Nernst(without charge Nernst) e?ect can occur by applying a transverse thermal bias. As the lead 4 transition from metal to ferromagnet, both the spin and charge Nernst e?ects can be obtained simultaneously. Moreover, the magnitude of the Nernst e?ect can be remarkably enhanced by the intra-dot and inter-dot Coulomb repulsion. The Nernst coe?cient is predicted to be more than 100 times larger than the case of zero Coulomb interaction.更多还原