【作者】 常博；

【导师】 梁九卿；

【作者基本信息】 山西大学 ， 理论物理， 2011， 博士

【摘要】 本文针对目前既有基础研究意义,又可能在未来具有重要应用价值的单分子磁体的量子输运做了较为系统的研究。研究的目的一方面为了揭示单分子磁体的宏观量子现象对电子输运的影响,另一方面为设计和实现以单分子磁体为电子元件的分子器件提供理论方面的依据。文章首先简单的介绍了介观输运、单分子磁体宏观量子现象以及单分子磁体量子输运的研究现状,接着在第二章中较为详细的介绍了本文所采用的理论研究工具——非平衡态的格林函数和隶玻色子平均场的方法,然后对嵌有单分子磁体的量子点的输运特性以及单分子磁体的Kondo效应做了研究：首先,我们提出低温下处于基态的分子磁体发生宏观量子相干时,就相当于一个旋转地磁矩,能够为和它有相互作用的电子提供-个自旋翻转的机制。而这个翻转机制可以通过加在难磁化平面内的磁场的大小和方向来进行调解。基于上述认识,我们对嵌有单分子磁体的量子点的输运作了研究。数值分析的结果显示,当量子点与完全极化的反平行的铁磁电极的耦合时,电流的大小会随着磁场发生振荡,在某些特殊磁场值下,电流会被完全抑制。另外,即使对于非完全极化的电极而言,分子磁体的宏观量子效应对输运的影响也不可忽略。利用上述性质期望可以实现由磁场控制的电流开关。其次,当外加磁场沿着分子磁体的易磁化轴时,会发生磁化矢量的共振隧穿。我们先将一个涉及到多态的问题映射为含有多个之间有耦合的子系统。然后利用非平衡态的格林函数对系统的电流、电导、隧穿磁阻和散粒噪声作了研究。分析结果显示,与分子磁体的耦合造成了造成了电导峰的劈裂和电流呈现出台阶状的特征,而磁化矢量的量子隧穿会进一步加强这个特征。特别是磁场的扫描速度通过分子磁体也会影响到输运。此外,我们还详细的分析了自旋的弹性与非弹性隧穿的问题,以及相应的非弹性隧穿的路径。对于隧穿磁阻来说,由于自旋非弹性的隧穿,造成零偏压处会有一个很小的非弹性电流,增加了隧穿电阻。散粒噪声和Fano因子也强烈的依赖于磁场强度、耦合强度和磁场的扫描速度。电子和单分子磁体的耦合会使散粒噪声和Fano因子得到明显的加强。而磁化矢量的量子隧穿所引起的非弹性隧穿又导致零偏压时散粒噪声的出现。最后,基于无限U下的Anderson模型,我们讨论了在强的电子关联下与金属电极耦合的单分子磁体的量子输运特性。通过隶玻色子平均场的方法,我们计算了零偏压附近的微分电导。数值结果显示,分子磁体的各向异性的等效于一个方向为一Z的外加磁场,造成了Kondo电导峰的劈裂。当外加磁场沿着分子磁体的易磁化轴以不同的速度c调节时,Kondo峰的行为也各不相同：c较大时,磁场只会造成Zeeman效应；c较小时,磁化矢量的共振隧穿使得已经劈裂的Kondo电导峰进一步的劈裂,并在自旋耦合较强时形成Kondo伴峰。此外,铁磁电极的极化率也能够影响到Kondo峰的大小和位置。更多还原

【Abstract】 The macroscopic quantum coherence (MQC) of single-molecule magnets(SMMs) has attracted a great deal of interest. One expects a rich interplay between quantum tunneling, phase coherence, and electronic correlations in the transport properties of SMMs. In this paper, we make a detailed investigation of the electron transport through the SMM manipulated by the MQC, which owns great potential application on nanoelctronics in the future. Our purpose is to explore the effect of the MQC on the electron transport throuth SMM; and on the other hand, to provide the theoretical support in the design of molecular devices. After brief review of the mescoscopic transport, MQC in SMM and electron transport through SMM, we present a detailed introduction of the method in this paper—nonequilibrium Green’s function and slave boson mean field approach(SBMFA) in the chapter 2. The dissertation is devoted a theoretical analysis of electron transport through a quantum dot with an embedded biaxial SMM and Kondo effect in SMM.First, We report a theoretical analysis of electron transport through a quantum dot with an embedded biaxial single-molecule magnet, which is coupled to ferromagnetic electrodes of parallel and antiparallel magnet-configurations. For the antiparallel configuration of complete polarization, it is shown that the originally prohibited electron transport can be opened up by the macroscopic quantum coherence of the molecular magnet, which provides a spin-flipping mechanism. The charge-current and differential conductance are controllable by variation of the magnitude and orientation of an external magnetic field, which in turn manipulates the macroscopic quantum coherence of the molecular magnet. Moreover, the transport can be switched off at particular values of the magnetic field. A new transport channel is found in the completely polarized parallel-configuration induced by the tunnel splitting of molecular magnet. For non-completely polarized leads, the tunneling effect of the SMM can not be ignored in high magnetic field regime.Then, we investigate a theoretical analysis of electron transport through a quantum dot with an embedded SMM based on mapping of the many-body interaction-system onto a one-body problem by means of the non-equilibrium Green’s function technique. It is found that the conducting current exhibits a stepwise behavior and the nonlinear differential conductance displays additional peaks with variations of the sweeping speed and the magnitude of magnetic field. This observation can be interpreted by the interaction of electron-spin with the SMM and the quantum tunneling of magnetization. The inelastic conductance and the corresponding tunneling processes are investigated with normal as well as ferromagnetic electrodes. In the case of ferromagnetic configuration, a sudden TMR-switch with the variations of magnetic field is observed, which is seen to be caused by the inelastic tunneling. Moreover, the shot noise and Fano factor are strongly dependent on the magnetic field, exchange coupling and sweeping speed of the field, and be enhanced by the coupling between electron and SMM. We also observe a suppression of shot noise is exhibited in the S-V diagram due to the suppression of inelastic tunneling.Finally, a Kondo-effect in single-molecule magnet is investigated based on combination of slave boson mean-field theory and the non-equilibrium Green function technique. It is found that the macroscopic quantum coherence of molecule-magnet results in the Kondo peak-split of nonlinear differential conductance with the help of interaction between electron-spin and molecular magnet. the peak height and position is sensitive to the sweeping speed of the applied magnetic field:In the fast sweeping field, the Kondo-peak shifts due to Zeeman effect. In the slow sweeping field, the quantum tunneling of magnetization results in a satellite peak from the 10-th channel at higher bias. It is also demonstrated that the Kondo peaks can be controlled by the electron-molecule coupling and the polarization parameter in the case of ferromagnetic electrodes.更多还原