【作者】 刘德；

【导师】 孔小均；

【作者基本信息】 河北师范大学 ， 凝聚态物理， 2009， 博士

【摘要】 本论文选择具有重要应用价值和基础理论研究意义的两端具有铁磁接触的分形半导体多层异质结构(F/FSM/F)作为研究对象,针对其中的电子自旋极化输运现象、关联系数、电流以及散粒噪声等问题进行较为细致的研究,通过与两端具有铁磁接触的周期半导体多层异质结构(F/PSM/F)中的相关结果比较,揭示了一些新效应及其物理机制。旨在探索新的量子结构,为可能的具有特殊性质的非线性自旋电子学器件的设计开发提供理论依据。本文得到的主要结论如下:(1)在自旋极化输运过程中,提出了一种具有自相似结构的F/FSM/F异质结构。F/FSM/F异质结构可以看作是属于F/S/F异质结构的一种带有缺陷的具有严格周期性的F/PSM/F异质结构。(2)基于相干量子输运理论,考虑Rashba自旋-轨道耦合相互作用,我们系统地研究了F/FSM/F和相应F/PSM/F异质结构中半导体层厚确定和随机变化两种情况下的自旋极化电子的输运性质;分别讨论了两种不同结构的隧穿几率与结构迭代次数n、半导体多层结构的总长度d、Rashba自旋-轨道耦合强度kR /k0和半导体层厚随机变化的随机度R之间的依赖关系。计算结果表明,F/FSM/F异质结构的隧穿谱与F/PSM/F异质结构的隧穿谱相比具有许多有趣的特性。例如,更显著的量子尺寸效应、更加尖锐的局域共振峰和半导体层厚随机波动的稳定性等等。同时,我们也发现,在半导体层厚存在随机变化的F/PSM/F和F/FSM/F异质结构中,自旋向上和自旋向下电子的隧穿几率不仅在左右铁磁体磁化方向平行(P)时可以分离,而且在磁化方向反平行(AP)时也会分离。这一结果与层厚没有随机变化的情况明显不同。(3)借助关联系数研究了自旋极化电子输运过程中的分形行为。通过计算隧穿几率的关联系数表明:与F/PSM/F异质结构的隧穿谱相比,F/FSM/F异质结构的隧穿谱具有与半导体多层中各层的分形分布有关的部分自相似结构。也就是说,共振隧穿谱与几何结构之间有一个清晰而直接的关联。(4)基于相干量子理论和Landauer-Büttiker散射理论,考虑Rashba自旋-轨道耦合相互作用,我们系统地研究了电场作用下电子隧穿第四代F/FSM/F异质结构和相应F/PSM/F异质结构的自旋相关的散粒噪声的性质。计算并讨论了两种不同结构的电流、散粒噪声和Fano因子与半导体多层结构的总长度、Rashba自旋-轨道耦合相互作用、外加偏压以及两铁磁接触磁矩间夹角的依赖关系。结果表明,电流、散粒噪声和Fano因子具有强自旋依赖性,不仅能够通过外加偏压和Rashba自旋-轨道耦合相互作用进行调制,而且也与半导体多层的总长度和两铁磁接触磁矩间的夹角等结构参数密切相关。(5)与F/PSM/F异质结构相比,F/FSM/F异质结构的电流、散粒噪声和Fano因子具有一些有趣的物理性质。例如,更显著的量子尺寸效应、随着偏压或Rashba自旋-轨道耦合强度的增加表现出的低频非周期振荡规律以及随角度的增加出现的对Rashba自旋-轨道耦合强度的弱依赖性等。我们的结果进一步表明,在准一维自旋晶体管器件的实现和量子相干自旋电子学器件的设计上,F/FSM/F异质结构可能比F/PSM/F异质结构更占有优势。因此,我们的研究结果可能会促进和激发人们对介观输运过程中精细物理机制的进一步研究,也有可能用于优化未来自旋电子学器件的设计。更多还原

【Abstract】 In this paper we investigate theoretically the spin-polarized transport properties of electrons tunneling through fractal semiconductor multilayers with two ferromagnetic contacts (F/FSM/F) in the presence of a spin-orbit interaction, which possess great potential in future applications. The spin-polarized transport properties of the F/FSM/F heterostructures are compared with that of periodic semiconductor multilayers again with ferromagnetic contacts (F/PSM/F). We expect to provide theoretical foundation for the development of nonlinear spintronic devices with our work. The main results obtained in this paper are listed below:(1) F/FSM/F heterostructures, a type of F/S/F having a self-similar structure have been introduced. A given F/FSM/F heterostructure can be understood as a strictly periodic F/PSM/F heterostructure with defects.(2) On the basis of coherent quantum theory, we have systematically investigated spin-polarized electron transport through the F/FSM/F and corresponding F/PSM/F heterostructures with and without randomly distributed variations in the semiconductor layer thicknesses but in the presence of a spin-orbit interaction. The dependence of the transmission coefficients for the two different structures on the degree of complexity, n , the total thickness of the SM structure, the strength of the Rashba spin-obit coupling and the degree of randomness in the semiconductor layer thicknesses has been discussed. The numerical results show that transmission spectra of the F/FSM/F heterostructures posses some distinctive properties, such as stability against the effects of randomness in the layer thicknesses, sharp localized resonances, and a more marked quantum size effect than in non-fractal structures. We have found that the transmission for spin-up and spin-down electrons can be separated not only in the P magnetizations but also in the AP magnetizations, which is different from the case for these structures without fluctuating layer thicknesses.(3) Fractal behavior in spin electron transport has been studied by means of the correlation coefficient. The correlation coefficient shows how the transmission coefficient for the F/FSM/F heterostructures has a self-similar structure associated with the fractal distribution of the SM structure. In other words, transmission spectra of the F/FSM/F heterostructures posses scalability than in non-fractal structures. This property illustrates a clear and direct correlation between the geometry of a structure and the spectra of resonant transmission.(4) On the basis of coherent quantum theory and the Landauer-Büttiker scattering formalism, we have systematically investigated the spin-dependent shot noise properties of electron tunneling through the fourth-stage F/FSM/F and the corresponding F/PSM/F heterostructures in the presence of the Rashba spin-orbit interaction and an electric field. The dependences of the current, shot noise and Fano factor for the two different structures on the total length of the SMs structure, the Rashba spin-obit interaction, the external bias voltage, and the angle between the magnetic moments in the two ferromagnetic contacts have been numerically calculated and discussed. The results indicate that the shot noise becomes strongly spin-dependent and can be greatly modulated not only by the external electric field and Rashba spin-orbit interaction, but also by the structural configuration and length of the semiconductor multilayers.(5) Compared to the F/PSM/F heterostructures, the current, shot noise, and Fano factor of the F/FSM/F heterostructures can exhibit a more marked quantum size effect, along with typical low-frequency aperiodic oscillations as the bias voltage or the Rashba spin-orbit coupling strength increases. There is little Rashba spin-orbit coupling strength dependence with increasing angle between the two magnetic moments of the left and right ferromagnets. Our results have further demonstrated that the F/FSM/F heterostructures have possible superiority over F/PSM/F heterostructures for the implementation of quasi-one-dimensional spin-transistor devices and the design of quantum coherent spintronic devices. Our results may thus shed light on and encourage further study of the subtle mechanisms in mesoscopic transport processes and help optimize the design of future spintronic devices.更多还原