【作者】 王海艳；

【导师】 周光辉；

【作者基本信息】 湖南师范大学 ， 凝聚态物理， 2012， 博士

【摘要】 二维电子气是在二维平面内能自由运动,而垂直于该平面方向运动受限的电子系统,它的独特性质使其成为凝聚态物理领域中研究热点之一.本文采用转移矩阵方法,分别对半导体异质结、石墨烯和拓扑绝缘体表面几种典型二维电子气体系在铁磁近邻作用下的自旋相关输运问题进行了较系统的研究,旨在为纳米电子器件和自旋量子器件的设计提供物理基础.全文共分为六章.第一章为绪论,简要介绍了几种典型二维电子气体系的实现及实验制备,以及它们的物理性质和应用背景.第二章,我们详细介绍了介观输运研究中常用的转移矩阵方法.第三章,我们根据薛定谔方程,研究半导体异质结二维电子气在外加两个不对称磁垒作用下的自旋相关输运性质.结果表明：不一致的磁垒可以使体系电导自旋分离,进而导致隧穿磁阻的自旋分离；磁垒间隔越小,不对称性越明显,自旋分离也越明显.因此,通过适当调节外加磁垒的结构参数,可以获得自旋明显分离的电导和磁阻.第四章,我们从Dirac方程出发,分别研究了石墨烯和扶手椅型边缘石墨烯纳米条带在两个可调磁垒作用下的自旋相关输运性质.结果表明：当两磁垒相对高度差较大时,体系低能量区域禁止导通,且反平行情况下的透射谱不再关于入射角对称；当两磁垒间隔增大时,中间区域左行波和右行波的干涉增强使透射谱更加离散化.当磁垒宽度变大时,由于衰减态的存在,凡是衰减长度小于势垒宽度的电子波将不能透过势垒.相应地,体系的隧穿电导和磁阻随结构参数变化的特征与透射谱一致,磁垒绝对和相对高度的增强或磁垒宽度变大都会使得电导减小,磁阻增大,而磁垒间隔增大只是导致电导和磁阻振荡加强.扶手椅型石墨烯条带在磁垒作用下的输运特性是：横向受限导致横向波矢离散化,所以磁化平行和反平行构型下体系都呈现出平台电导,由此导致了平台磁阻.同时,条带宽度不同,其能带结构和输运电导的特征也不相同.第五章是我们研究的重点内容.利用转移矩阵方法,我们研究了电磁复合超晶格作用下三维拓扑绝缘体表面的能带结构和输运性质,以及电子自旋极化的分布.结果表明：由于磁化方向从平行到反平行造成势垒的结构差异,在同样的能量窗口,反平行构型下子能级的数目多于平行情况.当磁垒足够强时,平行和反平行构型下低能量区域子能带都几乎平行于坐标轴,这意味着传输速度趋于零,所以电子的传输将被禁止.而电垒的变化对体系能带结构影响不大.体系的透射通道数目与能带结构是一致的,磁垒增强时反平行情况下存在更宽的禁止导通区域.电垒增强时,当能量与电垒高度匹配时,只存在小角度入射的透射通道.此外,我们还研究了表面电子的自旋极化分布.在动量空间,其自旋极化分布表明反射电子和透射电子出现的能量区域与透射谱都是相对应的.由于自旋与动量的锁定,反射电子自旋极化取向相对于入射电子旋转一个角度,而透射电子与入射电子一致.但在坐标空间,入射区域电子平面内自旋极化只随纵坐标周期性变化,但z分量不为零,打破了自旋平面的锁定.在透射区只有透射波,因此该区域内电子自旋极化取向与坐标无关,只随入射电子取向而变化.第六章,我们对本文的工作进行了总结和归纳,并对二维电子气体系在铁磁近邻作用下的输运研究进行了展望.更多还原

【Abstract】 Two-dimensional electron gas (2DEG) is the system that electron can move freely in two dimensional plane but is limited in the perpendicular direction, which has been a hot topic in condensed matter physics because of its special property. In this thesis, the spin-dependent transport properties for semiconductor heterostruc-ture, graphene and surface of three-dimensional topological insulator under ferro-magnetic modulation have been theoretically investigated with the transfer matrix method. The purpose of this study is for providing physical basis for the design of nano-electronical devices and spin quantum devices.The thesis is divided into six chapters. In the first chapter, we give a brief introduction about the discovery and fabrication techniques of three types of2DEG system, the physical properties and their application background. In the second chapter, we give a detailed introduction about the transfer matrix which are often used in the study of mesoscopic quantum transport.In the third chapter, according to the Schrodinger equation we theoretically investigate the transport property of semiconductor heterostructure2DEG with the modulation of two applied asymmetry magnetic barriers. It is demonstrated that, the spin-dependent conductance split as the magnetic barriers is inconsis-tent, which lead to the spin-splitting of the tunneling magnetic resistance (TMR). The shorter the space of the barriers and the more apparent the asymmetry, the splitting is more significant. Therefore, we can obtain the conductance and TMR which is spin-splitting remarkably by manipulating the structure parameters of the applied magnetic barriers.In the fourth chapter, based on the Dirac equation we respectively investi-gate the transport property of graphene and armchair-edged graphene nanoribbon (AGNR) under the modulation of two tunable magnetic barriers. It is demon-strated that, the transmission of system is forbidden in the lower energy region when the relatively height of magnetic barriers is stronger, and the transmission spectrum is not symmetry any more in AP configuration. With the increase of the space of the two barriers, the intervention of the left-going and right-going wave is enhanced, which result in the transmission channels more discrete. When the width of the magnetic barrier become longer, owing to the exist of the evanes-cent state the electron wave can not through the barrier as the decaying length is shorter than the width of the barrier. Accordingly, with the varied structure parameters the change of the conductance and TMR is consistent with the trans-mission spectrum. Both the increase of absolute (or relative) height and the width of the barrier can reduce the conductance and make the TMR lager. However, the resonant of conductance and TMR is enhanced as the space of the barriers become lager. In addition, the transport property of the AGNR is investigated. It is demonstrated that the transverse wave vector is discrete because of the lim-itation of transverse direction, and the system exhibit platform conductance in both P and AP configurations, which lead to the platform TMR. Meanwhile, the character of the energy band and conductance of the AGNRs with different width is different.Chapter five is our major work. By the transfer matrix method we investigate the band structure and transport property of the surface of three-dimensional topological insulator (3D TI) and the distribution of electron spin polarization under the modulation of electro-magnetic superlattice. It is demonstrated that, since the difference result from the system configuration changes from P to AP, the number of subband in AP configuration is lager than that in P configuration at the same energy window. As the height of the magnetic barriers become enough larger, the energy levels in lower energy region are almost parallel to the coordinate axis in both P and AP configuration, which means that the transverse velocity (?)E/(?)kx tend towards zero, and the transmission is forbidden. But as the electric barriers become stronger the influence is very slight. Further, the number of transmission channel is consistent with the band structure of system, and the transmission forbidden region is wider in AP configuration because of the stronger suppression. When the electric barriers increased, especially in the vicinity of E＝Vo, there is only small incident angle transmission channel. This property is different from the semiconductor heterostructure and graphene. Moreover, we investigate the character of the electron spin polarization. In momentum space, the incident energy and angle dependence of electron spin polarization is in good agreement with the transmission spectrum. Due to the spin-momentum locking of TI surface states, the spin orientation of reflected electrons is always rotated and transmitting electron is always along the direction of the incident electron. However, in real space, in incident region the in-plane spin polarization oscillates periodically only in x-direction, and the spin polarization Pz is not zero, which induced the spin-surface locking broken. Because there is only transmission wave in the transmitting region the spin polarization in the transmitting region is found not relevant to coordinates and varying with the incident electron.In chapter six, a summary of the work and a outlook of the transport prop-erties of two dimensional gas under the ferromagnetic modulation are given.更多还原