Tailoring spin and magnetism in systems of reduced dimensionality

与传统的块状材料的不同为研究和预测不寻常的基本现象以及实现新的应用提供了有趣的可能性。最近对自旋电子学和石墨烯的研究证明了在降维系统中可行的两个显著例子，这两个例子分别得到了2007年和2010年的诺贝尔物理学奖的认可。石墨烯由于其优异的传输和光学性质已经引起了人们的极大兴趣，这使得它成为可能应用于纳米电子学和光电子学的有吸引力的候选者。另一方面，在过去十年中，自旋电子学的进步使利用磁阻效应的金属结构中的计算机硬盘驱动器的容量增加了1000倍。然而，这只是冰山一角。对一大类材料及其纳米结构中自旋和磁性的控制有可能产生更广泛的影响。事实上，石墨烯的实验突破表明，它特别适合用于自旋电子学。对于特定的应用，石墨烯自旋电子学可能会显著优于现有的传统自旋电子学。因此，本文提出的研究试图从理论上解释一些降维系统中的新现象，如石墨烯、量子阱(QWS)和量子点(QD)：在第一个项目中，我们将研究基于石墨烯的系统的磁光性质。我们的初步工作表明，单层石墨烯的光学和磁光响应可以被衬底大幅度地修饰。因此，我们将系统地研究不同衬底对双层石墨烯、纳米带和纳米盘等石墨烯结构中磁光和自旋光电导的影响，并发展相关的克尔效应理论。在一个密切相关的项目中，我们还将研究此类结构中等离子体和声子之间的耦合及其对光学性质的影响。在我们之前对HgTe/CdTe量子阱的研究基础上，我们将研究边缘态和表面态对拓扑绝缘体(磁光)电导的影响。我们将寻找有限磁场中倒置和正常能带结构交叉的信号。最后，我们将开发一个量子点中磁极化子形成的微观模型，并为在布法罗大学进行的几个正在进行的实验提供理论输入，包括特殊的热增强磁性的前景。类似于HgTe/CdTe量子点的研究项目，我们还将探索量子点中能带反转的可能性及其光学特征，以揭示相关的非传统自旋和磁有序。

A departure from conventional, bulk-like materials offers intriguing possibilities to study and predict unusual fundamental phenomena as well as to realize novel applications. Two striking examples of what could be feasible in systems of reduced dimensionality have recently been demonstrated by the research on spintronics and graphene, also recognized by the 2007 and 2010 Nobel Prizes in Physics, respectively.Graphene has already attracted immense interest due to its excellent transport and optical properties, which make it an attractive candidate for possible applications in nanoscale electronics and optoelectronics. On the other hand, advances in spintronics have, over the past decade, enabled a 1,000-fold increase in the capacity of computer hard drives in metal-based structures that utilize magneto-resistive effects. However, this represents only the tip of the iceberg. The control of spin and magnetism in a wide class of materials and their nanostructures has the potential for a much broader impact. In fact, experimental breakthroughs in graphene suggest that it is particularly suitable for spintronics. For specific applications, graphene spintronics could significantly outperform the existing conventional counterparts.The research proposed here therefore seeks to theoretically elucidate novel phenomena in selected systems of reduced dimensionality, such as graphene, quantum wells (QWs), and quantum dots (QDs):In the first project, we will examine the magneto-optical properties of graphene-based systems. Our preliminary work suggests that the optical and magneto-optical response of monolayer graphene can be drastically modified by the substrate. Therefore we will systematically study the role of different substrates on magneto-optical and spin-optical conductivities in graphene-based structures, such as bilayer graphene, nanoribbons, and nanodiscs, as well as develop a related theory for the Kerr effect.In a closely related project, we will also study the coupling between plasmons and phonons in such structures and its effect on their optical properties.Building on our previous work on HgTe/CdTe QWs, we will study the effects of edge and surface states on the (magneto-)optical conductivity in topological insulators. We will look for signatures of the crossover between the inverted and normal band structures at a finite magnetic field.Finally, we will develop a microscopic model for the formation of magnetic polarons in QDs and provide theoretical input for several ongoing experiments conducted at the University at Buffalo, including the prospect for a peculiar thermally-enhanced magnetism. In analogy to the project on HgTe/CdTe QWs, we will also explore the possibility of band inversion in QDs and its optical signatures that could reveal related unconventional spin and magnetic ordering.