基于石墨烯的d电子属性Dirac系统中新奇量子现象的理论研究

Dirac system like graphene exhibits exotic quantum properties, which not only is an appealing model for fundamental physics but also implies wide applications. By combining first-principles calculations with tight-binding model, we investigate the intrinsic Dirac spectrum in transition metal (TM) intercalated epitaxial graphene on SiC(0001). Specially, we first illustrate the interactions among graphene, TM and SiC, as well as the roles they play in the formed d-electron Dirac spectrum. Its characteristics are compared with those dominantly composed of p electrons. We explore the dependence of the Dirac spectrum feature upon the TM properties, such as the outmost d-electron number and the vice quantum number; TM atoms distribution and the intercalated dose; structural defects or impurity, and so on. Second, according to the effects of the symmetries, TM population, d-electron character and spin-orbit coupling on the electronic structure, we construct an appropriate model Hamiltonian to mimic the interaction between SiC modulated TM d orbitals and graphene π states. We unravel the underlying mechanism of here obtained d-electron Dirac spectrum, and then study its response to the external applied mechanical, electric or magnetic field. Finally, based on these knowledge, we predict and well-interpret the relevant physical phenomena, especially those in terms of relativistic and spin-polarized effects arising from the unique d-electron character. We propose the means to manipulate and modulate the Dirac fermions as well as their spin for the ultimate design of the functional devices based on Dirac electrons.

以石墨烯为代表的Dirac系统展现出很多奇特的量子特性，具有深远的基础理论意义和广阔的应用前景。利用基于量子力学的第一性原理计算，结合凝聚态物理模型分析，对过渡金属插层的SiC外延石墨烯结构中本征存在的Dirac能谱进行理论研究：具体阐明石墨烯、过渡金属和SiC之间的相互作用及对Dirac能谱的内在影响,明确d与p电子属性Dirac能谱量子特性的异同，研究插层金属特征与系统Dirac能谱特性之间的依赖关系，如最外层d电子数目、壳层，金属空间分布、浓度，结构或异质缺陷等；分析体系的电子态特性与系统对称性、d电子布居和自旋轨道耦合作用之间的关系，建立有效模型，揭示d电子属性Dirac能谱产生的内在机理和对外力、电和磁场的响应，预言并解释相关的物理现象，尤其是d电子属性带来的独特相对论效应和自旋极化现象，提出改性和调控Dirac费米子及自旋的手段，最终设计出基于Dirac电子的功能性器件。

以石墨烯为代表的Dirac系统展现出很多奇特的量子特性，具有深远的基础理论意义和广阔的应用前景。在电子器件应用中需要将其置于半导体衬底之上并兼具良好的热学和力学稳定性。但是石墨烯和衬底之间的强的相互作用通常会损害石墨烯优异的线性能带结构。利用基于量子力学的第一性原理计算，结合凝聚态物理模型分析我们发现了一种强束缚石墨烯体系（石墨烯/过渡金属/碳化硅的三明治结构），呈现类石墨烯的能带结构。同时具有很好的结构稳定性（金属不团簇、不易氧化等）。基于该石墨烯基结构，我们还实现了100%自旋极化的狄拉克费米子（Dirac电子与磁性的结合）、非寻常的量子自旋霍尔效应（Dirac电子与自旋轨道耦合效应的结合）和利用半导体表面内建电场打开能隙。本质上，该体系中的狄拉克费米子源于石墨烯向过渡金属d电子的转移，因而展现出超越石墨烯自身的基础理论意义和更多潜在应用优势。理论层面：不同于石墨烯时的纯粹模型研究，该体系为狄拉克电子与强关联效应的结合——比如磁性、自旋轨道耦合效应等与线性狄拉克物理的融合研究提供了真实平台；应用层面：该体系能够获得本征磁性，易于实现磁调控和存储。再结合其本征的狄拉克费米子，具有同时实现逻辑运算与数据存储的巨大潜在优势。尤其是考虑到当下迟迟无法找到有效方法在石墨烯中打开能隙以获得足够高的开关比，用磁手段调控自旋极化的狄拉克费米子对于石墨烯基电子学的应用变得更有现实意义。