基于石墨烯量子点电子结构和磁学性质的理论研究

Gaphene, because of long spin relaxation time and high electon velocity, is a promising candidate for designing spintronic devices with high processing speed and low power consumption. However, the presence of vacancies in graphene is inevitable during electron or ion irradiation process. Aim to apply graphene based nanostructures on spintronic devices, we study the electronic structures and magnetic properties of graphene nanoflakes with different shapes and edge types in the presence of vacancies. The effects of vacancies are studied by using both the first-principle calculations and the tight binding method combined with the Hubbard model. By comparison between the energy levels and magnetic moments in the tight binding and DFT models, we obtain a set of parameters to describe actual material. We mainly focus on the effect of a in-plane electric field on the spin polarization density and magnetic ordering of the ground state in order to expore an efficient tool for the control of magnetic ordering in nanographene structures and make the spintronic application of them possible. Meantime, in the calculation of magnetic properties, it has to take into account the many-body effect of electrons. Our calculation start from the Coulomb repulsion, then exchange interaction, and final the correlation interaction between different electron configurations. By applying the configuration interaction method, we could obtain a further insight into the electric field induced magnetic phase transition in graphene nanodots. Moreover, the effect of excitons on the electronic structure and related properties also can be observed. Thus, the importance of the correlation interaction is found out.

石墨烯由于具有自旋驰豫时间长、电子运动速度快等特点，被认为是制备高速低耗的自旋电子器件的理想材料。但在实际石墨烯材料的制备过程中不可避免地伴随着空位缺陷的产生。本课题基于石墨烯纳米结构在自旋器件方面应用的目的，研究不同形状和边界类型的石墨烯量子点在出现空位缺陷时的电子结构和磁性情况。分别采用第一性原理计算方法和紧束缚近似结合哈伯德模型的方法来研究空位缺陷对体系的影响，通过比较两类方法计算的结果来拟合相关参数，得到考虑了原子弛豫现象的简便计算方法。由此我们着重考察外加电场对有空位缺陷存在的石墨烯量子点基态电子自旋极化分布和局域磁距的影响，从而找到能够有效调控实际石墨烯材料基态磁序的可行方法，使其在自旋电子学方面的应用成为可能。在研究体系磁性时，从只考虑电子间库仑作用出发，进而考察交换作用，最后采用组态相互作用将不同电子组态间的关联作用包括进来，由此揭示出电子关联效应的重要性。

本项目主要是采用由紧束缚近似结合哈伯德模型和基于密度泛函理论的第一性原理VASP计算软件的这两大类方法，系统研究了不同大小的六边形、三角形和领结形的锯齿型和扶手椅型边界石墨烯量子点的电子结构和磁性。通过拟合这两类方法的计算结果，修正紧束缚近似和哈伯德模型中的电子跳跃能和在位库仑能这两个参数的大小。计算表明，石墨烯量子点尺寸、形状和边界等因素对这两个参数均有影响。利用修正过参数的紧束缚近似方法和哈伯德模型进一步研究发现，石墨烯量子点体系在电场作用下基态磁序可在反铁磁、铁磁和无磁性等不同状态间变换。当体系中有空位缺陷出现时，需要考虑结构的弛豫情况，可由VASP拟合出更准确的电子跳跃能和在位库仑能大小。计算结果显示，空位缺陷的个数、所处的位置都对石墨烯量子点的电子结构和磁性有影响。当双空位出现同种子格上时，在电场作用下体系基态可由铁磁变为无磁性。可见，电场可以有效调控石墨烯量子点体系的磁性，可用于制备自旋器件。