类石墨烯材料中关联电子效应对能隙的影响

Many efforts have been spent on searching new field effect transistors based on two dimensional layered materials like graphene and silicene, or the van der Waals heterostructures composed of the above mentioned materials and substrates like hexagonal boron nitride, which might finally replace the silicon technology in the future. One of the key issues in this research area is how to open a gap or how large the gap is. However, it is difficult to resolve this problem. For example, in the field effect transistor made from bilayer graphene, the gap determined by transport measurements is much smaller than that obtained from optical studies, and in the heterostructure composed of graphene and hexagonal boron nitride, some experiments detect a gap while others not. Usually, these contradictions are ascribed to the structural imperfection or lattice mismatch while a more intrinsic effect, namely, the effect of electronic correlations, is unreasonably ignored. In the literature, both theoretical estimate of the interaction strength in graphene-like systems and many experimental results indicate that the electronic correlations can not be ignored. In this project, the effect of electronic correlations on the electronic states in the field effect transistors made from above mentioned two-dimensional layered materials and heterostructures will be investigated by means of coherent potential approximation and dynamical mean field theory, as well as their cluster extensions, in combination with density functional theory calculations. We will introduce a novel concept that the Coulomb interaction between electrons may either enhance or suppress the band gap to understand conflicting experiments. Furthermore, we will design new half metals based on graphene-like materials under the help of electronic interaction. Finally, we will reveal the role of electronic correlations on various topological phases predicted theoretically in silicene placed in external out-of-plane electric and magnetic fields.

用类石墨烯材料替代传统半导体工业中的硅是近年来科学界不懈努力的方向，其中关键的问题是如何打开能隙以及所获得的能隙宽度是多少？但是在解决这些关键问题时却遇到不少麻烦，如在由双层石墨烯制成的场效应晶体管中输运和光学测量给出的能隙宽度相差悬殊；又如在石墨烯-六方硼氮异质结中有些实验观测到了能隙而有些却没有，等等。通常人们都把这些矛盾归结为结构缺陷或晶格失配，但忽略了一个更本质且无法避免的因素：电子关联。而类石墨烯材料中电子相互作用强度的理论估算和大量的实验现象均表明在此类系统中电子关联效应不可忽略。本项目将结合密度泛函理论计算与相干势近似、动力学平均场理论以及它们的团簇拓展理论来研究电子关联对由类石墨烯材料组成的场效应晶体管和异质结中的电子态的影响，提出用电子相互作用既可增强亦可抑制能隙的新观点来解释实验；并利用电子相互作用设计新的类石墨烯半金属材料；以及阐明电子关联对硅烯中各类拓扑相的影响。

自石墨烯出现以来，科学界掀起了研究准二维系统的热潮。本项目针对准二维系统中物态，特别是能隙随调控参数的变化进行了深入的理论研究，主要研究内容包括，1）提出双层石墨烯中能隙随外加垂直于平面的电场出现非单调变化的新机制，并对层间有相对扭转角度的双层石墨烯中影响能隙的因素进行了分析，提出在石墨烯中不通过化学修饰或剪裁获得宽能隙的新途径，此外我们为可能合成的单层和双层二维半导体材料六方硼磷建立了紧束缚模型，我们的研究将为实现准二维纳米半导体器件奠定理论基础；2）基于对具有格点势的反铁磁模型的分析，提出在双层硅烯上可以实现完全极化的自旋电流的传输或过滤，并基于第一性原理计算提出，无需通过外加磁场、铁磁性材料或掺杂磁性元素等引入宏观磁矩的常规方式、只需对双层硅烯的平面进行弯曲或者施加垂直于平面的电场等可操作途径即可获得实现完全极化自旋电流的自旋电子学器件，我们的研究为推动自旋电子学发展做出了贡献；3）提出在考虑动力学关联的情况下，两轨道赫巴德模型中存在不同性质的轨道选择性相变，相变临界点可以通过计算纠缠熵来确定，轨道选择莫特相中是否存在非费米液体行为取决于体系是否存在近藤瓦解，即是否存在低能自旋涨落。我们还发现，在带有晶场劈裂且填充数为2/3的三轨道赫巴德模型中，自旋轨道耦合在相互作用较小时会增强系统的金属性，而在相互作用较大时会使莫特绝缘态更加稳定，同时自旋轨道耦合会诱导一个费米液体与非费米液体共存的轨道选择性非费米液体相，这一新物态是由于系统中出现了轨道选择的近藤瓦解引起的。此外我们还基于阻挫方格子上的单带赫巴德模型，给出了新的平均场近似下的相图。我们的研究为理论上统一地理解准二维高温超导材料的相图指出新的方向；4）提出铜铁砷材料中可能存在违反洪特规则现象，并由此产生的弱双条纹反铁磁态可用于解释铜铁砷材料中的实验现象。我们的研究有可能解决准二维铁磷族超导材料中弱磁性的起源问题。