量子反常霍尔效应的第一性原理研究

Materials showing quantum anomalous Hall (QAH) effect are topological insulators, in which the time-reversal symmetry is broken. Their bulk energy bands are insulating, while the edge states are conducting and robust, protected by the topology of the bulk materials. Since there is no back scattering in the edge states and the corresponding resistance is very low, QAH effects have potentially significant applications in future low-dissipation electronic devices. The reported QAH system in experiments is, however, far from applications, due to the required very low temperature (several dozens mK) and harsh criterions of the sample fabrications. It is meaningful to explore further the effect in both theories and experiments. In this project, we would like to theoretically study the QAH effect based on density-functional theory combined with Wannier function methods. The main research objects are two-dimensional systems with surfaces decorated and heterostructure systems. QAH states with large band gaps and/or high Chern numbers are expected to be achieved, which can be identified by calculating Berry curvatures, Chern numbers, edge states, and anomalous conductivities. The hidden mechanisms can be revealed by tight-binding etc models. We also plan to predict and investigate new topological states related to QAH, including quantum spin quantum anomalous Hall (QSQAH), valley-polarized QAH (VPQAH), and quantum valley Hall (QVH) states and the possible phase transitions between these topological states. Weyl semimetals will also be explored, based on the researches on QAH states. The performance of this project will help understand deeply this kind of exotic topological states and promote their developments in both experiments and device applications.

量子反常霍尔（QAH）体系是不具有时间反演对称的拓扑绝缘体，其体电子态绝缘而边缘是由体拓扑特性驱动的无能隙金属态。由于边缘态不存在杂质导致的背散射，因此其在低耗散电子器件应用方面具有重要前景。目前实验实现的QAH效应观测温度很低（几十mK）,对样品制备要求亦极高,离应用有较大距离，有必要对其开展进一步研究。本项目拟基于密度泛函理论结合Wannier函数方法对QAH态进行探索，研究对象主要为二维表面修饰体系和异质结。通过Berry曲率、陈数、边缘态、反常霍尔电导率等的计算，探索出具有较大能隙和/或高陈数的QAH新体系，通过紧束缚模型等分析机制。预言并探讨与QAH态相关的新拓扑电子态，如量子自旋量子反常霍尔态、谷极化QAH态,量子谷霍尔态等，及它们之间可能的相变。并在QAH态基础之上探讨Weyl半金属态。本项目开展将有助于人们深入理解这一奇异的拓扑态、促进其在实验上的观测和实际器件中的应用。

量子反常霍尔（QAH）体系是不具有时间反演对称的拓扑绝缘体，其体电子态绝缘而边缘是由体拓扑特性驱动的无能隙金属态。由于边缘态不存在杂质导致的背散射，因此其在低耗散电子器件应用方面具有重要前景。因为实现条件苛刻，在实验上观测到QAH效应的体系很少，因此，有必要对这一效应进行广泛探究并发现新的QAH系统。本项目基于密度泛函理论、Wannier函数方法及理论模型对QAH态、新颖机制、相关拓扑电子态及它们之间可能相变进行了深入、系统研究。我们开创性地提出了系列异质结体系，这些体系不仅具有QAH效应，还具有容易制备、大能隙、高陈数等优异性能，包括：没有能带翻转的QAH绝缘体PbC/MnSe异质结,带隙达到244 meV，带隙中具有高的费米速度、完全自旋极化的边缘态，其新颖的拓扑特性来源于two-meron结构的赝自旋涡旋；实验上容易制备、具有高陈数的石墨烯/Cr2Ge2Te6 van der Waals异质结，体系的陈数随着层厚增加而增加；无磁性原子参与的奇异QAH绝缘体SnH/PbI2。基于紧束缚模型我们给出了描述px,py轨道的Rashba自旋轨道耦合形式，并实现了对量子自旋量子反常Hall（QSQAH）杂化态的深入描述，首次给出其自旋流边缘态不同于电荷流边缘态的独特特性。在Bi单层中实现了破记录的500多meV的巨大谷极化效应及多重Hall效应。提出的拓扑态“相长”或“相消”耦合效应可很好解释Pb烯不同于其它四族单层具有的拓扑平庸电子态的原因。我们还发展算法及描述方法分别在Pb,Sn单层堆叠起来的体系及最近的二维明星材料黑磷中实现了Dirac和Nodal-line半金属特性，并对其拓扑特性进行了表征。项目所取得的这些成果能够很好地帮助人们理解QAH效应及相关拓扑电子结构，可大大促进拓扑电子态在实验上的观测和在未来低功耗电子器件中的应用。

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