二维薄膜堆垛形成的三维拓扑金属或半金属

Both topological materials and two-dimensional (2D) nanosheet materials are research spotlights of condensed matter physics in recent years. Although some topological phases exist in the 2D materials, many interesting topological phases cannot appear in them because of the limitation of dimensions. Therefore, the recent researches for the topological materials, especially for the topological metals and semimetals, mostly focused on the 3D materials. To break through the limitation of dimensions, we can stack the 2D nanosheets to 3D structures. Recent experiments reported that monolayer nanosheets can be successfully stacked to multi-layer Van Der Waals structures step by step. We carried out preliminary studies on the topological properties of stacking structures and found that these structures possess rich topological metal/semimetal phases. This indicated that stacking-nanosheet structures not only provide a new method to find topological materials and phases, but also extend the applications of nanosheets to a new field. However, systematic investigations on the topological properties of stacking-nanosheet structures are required, because of various structures of the 2D nanosheets, complicated interlayer couplings and different stacking forms. In this proposal, we will firstly predict all possible topological phases and phase transitions in the stacking structures by ideal models. Then, we will search real atomic structures corresponding to the ideal models, and propose new topological materials. Finally, we will study surface states and magnetic properties of the topological materials and find new transport phenomena in them.

拓扑材料和二维薄膜材料都是近年来凝聚态物理的研究热点。虽然在二维材料中存在一些拓扑相，但是由于维度的限制更多有趣的拓扑相都不可能出现在它们当中。因此，对拓扑材料特别是对新型拓扑金属和半金属的研究目前主要集中在三维材料上。为了突破维度的限制，可以把二维材料堆垛成三维结构。目前实验上已经能把单层薄膜依次堆垛成多层范德瓦尔斯薄膜结构。我们对薄膜堆垛结构的拓扑性质进行了初步的研究，发现其中蕴含着丰富的拓扑金属和半金属相。这说明薄膜堆垛不但提供了一种新的寻找拓扑材料和拓扑相的方法，还将薄膜材料的应用拓展到新的领域。然而，因为二维材料的结构繁多、层间堆垛的耦合复杂，所以对薄膜堆垛结构的拓扑性质还需要做系统的研究。在本项目中，首先通过描述堆垛结构的理想模型预测这类结构中可能存在的拓扑相及相变；然后寻找与理想模型对应的实际结构，提出新的拓扑材料；最后对拓扑材料的表面态和磁性质进行研究，发现新的输运现象。

拓扑材料和二维薄膜材料是目前凝聚态物理的研究热点。因受维度的限制，结构不可能出现很多有趣的拓扑相。从而对拓扑材料特别是对拓扑金属和半金属的研究主要是三维材料。为了突破维度的限制，项目组提出由二维材料堆垛成三维结构，并对堆垛结构性质进行了研究，得到一系列新型二维薄膜堆垛形成的三维拓扑金属或半金属，如α硼烯和六角氮化硼薄膜材料堆垛的三维拓扑金属、能够实现Hopf link网络的Sc3XC、Kagome石墨烯网络以及多个碳的三维网络，发现蕴含着丰富的拓扑金属和半金属相，如Hopf link网络、互锁节线链、拓扑节线环等；以及一些新奇的拓扑相变过程：三重简并点的node line变化、三重简并点与Hopf link混合相、狄拉克型到外尔型的拓扑相变、并发拉胀效应的拓扑相变等。此外，项目组还研究了其中部分拓扑材料的拓扑表面态，发现连接节点的费米弧等现象。通过研究，项目组提出了一种新的寻找拓扑材料的方法，也拓展了二维薄膜材料的应用领域；并系统地研究从二维过渡到三维空间后可能存在的拓扑相及相变；预测了一些新的薄膜结构，在薄膜堆垛结构中发现了新的拓扑相和新的电磁输运性质，并依此为提出新型电子器件提供理论依据和实验模型。