Three-Dirac-fermion approach to unexpected gapless surface states of van der Waals magnetic topological

报告题目：Three-Dirac-fermion approach to unexpected gapless surface states of van der Waals magnetic topological

报告人：张海军 教授（南京大学物理学院）

报告时间：2022年6月9日 10:00-12:00

腾讯会议：567-073-461

摘要：A diverse range of topological quantum phenomena and potential applications of three-dimensional topological insulators (TIs) hinge on opening an energy gap of Dirac-cone surface states. Layered van der Waals (vdW) topological materials, especially the recently discovered MnBi2Te4-family magnetic TIs, have aroused great attention, where the interlayer vdW gap is expected to play a crucial role in topological surface states. However, it remains a serious controversy whether the surface states are gapped or gapless for magnetic TI MnBi2Te4, which is a crucial issue for the prospect of various magnetic topological states. Here, a 3-Dirac-fermion approach is developed to generally describe surface states of nonmagnetic/magnetic vdW TIs under the interlayer vdW gap modulation. In particular, we apply this approach to solving controversial issues in the surface states of vdW antiferromagnetic (AFM) TIs. Remarkably, unexpected topologically protected gapless Dirac-cone surface states are found to arise due to the interlayer vdW gap expansion on the surface, when the surface ferromagnetic layer has a zero Chern number, while the surface states remain gapped for all other cases. The unexpected gapless Dirac-cone states are invaluable in solving the puzzle of the observed gapless surface states in MnBi2Te4. This work also provides a promising way for experiments to realize intrinsic magnetic quantum anomalous Hall effect with a large energy gap in MnBi2Te4 films.
References:
1)Phys. Rev. Lett. 122, 206401 (2019)
2)arXiv: 2205.08204 (2022)
简介：张海军，南京大学物理学院和固体微结构物理国家重点实验室教授，博士生导师。主要研究兴趣是，结合第一性原理方法、紧束缚近似方法和kp模型方法等理论方法，探索和理解凝聚态物理中的新奇物性，特别是凝聚态物理中的各种拓扑物质态。发表学术论文90余篇，总引用>14000次，单篇最高引用>4460次。