Abstract:The interplay between symmetry and topology led to the concept of symmetry-protected topological states, including all non-interacting and weakly interacting topological quantum states. Among them, recently proposed nodal line semimetal states with space-time inversion (PT) symmetry which are classified by the Stiefel-Whitney characteristic class associated with real vector bundles and can carry a nontrivial Z2 monopole charge have attracted widespread attention. However, we know less about such 3D Z2 nodal line (Z2NL) semimetals and do not know how to construct them. We layer construct 3D Z2 nodal line semimetals by stacking of 2D PT -symmetric Dirac semimetals via nonsymmorphic symmetries. Based on our construction scheme, numerous models can be produced. Combined with first-principles calculations, we predict abundant candidate electronic and phononic materials for Z2 nodal line semimetal. Our theoretical construction scheme can be directly applied to metamaterials and circuit systems.

Moreover, we demonstrate that a class of stable Z2 monopole charge Dirac point (Z2DP) phases can robustly exist in real materials, which surmounts the understanding: that is, a Z2DP is unstable and generally considered to be only the critical point of a Z2NL. We propose the alternating-twisted multilayer graphene as the first example to realize such stable Z2DP phase protected by crystal symmetries. We tabulate all possible space groups to allow the existence of the stable Z2DP phases, which will provide direct and strong guidance for the realization of the monopole semimetal phases in electronic materials, metamaterials and electrical circuits, etc.

主讲人简介：刘铖铖，北京理工大学物理学院教授，博导，计算物理系主任。从事计算物理与凝聚态理论研究。通过第一性原理计算方法，以及Green函数、Wannier函数、紧束缚近似和有效模型等理论方法研究真实材料的电子结构，重点关注由体系波函数的非平庸拓扑所导致的各种新颖量子态和新奇物性。同时关注以转角石墨烯为代表的转角体系。曾获“The 2018 New Journal of Physics Early Career Award”和2018年度国家自然科学奖二等奖（第二完成人），2019年入选基金委“优青”和教育部“青长”。New Journal of Physics编委（2020-至今）。发表论文40余篇（含11篇PRL），SCI引用7千余次，连续3年（2020-2022）入选爱思唯尔中国高被引学者。