北京大学

报告摘要：

In this work, we exhaust all the spin-space symmetries, which fully characterize collinear, non-collinear, commensurate spiral, as well as incommensurate spiral magnetism, etc., and investigate enriched features of electronic bands that respect these symmetries. We achieve this by systematically classifying the so-called spin space groups (SSGs) - joint symmetry groups of spatial and spin operations that leave the magnetic structure unchanged. By representing the SSGs as O(N) representations, we - for the first time - obtain the complete classifications of 1421, 9542, and 56512 distinct SSGs for collinear (N = 1), coplanar (N = 2), and non-coplanar (N = 3) magnetism, respectively. SSG not only fully characterizes the symmetry of spin d.o.f., but also gives rise to exotic electronic states, which, in general, form projective representations of magnetic space groups (MSGs). Surprisingly, electronic bands in SSGs exhibit features never seen in MSGs, such as (i) nonsymmorphic SSG Brillouin zone (BZ), where SSG operations behave as glide or screw when act on momentum, (ii) effective π-flux, where translation operators anti-commute with each other and yield duplicate bands, (iii) higher-dimensional representations unexplained by MSGs, and (iv) unconventional spin texture on Fermi surface, which is completely determined by SSG, independent of Hamiltonian details. To apply our theory, we identify the SSG for each of the 1595 published magnetic structures in the MAGNDATA database on the Bilbao Crystallographic Server. Material examples exhibiting the novel features (i)-(iv) are discussed with emphasis. We also investigate new types of SSG-protected topological electronic states that are unprecedented in MSGs. In particular, we propose a 3D Z2 topological insulator (TI) state with a four-fold degenerate Dirac point on the surface, and a new scenario of anomalous Z2 helical states that appear on magnetic domain walls.

报告人简介：

宋志达，2013年毕业于电子科技大学微电子学专业，2018年于中国科学院物理研究所获得理论物理博士学位，之后在普林斯顿大学做博士后研究，于2021年底入职北京大学物理学院量子材料中心，任助理教授。其主要研究方向包括拓扑物态、转角体系中的关联和超导物理、无序体系等，比较有代表性的工作包括：1. 拓扑晶体绝缘体的完整分类、对称性指标、高通量材料计算；2. 转角石墨烯中的拓扑、关联、超导，将转角石墨烯映射为重费米子问题；3. 磁性拓扑物态，三维自旋群的完整分类等。

报告地点：怀柔园区MA楼505会议室

腾讯会议：876-758-435

会议密码：2024

邀请人：王志俊 研究员

联系人：王慧颖 why@iphy.ac.cn

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