题目：Altermagnets design by twist and induced higher-order topological states

报告人：刘铖铖

时间：2024年7月29日（周一），上午10：00-12：00

地点：研究生大厦305会议室

摘要：

Recently,a type of collinear crystal-symmetry compensated magnetic order called altermagnetism,which uniquely combines the zero net magnetization and nonrelativistic spin splitting in the Brillouin zone,has emerged as an exciting research landscape. In the talk,I introduce our recent works on altermagnets.

In the first part of the talk, we propose a universal methodology for generating and manipulating altermagnetism in 2D magnetic van der Waals (MvdW) materials through twisting. We find that a key in-plane2-fold rotational operation can be achieved in a twisted bilayer of any 2D MvdW material, which takes one of all five 2D Bravais lattices, thereby inducing altermagnetism. By choosing the constituent MvdW monolayer with specific symmetry, our approach can tailor altermagnetism of any type, such as d-wave, g-wave, and i-wave. Furthermore, the properties of our twisted altermagnetic materials can be easily engineered. Taking a transition-metal oxyhalide VOBr as an example, we find that by tuning the twist angle and Fermi level a giant spin Hall angle can be obtained, much larger than the experimentally reported. This approach establishes a general, robust, and adjustable platform to explore altermagnetism, and provides a new efficient way to generate and manipulate the spin current.

In the second part, we propose to utilize altermagnets’ unique zero net magnetization and nonrelativistic band spin splitting to create and manipulate higher-order topological states. On one hand, based on symmetry analysis and effective edge theory, we show that the special spin splitting in altermagnets with different symmetries, such as d wave, can introduce Dirac mass terms with opposite signs on the adjacent boundaries of a topological insulator, resulting in a higher-order topological state with mass-domain-bound corner states. Moreover, by adjusting the direction of the Néel vector, we can manipulate such topological corner states by moving their positions. By first-principles calculations, we predict a 2D topological insulator bismuthene with a square lattice on an altermagnet MnF₂ as an example. On the other hand, we also find that the altermagnet can induce mass terms at the edges that compete with electron pairing, and mass domains are formed at the corners of the sample, resulting in zero-energy Majorana corner modes (MCMs). The presence or absence of MCMs can be engineered by changing the direction of the Néel vector. Moreover, uniaxial strain can effectively manipulate the patterns of the MCMs, such as moving and interchanging MCMs.

报告人简介：

刘铖铖，北京理工大学物理学院教授，计算物理系首任系主任。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）。从事凝聚态理论和计算物理研究，在硅烯、锗烯、锡烯、转角石墨烯、弱拓扑绝缘体和拓扑超导体等方面的研究成果有重要影响。发表学术论文50余篇（含11篇PRL），SCI引用7900余次，连续四年Elsevier中国高被引学者（物理学）（2020-2023）。