Abstract:

Kagome lattice provides a rich platform for exploring novel quantum states, emerging from the interplay between its frustrated corner-sharing triangular geometry and intriguing electronic structure. Co₃Sn₂S₂ is a kagome lattice ferromagnet, exhibiting a unique interplay between its electronic wavefunction topology and magnetic spin configuration. This interaction results in several intriguing properties, including Weyl points, a colossal anomalous Hall effect, and a pronounced magneto-optical response.

In the first part of the talk, I will discuss our recent ultrafast study of Co₃Sn₂S₂ [1]. To our surprise, we directly observe two magnon modes in the terahertz range in the time domain. These frequencies exceed typical ferromagnetic resonance frequencies by 1-2 orders of magnitude. These dual modes originate from the strong coupling of localized spin and orbital magnetic moments. These findings unveil an unconventional category of magnons in a ferromagnet stemming from orbital magnetic moments, and position Co₃Sn₂S₂ as a promising candidate for high-speed terahertz spintronic applications.

In the second part, I will report the discovery of chiral phonon modes in Co₃Sn₂S₂ [2]. Using helicity-resolved magneto-Raman spectroscopy, we observe the spontaneous splitting of the doubly degenerate in-plane E_g modes into two distinct chiral phonon modes of opposite helicity when the sample is zero-field cooled below the Curie temperature, in the absence of an external magnetic field. As we sweep the out-of-plane magnetic field, this E_g phonon splitting exhibits a well-defined hysteresis loop directly correlated with the material’s magnetization. Our findings highlight the role of the magnetic order in inducing chiral phonons, paving the way for novel methods to manipulate chiral phonons through magnetization and vice versa.

References:

[1] M. Che et al., Discovery of terahertz-frequency orbitally-coupled magnons in a kagome ferromagnet, Science Advances 11, eadw1182 (2025).

[2] M. Che et al., Magnetic order induced chiral phonons in a ferromagnetic Weyl semimetal, Physical Review Letters 134, 196906 (2025).

主讲人简介：

杨鲁懿，2007年获清华大学学士，2013年获美国加州大学伯克利分校博士。2013-2016年在美国强磁场国家实验室从事博士后研究（LANL Director’s Postdoctoral Fellow）。2016-2019年加拿大多伦多大学任助理教授，获得Canada Research Chair、CIFAR Azeieli Global Scholar等荣誉。2019年入选国家级青年人才计划并任清华大学物理系副教授。她的专长是发展和应用先进的光学技术，以超高的时间和空间分辨率探测凝聚态系统中的超快动力学。回清华后，作为独立PI，她的课题组自主研发并搭建了多个量子材料超快动力学测量平台，并在磁性拓扑材料、钙钛矿光伏材料以及氧化物功能材料的光学性质和超快动力学等方面取得一系列进展。