Title: Kagome materials in SG 191: LEGO building block of band structure, soft flat phonons, and CDW formation

Speaker: Dr. Haoyu, Hu (Donostia International Physics Center, Spain)
Time: June 14 (Wensday), 10:00
Place: Rm M830, IOP-CAS

Abstract: 
Kagome materials with flat bands exhibit wildly different physical properties depending on symmetry group, and electron number. Their complicated physics and even the one-particle ”spaghetti” of electron/phonon bands are so far amenable only to phenomenological interpretation. In this study, we aim to enhance our comprehension of Kagome materials in SG 191 by employing a comprehensive approach that combines ab initio calculations and analytical approach.

Firstly, we demonstrate that the complex electron bands observed in the 11 material (FeGe class) can be effectively dissected into three distinct groups [1]. This decomposition provides an analytical framework for understanding the presence of flat bands in these materials. Additionally, we investigate the properties of the 166 material (MgFe6Ge6 class) by treating the 11 material as a fundamental "LEGO"-like building block, enabling further insights into its behavior [1].

Secondly, we delve into the formation of charge density waves (CDWs) within a specific 166 material, ScV6Sn6 [2]. Our analysis reveals that a flat and soft phonon band collapses at (1/3, 1/3, 1/2) as the temperature decreases. This collapse is attributed to the coupling between the mirror-even Wannier orbitals of the phonons and electrons, which share the same Wannier center. Moreover, we highlight that the flatness of the soft phonon band induces strong fluctuations, ultimately stabilizing a CDW phase at a different wave vector (1/3, 1/3, 1/3) through a first-order transition.

[1] To be published
[2] H. Hu, Y. Jiang, et al, arXiv:2305.15469 (2023).
More details can be found in http://seminar.cpsjournals.cn/