报告人：Feng Liu

报告题目：Negative Force Constant Manifested by Topological Phonons

报告摘要：Phonons as bosons have notably fundamental differences from electrons as fermions. Unlike interatomic electron hopping that can be either positive or negative, interatomic force constant must be positive, otherwise the atomic structure would be unstable without restoring forces. Strikingly, we demonstrate that multi-atom cluster-mode vibrations can be coupled with a negative force constant (NFC), which is in fact a direct physical manifestation of topological phonons. Using the spring-mass lattice model, we show that topological phonon flat band (FB), in analogy of topological electron FB hosted in line-graph lattices, can have either a positive or negative Chern number manifesting a positive FC or NFC, respectively; while a phononic topological insulator, characterized with “mode inversion” around a phonon Dirac point, implies always an NFC. Guided by this fundamental correspondence between NFC and nontrivial phonon topology, we perform first-principles calculations and model analyses to elucidate the existence of phonon FB of NFC in a real material of 2D Kagome-BN. Our work presents a conceptual advance in understanding topological phonons in association with NFC, which can be generalized to meta-materials to generate topological mechanical and sound waves by engineering negative compressibility.

报告简介：Feng Liu, Distinguished and Ivan B. Cutler Professor of Department of Materials Science and Engineering, University of Utah. He received his PhD in Chemical Physics from Virginia Commonwealth University in 1990. Prof. Liu is a fellow of American Physical Society and recipient of Senior Humboldt Award. His research interest lies in theoretical and computational studies of low-dimensional nano and quantum materials, with a most recent focus on topological and flat-band materials.