报告题目 (Title)：Beyond Closed Wave Systems: Non-Hermiticity and Vacuum Fluctuations（超越封闭波动体系：非厄米性和真空涨落）
报告人 (Speaker)：丁鲲 青年研究员（复旦大学）
报告时间 (Time)：2021年5月11日(周二) 10:00
The classical wave system has demonstrated itself as an excellent platform to manipulate classical waves (light, sound, etc.) and realize novel phenomena which are not easy to experimentally investigate in condensed matters. All of these are based on the scattering and eigenvalue forms of the wave equation in which the core quantities are the macroscopic ε and µ (or ρ and Β) obtained from the homogenization or mean field treatment. However, this framework not only ignores the non-Hermiticity coming from the interaction between different (quasi-)particles but also averages out the fast-varying parts and rapid variations of the electrons.
Therefore, the presentation will cover two topics: non-Hermitian physics and Casimir effect. The first part will focus on the exceptional nexus (EX), which is not only a higher-order exceptional point (EP) but also the cusp singularity of multiple exceptional arcs, which are composed of order-2 EPs. The EX possesses a hybrid topological invariant, which consists of distinct winding numbers associated with Berry phases accumulated by cyclic paths on different complex planes . The second part will talk about Casimir induced instabilities at metallic surfaces and interfaces. Surface plasmons subject to a surface distortion split asymmetrically in energy resulting in a net lowering of zero-point energy, which gives rise to the instabilities of planar structures. This is because surface plasmon eigenvalues are the square of frequencies, a statement generally true for electromagnetic excitations. This mechanism provides a fundamental length scale limit to planar nanostructures .
 Physical Review X 6, 021007 (2016); Science 370, 1077 (2020); arXiv: 2010.10118.
 Physical Review Letters 126, 046802 (2021).