报告时间：2025年10月14日 上午10:30 (10:30, Oct.14, 2025)

报告地点：物质科研楼A309会议室 Room A309 Material Science Building)

报告人： 马冠聪教授  香港浸会大学物理学院

报告题目/Title:Generalization of exceptional points: high-dimensional coalescing non-Hermitian spaces

摘要/Abstract:Exceptional points (EPs) are spectral singularities in non-Hermitian systems, at which individual eigenvalues and eigenvectors become identical. The successful realization of EPs in diverse physical systems, such as photonics, acoustics, cold atoms, and superconducting circuits, has triggered tremendous interest in fundamental physics and inspired a vast array of novel applications. However, as the term “exceptional” suggests, EPs are usually isolated spectral points. Consequently, functionalities rooted in EPs face several inherent limitations, such as narrow bandwidth. Here, we present a generalization of the concept of EPs characterized by the coalescence of multi-dimensional Hilbert subspaces and overlapped spectra. This condition is denoted as “exceptional deficiency” (ED). When this happens, two Hilbert subspaces of identical but arbitrarily large dimensions exactly align, accompanied by the coincidence of their spectra.  Consequently, the full Hilbert space is highly defective by “missing” half of its span. In the meantime, the spectrum is entirely composed of EPs.

The condition of ED can be reached in a large variety of systems. We present its realization in a one-dimensional double-chain lattice. Intriguingly, the ED can break the bulk-edge correspondence of non-Hermitian skin effects, which relates nontrivial point-gap topology to the existence of skin modes. As a consequence, we find that the ED underpins new non-Hermitian phenomena characterized by the synergy of skin effects and propagation.

报告人简介/Curriculum Vitae:Prof. Guancong Ma is currently a professor of physics at Hong Kong Baptist University. He received B.Sc. in applied physics at the South China University of Technology in 2007 and then Ph.D. in physics at the Hong Kong University of Science and Technology in 2012. After that, he became a postdoc fellow at the Institute for Advanced Study and the Department of Physics at the same institution until 2017, when he joined the Department of Physics at Hong Kong Baptist University. He now serves as a member of the Executive Committee of the Physics Society of Hong Kong. Prof. Ma was awarded the “Young Investigator Award 2021” by the International Phononics Society, and was selected as one of the “Top 10 Rising Stars in Science and Technology 2021” by the China Association for Science and Technology. He is the recipient of the “C. N. Yang Award” in 2022 –  a prize from the Association of Asia Pacific Physical Societies and the Asia Pacific Center for Theoretical Physics. Prof. Ma has obtained the support from the National Natural Science Foundation of China’s Excellent Young Scientists Scheme (Hong Kong & Macao).