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[固态量子及光物理实验室学术报告] Symmetry and Topology in Non-Hermitian Systems 非厄米系统中的对称性与拓扑
时间: 2019年06月13日 10:00
地点: 新萄京科学院物理研究所M236
报告人: 周恒昀

(MIT及哈佛大学)

摘要:The ideas of topology have found great success in Hermitian physical systems, but the incorporation of non-Hermitian effects may lead to even richer possibilities. Here, we present three results regarding the roles of symmetry and topology in non-Hermitian physical systems. First, we theoretically propose and experimentally demonstrate a bulk Fermi arc that develops from non-Hermitian radiative losses in an open system of photonic crystal slabs. Moreover, we discover half-integer topological charges in the polarization of far-field radiation around the bulk Fermi arc. Next, we provide a systematic classification of non-Hermitian symmetry protected topological phases in arbitrary spatial dimension, based on the Bernard-LeClair symmetry classes. We discuss the physical insights provided by such a classification, and how it can serve as an important guide for future searches of non-Hermitian topological systems. Finally, we discuss how symmetries can protect the existence of a surface of exceptional points, which are a natural generalization of Hermitian topological nodal phases.

近年来,拓扑学在厄米物理系统中获得了广泛的应用,而非厄米系统中的拓扑现象可能会带来更加丰富的可能性。本报告将介绍我们近期三个关于非厄米物理系统中的对称性与拓扑的结果。首先,我们在理论上提出并在光子晶体系统中实验验证由非厄米效应所导致的体费米弧。相较于厄米系统表面的费米弧,这里的体费米弧直接来自二维系统自身,因而是非厄米效应的一种独特展现。此外,我们发现在费米弧周围的远场辐射中可以观测到特殊的偏振态半核。接下来,我们基于Bernard-LeClair对称性分类,给出在任意空间维度上非厄米对称保护拓扑相的系统性分类,并详细讨论这些拓扑分类相对应的具体拓扑不变量,以及非厄米系统中对Kramers简并的推广。最后,我们将厄米拓扑系统中的节线简并推广到非厄米系统,以此构造二维节面简并 (nodal surface)。

References:

[1] “Observation of bulk Fermi arc and polarization half charge from paired exceptional points”, H. Zhou*, C. Peng*, Y. Yoon, C. W. Hsu, K. A. Nelson, L. Fu, J. D. Joannopoulos, M. Soljacic, B. Zhen, Science 359, 1009-1012 (2018)
[2] “Exceptional surfaces in PT-symmetric non-Hermitian photonic systems”, H. Zhou, J. Y. Lee, S. Liu, and B. Zhen, Optica 6, 190 (2019)
[3] “Periodic table for topological bands with non-Hermitian Bernard-LeClair symmetries”, H. Zhou, J. Y. Lee, arXiv:1812.10490 (2018)

报告人简历:周恒昀曾获2012年物理国际奥赛金牌,2012年进入北京大学物理系本科学习,2013年转学进入MIT物理系本科学习,本科毕业后进入哈佛大学物理系攻读博士学位, 

参与了离散时间晶体实验工作(Nature 2017),最近关注非厄米及拓扑物理。

邀请人及联系人:范桁(82649518)、陆凌(82649203)


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