Exceptional Fermion Doubling of Non-Hermitian Systems

Date

Feb 24, 2026

Time

11:00 AM - 12:00 PM

Speaker

Lukas König

Affiliation

Stockholm University

Language

en

Main Topic

Materialien

Host

Grit Rötzer

Description

Nielsen and Ninomiya's fermion doubling theorem arises from the topological classification of chiral fermions in lattice systems, mandating that their total chirality across the Brillouin zone must vanish. It has recently been shown to break down in two distinct ways. First, in non-Hermitian systems, the "fermions" are exceptional points that are topologically classified by non-Abelian braids. This complicates the total chirality requirement, leading to the possibility of monopoles. Second, in systems with non-symmorphic symmetries, the underlying Brillouin zone can reduce to a non-orientable manifold, weakening the notion of chirality itself. Using simple examples as well as homotopy theory, I will show how the classification of gapped phases has to be adapted, and what remains of fermion doubling in such systems. I will highlight two recent photonic experiments in which the aforementioned monopoles were observed directly. Time permitting, I will discuss extensions of these homotopy tools to more complicated problems. --- [1] König, J. L. K. et al., Braid-protected topological band structures with unpaired exceptional points. Phys. Rev. Res. 5, L042010 (2023). [2] König, J. L. K. et al., Exceptional Topology on Nonorientable Manifolds. To appear in Phys. Rev. Research (2026). [3] Wang, K. et al. Observation of Braid-Protected Unpaired Exceptional Points. Phys. Rev. Lett. 136, 056602 (2026). [4] Xu, Z.-S. et al. Non-Hermitian Exceptional Topology on a Klein Bottle Photonic Circuit. Preprint at arXiv:2512.20273. .

Links

• Seminars at the IFW

Last modified: Feb 23, 2026, 7:40:27 AM