Symmetry and Higher-Order Exceptional Points

Exceptional points (EPs), at which both eigenvalues and eigenvectors coalesce, are ubiquitous and unique features of non-Hermitian systems. Second-order EPs are by far the most studied due to their abundance, requiring only the tuning of two real parameters, which is less than the three parameters needed to generically find ordinary Hermitian eigenvalue degeneracies. Higher-order EPs generically require more fine-tuning, and are thus assumed to play a much less prominent role. Here, however, we illuminate how physically relevant symmetries make higher-order EPs dramatically more abundant and conceptually richer. More saliently, third-order EPs generically require only two real tuning parameters in the presence of either a parity-time (PT) symmetry or a generalized chiral symmetry. Remarkably, we find that these different symmetries yield topologically distinct types of EPs. We illustrate our findings in simple models, and show how third-order EPs with a generic similar to k1=3 dispersion are protected by PT symmetry, while thirdorder EPs with a similar to k1=2 dispersion are protected by the chiral symmetry emerging in non-Hermitian Lieb lattice models. More generally, we identify stable, weak, and fragile aspects of symmetry-protected higherorder EPs, and tease out their concomitant phenomenology.

Automated summary

Exceptional points (EPs), where eigenvalues and eigenvectors coalesce, are common and unique features of non-Hermitian systems. Higher-order EPs, which typically require more fine-tuning, can become more abundant and conceptually richer with physically relevant symmetries.