Unconventional bulk-Fermi-arc links paired third-order exceptional points splitting from a defective triple point

Abstract

Exceptional degeneracies, unique to open systems, are important in non-Hermitian topology. While bulk-Fermi-arcs connecting second-order exceptional points (EP2s) have been observed, the existence of bulk-Fermi-arcs linking higher-order exceptional points remains unexplored. Here, we introduce an unconventional bulk-Fermi-arc in systems with parity-time and pseudo-Hermitian symmetries, which links paired third-order exceptional points (EP3s), where three eigenvalues share identical real parts but distinct imaginary parts. We realize these systems using topological circuits and experimentally demonstrate this unconventional bulk-Fermi-arc. A winding number defined from resultant vector shows that the bulk-Fermi-arc is stabilized by the exchange of Riemannian sheets. Furthermore, analysis via eigenframe deformation and rotation reveals that the EP3 pair is topologically nontrivial and equivalent to a single defective triple point. The EP3s can split from the triple point by varying system parameters, with this splitting protected by topological equivalence. This finding offers insights into non-Hermitian topology with potential applications in wave engineering.

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