Fig. 3: Planar exceptional chain.
From: Symmetry-protected topological exceptional chains in non-Hermitian crystals
a Schematic of a planar exceptional chain protected by two mirror-adjoint symmetries (M1,2-†). K0 is the non-defective chain point confined on the intersection of two mirror planes Π1 and Π2. A and B are two regions of exact phase in Π1 with opposite M1-†parities \({\tilde{p}}_{{{{{{{{\rm{A}}}}}}}}}^{+}{\tilde{p}}_{{{{{{{{\rm{B}}}}}}}}}^{+} =-1\). b, c Two plausible evolutions compatible with the source-free principle of exceptional lines, which are, however, forbidden by (b) the same M2-†-parity of two bands and by (c) the quantized Berry phase along ΓM, respectively. In each plane, the lighter (darker) region denotes the exact (broken) phase.
