Fermi arcs connect topological degeneracies

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Science  02 Mar 2018:
Vol. 359, Issue 6379, pp. 995-996
DOI: 10.1126/science.aar8210

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In condensed matter and photonics, topology is defined with respect to the energy bands in momentum space (1). The boundary between two topologically different phases (for example, supporting right- versus left-handed particles) appears as degeneracies where two linear dispersion bands intersect. Fundamental point degeneracies in two-dimensional (2D) and 3D Hermitian systems—known as Dirac and Weyl points, respectively—have been observed in photonic structures (26) but not the ideal Weyl points and the helicoidal dispersion, which leads to the open Fermi arcs connecting points of opposite chirality. On page 1013 of this issue, Yang et al. (7) demonstrate an ideal Weyl system with four Weyl points (8, 9) and helicoidal surface Fermi arcs interconnecting them in a 3D photonic crystal composed of metallic inclusions. On page 1009 of this issue, Zhou et al. (10) explore non-Hermitian topological photonics in which radiative losses come into play, and demonstrate the emergence of bulk Fermi arc and half topological charges in a 2D-periodic photonic crystal.