Extended Data Fig. 10: Formation of Hopf link surface states.
From: Topological Chern vectors in three-dimensional photonic crystals
a, b, Illustrations for surface Fermi loops induced by Chern vectors \({{\bf{C}}}_{1}=2\hat{z}\) and \({{\bf{C}}}_{2}=2\hat{x}\), respectively. c, Surface Fermi loops rearranged in the presence of coupling between two photonic crystals with \({{\bf{C}}}_{1}=2\hat{z}\) and \({{\bf{C}}}_{2}=2\hat{x}\). Blue and red solid lines depict the resulted two Fermi loops around the BZ. d, Blue and red lines individually form a loop in a torus geometry, and the two loops form a Hopf link.
