Abstract
Dirac cones on the Γ point of periodic metamaterials have a unique feature in which the electromagnetic
wave propagates in the medium without a phase change, which realizes such novel optical phenomena as
cloaking, scatter-free waveguides, and the lenses of arbitrary shapes. In this article, we analyze the
mechanism of Dirac-cone formation by tight binding approximation and k・p perturbation to show that
its essence is the accidental degeneracy of two modes with particular symmetry combinations. The
Dirac cone can be materialized for any waves in the periodic structures, which include electromagnetic,
electronic, phonon, and magnon waves.
