  We study the propagation of electromagnetic waves in layered photonic
crystals formed by materials with a spontaneous nonreciprocal response, such as
Tellegen (axion) media or topological insulators. Surprisingly, it is proven
that stratified Tellegen photonic crystals that break simultaneously the space
inversion and time reversal symmetries have always symmetric dispersion
diagrams. Interestingly, we show that by combining chiral and nonreciprocal
materials the photonic band diagrams can exhibit a spectral asymmetry such that
. Furthermore, it is demonstrated that in some conditions two juxtaposed
Tellegen medium layers have an electromagnetic response analogous to that of a
biased ferrite slab.
