  We show that Parity-time symmetry in matching electric permittivity to
magnetic permeability can be established by considering an effective Parity
operator involving both mirror symmetry and coupling between electric and
magnetic fields. This approach extends the discussion of Parity-time symmetry
to the situation with more than one material potential. We show that the band
structure of a one-dimensional photonic crystal with alternating purely
dielectric and purely magnetic slabs can undergo a phase transition between
propagation modes and evanescent modes when the balanced gain/loss parameter is
varied. The cross-matching between different material potentials also allows
exceptional points of the constitutive matrix to appear in the long wavelength
limit where they can be used to construct ultrathin metamaterials with
unidirectional reflection.
