  We consider a junction between two topological insulators, and calculate the
properties of the interface states with an effective low energy Hamiltonian for
topological insulators with a single cone on the surface. This system bears a
close resemblance to bilayer graphene, as both result from the hybridization of
Dirac cones. We find gapless interface states not only when the helicity
direction of the topological surface states are oppositely oriented, but they
can also exist if they are equally oriented. Furthermore, we find that the
existence of the interface states can be understood from the closing of the
bulk gap when the helicity changes orientation. Recently, superluminal
tachyonic excitations were also claimed to exist at the interface between
topological insulators. However, here we show that these interface states do
not exist.
