  We have considered a system of two topological insulators and have determined
the properties of the surface states at the junction. Here we report that these
states, under certain conditions exhibit superluminous (tachyonic) dispersion
of the Dirac fermions. Although superluminal excitations are known to exist in
optical systems, this is the first demonstration of possible tachyonic
excitations in a purely electronic system. The first ever signature of tachyons
could therefore be found experimentally in a topological insulator junction.
