  We show that a smooth interface between two insulators of opposite
topological Z2 indices possesses multiple surface states, both massless and
massive. While the massless surface state is non-degenerate, chiral and
insensitive to the interface potential, the massive surface states only appear
for a sufficiently smooth heterojunction. The surface states are particle-hole
symmetric and a voltage drop reveals their intrinsic relativistic nature,
similarly to Landau bands of Dirac electrons in a magnetic field. We discuss
the relevance of the massive Dirac surface states in recent ARPES and transport
experiments.
