  A new type of topological spin-helical surface states was discovered in
layered van der Waals bonded (SnTe)$_{n=2,3}$(Bi$_2$Te$_3$)$_{m=1}$ compounds
which comprise two covalently bonded band inverted subsystems, SnTe and
Bi$_2$Te$_3$, within a building block. This novel topological states
demonstrate non-Dirac dispersion within the band gap. The dispersion of the
surface state has two linear sections of different slope with shoulder feature
between them. Such a dispersion of the topological surface state enables
effective switch of the velocity of topological carriers by means of applying
an external electric field.
