  Spin-split two-dimensional electronic states have been observed on ultrathin
Sn(001) films grown on InSb(001) substrates. Angle-resolved photoelectron
spectroscopy (ARPES) performed on these films revealed Dirac-cone-like linear
dispersion around the $\bar{\Gamma}$ point of surface Brillouin zone,
suggesting nearly massless electrons belonging to 2D surface states. The states
disperse across a bandgap between bulk-like quantum well states in the films.
Moreover, both circular dichroism of ARPES and spin-resolved ARPES studies show
helical spin polarization of the Dirac-cone-like surface states, suggesting a
topologically protected character as in a bulk topological insulator (TI).
These results indicate that a quasi-3D TI phase can be realized in ultrathin
films of zero-gap semiconductors.
