  Gray tin, also known as $\alpha$-Sn, can be turned into a three-dimensional
topological insulator (3D-TI) by strain and finite size effects. Such room
temperature 3D-TI is peculiarly interesting for spintronics due to the
spin-momentum locking along the Dirac cone (linear dispersion) of the surface
states. Angle resolved photoemission spectroscopy (ARPES) has been used to
investigate the dispersion close to the Fermi level in thin (0\,0\,1)-oriented
epitaxially strained films of $\alpha$-Sn, for different film thicknesses as
well as for different capping layers (Al, AlO$_x$ and MgO). Indeed a proper
capping layer is necessary to be able to use $\alpha$-Sn surface states for
spintronics applications. In contrast with free surfaces or surfaces coated
with Ag, coating the $\alpha$-Sn surface with Al or AlO$_x$ leads to a drop of
the Fermi level below the Dirac point, an important consequence for transport
is the presence of bulk states at the Fermi level. $\alpha$-Sn films coated by
AlO$_x$ are studied by electrical magnetotransport: despite clear evidence of
surface states revealed by Shubnikov-de Haas oscillations, an important part of
the magneto-transport properties is governed by "bulk" electronic states
attributed to the $\Gamma 8$ band, as suggested by {\it ab-initio}
calculations.
