  We report on the epitaxial fabrication and electronic properties of a
topological phase in strained \alpha-Sn on InSb. The topological surface state
forms in the presence of an unusual band order not based on direct spin-orbit
coupling, as shown in density functional and GW slab-layer calculations.
Angle-resolved photoemission including spin detection probes experimentally how
the topological spin-polarized state emerges from the second bulk valence band.
Moreover, we demonstrate the precise control of the Fermi level by dopants.
