  Identifying the effects of surface-bulk coupling is a key challenge towards
exploiting the topological nature of the surface states in many available
three-dimensional topological `metals'. Here we combine an effective-model
calculation and an ab-initio slab calculation to study the effects of the
lowest order surface-bulk interaction: the hybridization. In the
effective-model study, we discretize an established low-energy effective
four-band model and introduce the hybridization between the surface bands and
bulk bands in the spirit of the Fano model. We find that the hybridization
enhances the energy gap between bulk and Dirac surface states and preserves the
latter's spin texture qualitatively albeit with a reduced spin-polarization
magnitude. Our ab-initio study finds the energy gap between the bulk and
surface states to grow upon an increase in the slab thickness, very much in
qualitative agreement with the effective model study. Comparing the results of
our two approaches, we deduce that the experimentally observed low magnitude of
spin polarization can be attributed to a hybridization-type surface-bulk
interaction. We discuss evidences for such hybridization in existing ARPES
data.
