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Effects of surface-bulk hybridization in 3D topological `metals'

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.