  HgTe is a versatile topological material and has enabled the realization of a
variety of topological states, including two- and three-dimensional (3D)
topological insulators and topological semimetals. Nevertheless, a quantitative
understanding of its electronic structure remains challenging, in particular
due to coupling of the Te 5p-derived valence electrons to Hg 5d core states at
shallow binding energy. We present a joint experimental and theoretical study
of the electronic structure in strained HgTe(001) films in the 3D
topological-insulator regime, based on angle-resolved photoelectron
spectroscopy and density functional theory. The results establish detailed
agreement in terms of (i) electronic band dispersions and orbital symmetries,
(ii) surface and bulk contributions to the electronic structure, and (iii) the
importance of Hg 5d states in the valence-band formation. Supported by theory,
our experiments directly image the paradigmatic band inversion in HgTe,
underlying its non-trivial band topology.
