In the flurry of experiments looking for topological insulator materials, it
has been recently discovered that some bulk metals very close to topological
insulator electronic states, support the same topological surface states that
are the defining characteristic of the topological insulator. First observed in
spin-polarized ARPES in Sb (D. Hsieh et al. Science 323, 919 (2009)), the
helical surface states in the metallic systems appear to be robust to at least
mild disorder. We present here a theoretical investigation of the nature of
these "helical metals" - bulk metals with helical surface states. We explore
how the surface and bulk states can mix, in both clean and disordered systems.
Using the Fano model, we discover that in a clean system, the helical surface
states are \emph{not} simply absorbed by hybridization with a non-topological
parasitic metallic band. Instead, they are pushed away from overlapping in
momentum and energy with the bulk states, leaving behind a finite-lifetime
surface resonance in the bulk energy band. Furthermore, the hybridization may
lead in some cases to multiplied surface state bands, in all cases retaining
the helical characteristic. Weak disorder leads to very similar effects -
surface states are pushed away from the energy bandwidth of the bulk, leaving
behind a finite-lifetime surface resonance in place of the original surface
states
