  Having previously been the subject of decades of semiconductor research,
cadmium arsenide has now reemerged as a topological material, realizing ideal
three-dimensional Dirac points at the Fermi level. These topological Dirac
points lead to a number of extraordinary transport phenomena, including strong
quantum oscillations, large magnetoresistance, ultrahigh mobilities, and Fermi
velocities exceeding graphene. The large mobilities persist even in thin films
and nanowires of cadmium arsenide, suggesting the involvement of topological
surface states. However, computational studies of the surface states in this
material are lacking, in part due to the large 80-atom unit cell. Here we
present the computed Fermi arc surface states of a cadmium arsenide thin film,
based on a tight-binding model derived directly from the electronic structure.
We show that despite the close proximity of the Dirac points, the Fermi arcs
are very long and straight, extending through nearly the entire Brillouin zone.
The shape and spin properties of the Fermi arcs suppress both back- and side-
scattering at the surface, which we show by explicit integrals over the phase
space. The introduction of a small symmetry-breaking term, expected in a strong
electric field, gaps the electronic structure, creating a weak topological
insulator phase that exhibits similar transport properties. Crucially, the
mechanisms suppressing scattering in this material differ from those in other
topological materials such as Weyl semimetals and topological insulators,
suggesting a new route for engineering high-mobility devices based on Dirac
semimetal surface states.
