  The unusual electronic states found in topological materials can enable a new
generation of devices and technologies, yet a long-standing challenge has been
finding materials without deleterious parallel bulk conduction. This can arise
either from defects or thermally activated carriers. Here, I clarify the
criteria that materials need to meet to realize transport properties dominated
by the topological states, a necessity for a topological device. This is
demonstrated for 3-dimensional topological insulators, 3D Dirac materials, and
1D quantum anomalous Hall insulators, though this can be applied to similar
systems. The key parameters are electronic band gap, dielectric constant, and
carrier effective mass, which dictate under what circumstances (defect density,
temperature, etc.) the unwanted bulk state will conduct in parallel to the
topological states. As these are fundamentally determined by the basic atomic
properties, simple chemical arguments can be used to navigate the phase space
to ultimately find improved materials. This will enable rapid identification of
new systems with improved properties, which is crucial to design new materials
systems and push into a new generation of topological technologies.
