  Bandedge states in the indirect-gap group-IV metal monochalcogenide
monolayers ('four-six-enes' such as SnS, GeTe, etc.) inherit the properties of
nearby reciprocal space points of high symmetry at the Brillouin zone edge. We
employ group theory and the method of invariants to capture these essential
symmetries in effective Hamiltonians including spin-orbit coupling, and use
perturbation theory to shed light on the nature of the bandedge states. In
particular, we show how the structure of derived wavefunctions leads to
specific dominant momentum and spin scattering mechanisms for both valence
holes and conduction electrons, we analyze the direct optical transitions
across the bandgap, and expose the interactions responsible for subtle features
of the local dispersion relations.
