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Electrons, holes, and spin in the IV-VI monolayer 'four-six-enes'

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.