  The electronic structure of (group-III) metal-monochalcogenide monolayers
exhibits many unusual features. Some, such as the unusually distorted upper
valence band dispersion we describe as a 'caldera', are primarily the result of
purely orbital interactions. Others, including spin splitting and wavefunction
spin-mixing, are directly driven by spin-orbit coupling. We employ elementary
group theory to explain the origins of these properties, and use a
tight-binding model to calculate the phenomena enabled by them, such as
band-edge carrier effective g-factors, optical absorption spectrum, conduction
electron spin orientation, and a relaxation-induced upper valence band
population inversion and spin polarization mechanism.
