Electronic and optical excitations of K-Sb and Na-Sb crystals

Recent advances in experimental techniques and computational methods have significantly expanded the family of alkali antimonides, a class of semiconducting materials used as photocathodes in particle accelerators, unveiling new crystal structures and stoichiometries with improved stability and quantum efficiency. This work investigates the electronic and optical properties of eight Na- and K-based alkali antimonide binary crystals with 3:1 and 1:1 alkali-to-antimony ratios, which were predicted to be stable in a recent high-throughput screening study. Employing density functional theory and many-body perturbation theory, we find that all systems exhibit direct band gaps, except for monoclinic Na$_8$Sb$_8$, which has a nearly degenerate indirect gap. Optical spectra are characterized by near-infrared absorption onsets and intense visible excitations. Our analysis highlights the significant role of electron-hole correlations, particularly in K-based compounds, leading to exciton binding energies above 100~meV and sharper absorption peaks. An in-depth analysis of the electronic contributions to the excited states provides additional insight into the role of excitonic effects. By shedding light on the fundamental properties of alkali antimonide binary crystals, our results are relevant for the design and optimization of next-generation electron sources for particle accelerators.