Electronic structure and optical properties of Na<sub>2</sub>KSb and NaK<sub>2</sub>Sb from first-principles many-body theory
Raymond Amador, Holger-Dietrich Saßnick, Caterina Cocchi
Abstract
Abstract In the search for novel materials for vacuum electron sources, multi-alkali antimonides and in particular sodium-potassium-antimonides have been recently regarded as especially promising due to their favorable electronic and optical properties. In the framework of density-functional theory and many-body perturbation theory, we investigate the electronic structure and the dielectric response of two representative members of this family, namely Na 2 KSb and NaK 2 Sb. We find that both materials have a direct gap, which is on the order of 1.5 eV in Na 2 KSb and 1.0 eV in NaK 2 Sb. In either system, valence and conduction bands are dominated by Sb states with p - and s -character, respectively. The imaginary part of the dielectric function, computed upon explicit inclusion of electron–hole interactions to characterize the optical response of the materials, exhibits maxima starting from the near-infrared region, extending up to the visible and the ultraviolet band. With our analysis, we clarify that the lowest-energy excitations are non-excitonic in nature and that their binding energy is on the order of 100 meV. Our results confirm the potential of Na 2 KSb and NaK 2 Sb as photoemissive materials for vacuum electron sources, photomultipliers, and imaging devices.