Electronic structure and optical properties of Na$_2$KSb and NaK$_2$Sb from first-principles many-body theory

TL;DR

This study uses first-principles many-body theory to analyze the electronic and optical properties of Na$_2$KSb and NaK$_2$Sb, revealing their potential as photoemissive materials with direct band gaps and specific optical responses.

Contribution

It provides detailed electronic structure and optical response data for Na$_2$KSb and NaK$_2$Sb using advanced theoretical methods, highlighting their suitability for vacuum electron sources.

Findings

Both materials have direct band gaps (~1.5 eV and 1.0 eV).

Valence and conduction bands are Sb p- and s-character.

Optical response peaks extend from near-infrared to ultraviolet.

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.