Ab initio calculations of the dielectric function of Eu₅In₂Sb₆ for light dark matter detection

Quantum materials with narrow meV-scale band gaps have recently emerged as promising targets for the detection of light dark matter (DM) in the largely unexplored mass range of keV-MeV DM. Eu₅In₂Sb₆, a narrow band gap material with complex antiferromagnetic ordering and potentially non-trivial band topology, is currently under investigation for DM detection, but many questions about its electronic structure remain open. Here, we present first-principles density functional theory calculations of the electronic structure and complex dielectric function of Eu₅In₂Sb₆, addressing the effects of magnetic ordering, spin-orbit coupling, and GW self-energy corrections. We will discuss the challenges of accurately calculating the electronic structure and dielectric function of narrow band gap quantum materials, such as Eu₅In₂Sb₆, and the implications this has for accurate prediction of the projected reach of light DM scattering and absorption.